OpenGL Roundup, September 19, 2013

Here’s the beginning of a new series on OpenGL ES 2.0 for iOS, using Apple’s GLKit.

ROBOVM BACKEND IN LIBGDX NIGHTLIES AND FIRST PERFORMANCE FIGURES! – Libgdx is moving to a new backend for iOS that uses RoboVM, a Java to machine code compiler for iOS. Initial performance figures look good!

Zero to Sixty in One Second – the developer & designer behind acko.net has redesigned his header and website using WebGL, and I have to say that it looks very cool.

And now for something completely different…

Using runtime-compiled C++ code as a scripting language.

Adding a 3d Perspective and Object Rendering to Our Air Hockey Project in Native C Code

For this post in the air hockey series, we’ll learn how to render our scene from a 3D perspective, as well as how to add a puck and two mallets to the scene. We’ll also see how easy it is to bring these changes to Android, iOS, and emscripten.

Prerequisites

This lesson continues the air hockey project series, building upon the code from GitHub for ‘article-2-loading-png-file’. Here are the previous posts in this series:

Setting up a simple build system

Adding support for PNG loading into a texture

Adding support for a matrix library

The first thing we’ll do is add support for a matrix library so we can use the same matrix math on all three platforms, and then we’ll introduce the changes to our code from the top down. There are a lot of libraries out there, so I decided to use linmath.h by Wolfgang Draxinger for its simplicity and compactness. Since it’s on GitHub, we can easily add it to our project by running the following git command from the root airhockey/ folder:

git submodule add https://github.com/datenwolf/linmath.h.git src/3rdparty/linmath

Updating our game code

We’ll introduce all of the changes from the top down, so let’s begin by replacing everything inside game.c as follows:

Headers and declarations

#include "game.h"
#include "game_objects.h"
#include "asset_utils.h"
#include "buffer.h"
#include "image.h"
#include "linmath.h"
#include "math_helper.h"
#include "matrix.h"
#include "platform_gl.h"
#include "platform_asset_utils.h"
#include "program.h"
#include "shader.h"
#include "texture.h"

static const float puck_height = 0.02f;
static const float mallet_height = 0.15f;

static Table table;
static Puck puck;
static Mallet red_mallet;
static Mallet blue_mallet;

static TextureProgram texture_program;
static ColorProgram color_program;

static mat4x4 projection_matrix;
static mat4x4 model_matrix;
static mat4x4 view_matrix;

static mat4x4 view_projection_matrix;
static mat4x4 model_view_projection_matrix;

static void position_table_in_scene();
static void position_object_in_scene(float x, float y, float z);

We’ve added all of the new includes, constants, variables, and function declarations that we’ll need for our new game code. We’ll use Table, Puck, and Mallet to represent our drawable objects, TextureProgram and ColorProgram to represent our shader programs, and the mat4x4 (a datatype from linmath.h) matrices for our OpenGL matrices. In our draw loop, we’ll call position_table_in_scene() to position the table, and position_object_in_scene() to position our other objects.

For those of you who have also followed the Java tutorials from OpenGL ES 2 for Android: A Quick-Start Guide, you’ll recognize that this has a lot in common with the air hockey project from the first part of the book. The code for that project can be freely downloaded from The Pragmatic Bookshelf.

on_surface_created()

void on_surface_created() {
	glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
	glEnable(GL_DEPTH_TEST);

	table = create_table(
		load_png_asset_into_texture("textures/air_hockey_surface.png"));

	vec4 puck_color = {0.8f, 0.8f, 1.0f, 1.0f};
	vec4 red = {1.0f, 0.0f, 0.0f, 1.0f};
	vec4 blue = {0.0f, 0.0f, 1.0f, 1.0f};

	puck = create_puck(0.06f, puck_height, 32, puck_color);
	red_mallet = create_mallet(0.08f, mallet_height, 32, red);
	blue_mallet = create_mallet(0.08f, mallet_height, 32, blue);

	texture_program = get_texture_program(build_program_from_assets(
		"shaders/texture_shader.vsh", "shaders/texture_shader.fsh"));
	color_program = get_color_program(build_program_from_assets(
		"shaders/color_shader.vsh", "shaders/color_shader.fsh"));
}

Our new on_surface_created() enables depth-testing, initializes the table, puck, and mallets, and loads in the shader programs.

on_surface_changed(int width, int height)

void on_surface_changed(int width, int height) {
	glViewport(0, 0, width, height);
	mat4x4_perspective(projection_matrix, 45, (float) width / (float) height, 1, 10);
	mat4x4_look_at(view_matrix, 0.0f, 1.2f, 2.2f, 0.0f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f);
}

Our new on_surface_changed(int width, int height) now takes in two parameters for the width and the height, and it sets up a projection matrix, and then sets up the view matrix to be slightly above and behind the origin, with an eye position of (0, 1.2, 2.2).

on_draw_frame()

void on_draw_frame() {
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    mat4x4_mul(view_projection_matrix, projection_matrix, view_matrix);

	position_table_in_scene();
    draw_table(&table, &texture_program, model_view_projection_matrix);

	position_object_in_scene(0.0f, mallet_height / 2.0f, -0.4f);
	draw_mallet(&red_mallet, &color_program, model_view_projection_matrix);

	position_object_in_scene(0.0f, mallet_height / 2.0f, 0.4f);
	draw_mallet(&blue_mallet, &color_program, model_view_projection_matrix);

	// Draw the puck.
	position_object_in_scene(0.0f, puck_height / 2.0f, 0.0f);
	draw_puck(&puck, &color_program, model_view_projection_matrix);
}

Our new on_draw_frame() positions and draws the table, mallets, and the puck.

Because we changed the definition of on_surface_changed(), we also have to change the declaration in game.h. Change void on_surface_changed(); to void on_surface_changed(int width, int height);.

Adding new helper functions

static void position_table_in_scene() {
	// The table is defined in terms of X & Y coordinates, so we rotate it
	// 90 degrees to lie flat on the XZ plane.
	mat4x4 rotated_model_matrix;
	mat4x4_identity(model_matrix);
	mat4x4_rotate_X(rotated_model_matrix, model_matrix, deg_to_radf(-90.0f));
	mat4x4_mul(
		model_view_projection_matrix, view_projection_matrix, rotated_model_matrix);
}

static void position_object_in_scene(float x, float y, float z) {
	mat4x4_identity(model_matrix);
	mat4x4_translate_in_place(model_matrix, x, y, z);
	mat4x4_mul(model_view_projection_matrix, view_projection_matrix, model_matrix);
}

These functions update the matrices to let us position the table, puck, and mallets in the scene. We’ll define all of the extra functions that we need soon.

Adding new shaders

Now we’ll start drilling down into each part of the program and make the changes necessary for our game code to work. Let’s begin by updating our shaders. First, let’s rename our vertex shader shader.vsh to texture_shader.vsh and update it as follows:

texture_shader.vsh

uniform mat4 u_MvpMatrix;

attribute vec4 a_Position;
attribute vec2 a_TextureCoordinates;

varying vec2 v_TextureCoordinates;

void main()
{
    v_TextureCoordinates = a_TextureCoordinates;
    gl_Position = u_MvpMatrix * a_Position;
}

We can rename our fragment shader shader.fsh to texture_shader.fsh without making any other changes.

We’ll also need a new set of shaders to render our puck and mallets. Let’s add the following new shaders:

color_shader.vsh

uniform mat4 u_MvpMatrix;
attribute vec4 a_Position;
void main()
{
    gl_Position = u_MvpMatrix * a_Position;
}

color_shader.fsh

precision mediump float;
uniform vec4 u_Color;
void main()
{
    gl_FragColor = u_Color;
}

Creating our game objects

Now we’ll add support for generating and drawing our game objects. Let’s begin with game_objects.h:

#include "platform_gl.h"
#include "program.h"
#include "linmath.h"

typedef struct {
	GLuint texture;
	GLuint buffer;
} Table;

typedef struct {
	vec4 color;
	GLuint buffer;
	int num_points;
} Puck;

typedef struct {
	vec4 color;
	GLuint buffer;
	int num_points;
} Mallet;

Table create_table(GLuint texture);
void draw_table(const Table* table, const TextureProgram* texture_program, mat4x4 m);

Puck create_puck(float radius, float height, int num_points, vec4 color);
void draw_puck(const Puck* puck, const ColorProgram* color_program, mat4x4 m);

Mallet create_mallet(float radius, float height, int num_points, vec4 color);
void draw_mallet(const Mallet* mallet, const ColorProgram* color_program, mat4x4 m);

We’ve defined three C structs to hold the data for our table, puck, and mallets, and we’ve declared functions to create and draw these objects.

Drawing a table

Let’s continue with game_objects.c:

#include "game_objects.h"
#include "buffer.h"
#include "platform_gl.h"
#include "program.h"
#include "linmath.h"
#include <math.h>

// Triangle fan
// position X, Y, texture S, T
static const float table_data[] = { 0.0f,  0.0f, 0.5f, 0.5f,
        						   -0.5f, -0.8f, 0.0f, 0.9f,
        						   	0.5f, -0.8f, 1.0f, 0.9f,
        						   	0.5f,  0.8f, 1.0f, 0.1f,
        						   -0.5f,  0.8f, 0.0f, 0.1f,
        						   -0.5f, -0.8f, 0.0f, 0.9f};

Table create_table(GLuint texture) {
	return (Table) {texture, 
		create_vbo(sizeof(table_data), table_data, GL_STATIC_DRAW)};
}

void draw_table(const Table* table, const TextureProgram* texture_program, mat4x4 m)
{
	glUseProgram(texture_program->program);

	glActiveTexture(GL_TEXTURE0);
	glBindTexture(GL_TEXTURE_2D, table->texture);
	glUniformMatrix4fv(texture_program->u_mvp_matrix_location, 1, 
		GL_FALSE, (GLfloat*)m);
	glUniform1i(texture_program->u_texture_unit_location, 0);

	glBindBuffer(GL_ARRAY_BUFFER, table->buffer);
	glVertexAttribPointer(texture_program->a_position_location, 2, GL_FLOAT,
		GL_FALSE, 4 * sizeof(GL_FLOAT), BUFFER_OFFSET(0));
	glVertexAttribPointer(texture_program->a_texture_coordinates_location, 2, GL_FLOAT,
		GL_FALSE, 4 * sizeof(GL_FLOAT), BUFFER_OFFSET(2 * sizeof(GL_FLOAT)));
	glEnableVertexAttribArray(texture_program->a_position_location);
	glEnableVertexAttribArray(texture_program->a_texture_coordinates_location);
	glDrawArrays(GL_TRIANGLE_FAN, 0, 6);

	glBindBuffer(GL_ARRAY_BUFFER, 0);
}

After the imports, this is the code to create and draw the table data. This is essentially the same as what we had before, with the coordinates adjusted a bit to change the table into a rectangle.

Generating circles and cylinders

Before we can draw a puck or a mallet, we’ll need to add some helper functions to draw a circle or a cylinder. Let’s define those now:

static inline int size_of_circle_in_vertices(int num_points) {
	return 1 + (num_points + 1);
}

static inline int size_of_open_cylinder_in_vertices(int num_points) {
	return (num_points + 1) * 2;
}

We first need two helper functions to calculate the size of a circle or a cylinder in terms of vertices. A circle drawn as a triangle fan has one vertex for the center, num_points vertices around the circle, and one more vertex to close the circle. An open-ended cylinder drawn as a triangle strip doesn’t have a center point, but it does have two vertices for each point around the circle, and two more vertices to close off the circle.

static inline int gen_circle(float* out, int offset, 
	float center_x, float center_y, float center_z, 
	float radius, int num_points)
{
	out[offset++] = center_x;
	out[offset++] = center_y;
	out[offset++] = center_z;

	int i;
	for (i = 0; i <= num_points; ++i) {
		float angle_in_radians = ((float) i / (float) num_points) 
                               * ((float) M_PI * 2.0f);
		out[offset++] = center_x + radius * cos(angle_in_radians);
		out[offset++] = center_y;
		out[offset++] = center_z + radius * sin(angle_in_radians);
	}

	return offset;
}

This code will generate a circle, given a center point, a radius, and the number of points around the circle.

static inline int gen_cylinder(float* out, int offset, 
	float center_x, float center_y, float center_z, 
	float height, float radius, int num_points)
{
	const float y_start = center_y - (height / 2.0f);
	const float y_end = center_y + (height / 2.0f);

	int i;
	for (i = 0; i <= num_points; i++) {
		float angle_in_radians = ((float) i / (float) num_points) 
                               * ((float) M_PI * 2.0f);

		float x_position = center_x + radius * cos(angle_in_radians);
		float z_position = center_z + radius * sin(angle_in_radians);

		out[offset++] = x_position;
		out[offset++] = y_start;
		out[offset++] = z_position;

		out[offset++] = x_position;
		out[offset++] = y_end;
		out[offset++] = z_position;
	}

	return offset;
}

This code will generate the vertices for an open-ended cylinder. Note that for both the circle and the cylinder, the loop goes from 0 to num_points, so the first and last points around the circle are duplicated in order to close the loop around the circle.

Drawing a puck

Let’s add the code to generate and draw the puck:

Puck create_puck(float radius, float height, int num_points, vec4 color)
{
	float data[(size_of_circle_in_vertices(num_points) 
              + size_of_open_cylinder_in_vertices(num_points)) * 3];

	int offset = gen_circle(data, 0, 0.0f, height / 2.0f, 0.0f, radius, num_points);
	gen_cylinder(data, offset, 0.0f, 0.0f, 0.0f, height, radius, num_points);

	return (Puck) {{color[0], color[1], color[2], color[3]},
				   create_vbo(sizeof(data), data, GL_STATIC_DRAW),
				   num_points};
}

A puck contains one open-ended cylinder, and a circle to top off that cylinder.

void draw_puck(const Puck* puck, const ColorProgram* color_program, mat4x4 m)
{
	glUseProgram(color_program->program);

	glUniformMatrix4fv(color_program->u_mvp_matrix_location, 1, GL_FALSE, (GLfloat*)m);
	glUniform4fv(color_program->u_color_location, 1, puck->color);

	glBindBuffer(GL_ARRAY_BUFFER, puck->buffer);
	glVertexAttribPointer(color_program->a_position_location, 3, GL_FLOAT, 
		GL_FALSE, 0, BUFFER_OFFSET(0));
	glEnableVertexAttribArray(color_program->a_position_location);

	int circle_vertex_count = size_of_circle_in_vertices(puck->num_points);
	int cylinder_vertex_count = size_of_open_cylinder_in_vertices(puck->num_points);

	glDrawArrays(GL_TRIANGLE_FAN, 0, circle_vertex_count);
	glDrawArrays(GL_TRIANGLE_STRIP, circle_vertex_count, cylinder_vertex_count);
	glBindBuffer(GL_ARRAY_BUFFER, 0);
}

To draw the puck, we pass in the uniforms and attributes, and then we draw the circle as a triangle fan, and the cylinder as a triangle strip.

Drawing a mallet

Let’s continue with the code to create and draw a mallet:

Mallet create_mallet(float radius, float height, int num_points, vec4 color)
{
	float data[(size_of_circle_in_vertices(num_points) * 2 
	          + size_of_open_cylinder_in_vertices(num_points) * 2) * 3];

	float base_height = height * 0.25f;
	float handle_height = height * 0.75f;
	float handle_radius = radius / 3.0f;

	int offset = gen_circle(data, 0, 0.0f, -base_height, 0.0f, radius, num_points);
	offset = gen_circle(data, offset, 
		0.0f, height * 0.5f, 0.0f, 
		handle_radius, num_points);
	offset = gen_cylinder(data, offset, 
		0.0f, -base_height - base_height / 2.0f, 0.0f, 
		base_height, radius, num_points);
	gen_cylinder(data, offset, 
		0.0f, height * 0.5f - handle_height / 2.0f, 0.0f, 
		handle_height, handle_radius, num_points);

	return (Mallet) {{color[0], color[1], color[2], color[3]},
					 create_vbo(sizeof(data), data, GL_STATIC_DRAW),
				     num_points};
}

A mallet contains two circles and two open-ended cylinders, positioned and sized so that the mallet’s base is wider and shorter than the mallet’s handle.

void draw_mallet(const Mallet* mallet, const ColorProgram* color_program, mat4x4 m)
{
	glUseProgram(color_program->program);

	glUniformMatrix4fv(color_program->u_mvp_matrix_location, 1, GL_FALSE, (GLfloat*)m);
	glUniform4fv(color_program->u_color_location, 1, mallet->color);

	glBindBuffer(GL_ARRAY_BUFFER, mallet->buffer);
	glVertexAttribPointer(color_program->a_position_location, 3, GL_FLOAT, 
	GL_FALSE, 0, BUFFER_OFFSET(0));
	glEnableVertexAttribArray(color_program->a_position_location);

	int circle_vertex_count = size_of_circle_in_vertices(mallet->num_points);
	int cylinder_vertex_count = size_of_open_cylinder_in_vertices(mallet->num_points);
	int start_vertex = 0;

	glDrawArrays(GL_TRIANGLE_FAN, start_vertex, circle_vertex_count); 
	start_vertex += circle_vertex_count;
	glDrawArrays(GL_TRIANGLE_FAN, start_vertex, circle_vertex_count); 
	start_vertex += circle_vertex_count;
	glDrawArrays(GL_TRIANGLE_STRIP, start_vertex, cylinder_vertex_count); 
	start_vertex += cylinder_vertex_count;
	glDrawArrays(GL_TRIANGLE_STRIP, start_vertex, cylinder_vertex_count);
	glBindBuffer(GL_ARRAY_BUFFER, 0);
}

Drawing the mallet is similar to drawing the puck, except that now we draw two circles and two cylinders.

Adding math helper functions

We’ll need to add a helper function that we’re currently using in game.c; create a new header file called math_helper.h, and add the following code:

#include <math.h>

static inline float deg_to_radf(float deg) {
	return deg * (float)M_PI / 180.0f;
}

Since C’s trigonometric functions expect passed-in values to be in radians, we’ll use this function to convert degrees into radians, where needed.

Adding matrix helper functions

While linmath.h contains a lot of useful functions, there’s a few missing that we need for our game code. Create a new header file called matrix.h, and begin by adding the following code, all adapted from Android’s OpenGL Matrix class:

#include "linmath.h"
#include <math.h>
#include <string.h>

/* Adapted from Android's OpenGL Matrix.java. */

static inline void mat4x4_perspective(mat4x4 m, float y_fov_in_degrees, 
	float aspect, float n, float f)
{
	const float angle_in_radians = (float) (y_fov_in_degrees * M_PI / 180.0);
	const float a = (float) (1.0 / tan(angle_in_radians / 2.0));

	m[0][0] = a / aspect;
	m[1][0] = 0.0f;
	m[2][0] = 0.0f;
	m[3][0] = 0.0f;

	m[1][0] = 0.0f;
	m[1][1] = a;
	m[1][2] = 0.0f;
	m[1][3] = 0.0f;

	m[2][0] = 0.0f;
	m[2][1] = 0.0f;
	m[2][2] = -((f + n) / (f - n));
	m[2][3] = -1.0f;

	m[3][0] = 0.0f;
	m[3][1] = 0.0f;
	m[3][2] = -((2.0f * f * n) / (f - n));
	m[3][3] = 0.0f;
}

We’ll use mat4x4_perspective() to setup a perspective projection matrix.

static inline void mat4x4_translate_in_place(mat4x4 m, float x, float y, float z)
{
	int i;
    for (i = 0; i < 4; ++i) {
        m[3][i] += m[0][i] * x
        		+  m[1][i] * y
        		+  m[2][i] * z;
    }
}

This helper function lets us translate a matrix in place.

static inline void mat4x4_look_at(mat4x4 m,
		float eyeX, float eyeY, float eyeZ,
		float centerX, float centerY, float centerZ,
		float upX, float upY, float upZ)
{
	// See the OpenGL GLUT documentation for gluLookAt for a description
	// of the algorithm. We implement it in a straightforward way:

	float fx = centerX - eyeX;
	float fy = centerY - eyeY;
	float fz = centerZ - eyeZ;

	// Normalize f
	vec3 f_vec = {fx, fy, fz};
	float rlf = 1.0f / vec3_len(f_vec);
	fx *= rlf;
	fy *= rlf;
	fz *= rlf;

	// compute s = f x up (x means "cross product")
	float sx = fy * upZ - fz * upY;
	float sy = fz * upX - fx * upZ;
	float sz = fx * upY - fy * upX;

	// and normalize s
	vec3 s_vec = {sx, sy, sz};
	float rls = 1.0f / vec3_len(s_vec);
	sx *= rls;
	sy *= rls;
	sz *= rls;

	// compute u = s x f
	float ux = sy * fz - sz * fy;
	float uy = sz * fx - sx * fz;
	float uz = sx * fy - sy * fx;

	m[0][0] = sx;
	m[0][1] = ux;
	m[0][2] = -fx;
	m[0][3] = 0.0f;

	m[1][0] = sy;
	m[1][1] = uy;
	m[1][2] = -fy;
	m[1][3] = 0.0f;

	m[2][0] = sz;
	m[2][1] = uz;
	m[2][2] = -fz;
	m[2][3] = 0.0f;

	m[3][0] = 0.0f;
	m[3][1] = 0.0f;
	m[3][2] = 0.0f;
	m[3][3] = 1.0f;

	mat4x4_translate_in_place(m, -eyeX, -eyeY, -eyeZ);
}

We can use mat4x4_look_at() like a camera, and use it to position the scene in a certain way.

Adding shader program wrappers

We’re almost done the changes to our core code. Let’s wrap up those changes by adding the following code:

program.h

#pragma once
#include "platform_gl.h"

typedef struct {
	GLuint program;

	GLint a_position_location;
	GLint a_texture_coordinates_location;
	GLint u_mvp_matrix_location;
	GLint u_texture_unit_location;
} TextureProgram;

typedef struct {
	GLuint program;

	GLint a_position_location;
	GLint u_mvp_matrix_location;
	GLint u_color_location;
} ColorProgram;

TextureProgram get_texture_program(GLuint program);
ColorProgram get_color_program(GLuint program);

program.c

#include "program.h"
#include "platform_gl.h"

TextureProgram get_texture_program(GLuint program)
{
	return (TextureProgram) {
			program,
			glGetAttribLocation(program, "a_Position"),
			glGetAttribLocation(program, "a_TextureCoordinates"),
			glGetUniformLocation(program, "u_MvpMatrix"),
			glGetUniformLocation(program, "u_TextureUnit")};
}

ColorProgram get_color_program(GLuint program)
{
	return (ColorProgram) {
			program,
			glGetAttribLocation(program, "a_Position"),
			glGetUniformLocation(program, "u_MvpMatrix"),
			glGetUniformLocation(program, "u_Color")};
}

Adding support for Android

We first need to update Android.mk and add the following to LOCAL_SRC_FILES:

				   $(CORE_RELATIVE_PATH)/game_objects.c \
                   $(CORE_RELATIVE_PATH)/program.c \

We also need to add a new LOCAL_C_INCLUDES:

LOCAL_C_INCLUDES += $(PROJECT_ROOT_PATH)/3rdparty/linmath/

We then need to update renderer_wrapper.c and change the call to on_surface_changed(); to on_surface_changed(width, height);. Once we’ve done that, we should be able to run the app on our Android device, and it should look similar to the following image:

Air hockey, running on a Galaxy Nexus
Air hockey, running on a Galaxy Nexus

Adding support for iOS

For iOS, we just need to open up the Xcode project and add the necessary references to linmath.h and our new core files to the appropriate folder groups, and then we need to update ViewController.m and change on_surface_changed(); to the following:

on_surface_changed([[self view] bounds].size.width, [[self view] bounds].size.height);

Once we run the app, it should look similar to the following image:

Air hockey, running on the iPhone simulator
Air hockey, running on the iPhone simulator

Adding support for emscripten

For emscripten, we need to update the Makefile and add the following lines to SOURCES:

		  ../../core/game_objects.c \
		  ../../core/program.c \

We’ll also need to add the following lines to OBJECTS:

		  ../../core/game_objects.o \
		  ../../core/program.o \

We then just need to update main.c, move the constants width and height from inside init_gl() to outside the function near the top of the file, and update the call to on_surface_changed(); to on_surface_changed(width, height);. We can then build the file by calling emmake make, which should produce a file that looks as follows:

See how easy that was? Now that we have a minimal cross-platform framework in place, it’s very easy for us to bring changes to the core code across to each platform.

Exploring further

The full source code for this lesson can be found at the GitHub project. In the next post, we’ll take a look at user input so we can move our mallet around the screen.

OpenGL Roundup, September 12, 2013

As some of you may already know, Apple recently announced the iPhone 5s & 5c at their annual iPhone event, and one of the new updates is that the iPhone 5s will also be coming with support for OpenGL ES 3.0! Google announced support for OpenGL ES 3.0 with their release of Android 4.3 not too long ago, so the new version is slowly making its way onto mobile devices.

OpenGL ES 3.0 is backwards-compatible with OpenGL ES 2.0, so everything you learned about OpenGL ES 2.0 still applies. This post from Phoronix goes into more detail about what the new version brings: A Look At OpenGL ES 3.0: Lots Of Good Stuff.

On to the roundup:

Ghoshehsoft’s Blog – A look at many topics related to OpenGL ES 2.0 on Android.

Project Anarchy – “Project Anarchy is a complete end to end game engine and state-of-the-art toolset for mobile. Project Anarchy also comprises a vibrant game development community centered right here at www.projectanarchy.com. Project Anarchy includes an entirely free license to ship your game on iOS, Android and Tizen platforms.”

emscripten and PNaCl: Build Systems

What Does gpus_ReturnGuiltyForHardwareRestart Mean?

Theoretical Engineering: Occlusion Culling for BrickSmith

Android 4.3 Jelly Bean Introduces Support for OpenGL ES 3.0

From the Android Developers Website:

OpenGL ES 3.0 for High-Performance Graphics

Android 4.3 introduces platform support for Khronos OpenGL ES 3.0, providing games and other apps with highest-performance 2D and 3D graphics capabilities on supported devices. You can take advantage of OpenGL ES 3.0 and related EGL extensions using either framework APIs or native API bindings through the Android Native Development Kit (NDK).

Key new functionality provided in OpenGL ES 3.0 includes acceleration of advanced visual effects, high quality ETC2/EAC texture compression as a standard feature, a new version of the GLSL ES shading language with integer and 32-bit floating point support, advanced texture rendering, and standardized texture size and render-buffer formats.

You can use the OpenGL ES 3.0 APIs to create highly complex, highly efficient graphics that run across a range of compatible Android devices, and you can support a single, standard texture-compression format across those devices.

OpenGL ES 3.0 is an optional feature that depends on underlying graphics hardware. Support is already available on Nexus 7 (2013), Nexus 4, and Nexus 10 devices.

Loading a PNG into Memory and Displaying It as a Texture with OpenGL ES 2, Using (Almost) the Same Code on iOS, Android, and Emscripten

In the last post in this series, we setup a system to render OpenGL to Android, iOS and the web via WebGL and emscripten. In this post, we’ll expand on that work and add support for PNG loading, shaders, and VBOs.

TL;DR

We can put most of our common code into a core folder, and call into that core from a main loop in our platform-specific code. By taking advantage of open source libraries like libpng and zlib, most of our code can remain platform independent. In this post, we cover the new core code and the new Android platform-specific code.

To check out the completed project for this part of the series, head over to GitHub and download the files for ‘article-2-loading-png-file’.

Prerequisites

Before we begin, you may want to check out the previous posts in this series so that you can get the right tools installed and configured on your local development machine:

You can setup a local git repository with all of the code by cloning ‘article-1-clearing-the-screen’ or by downloading it as a ZIP from GitHub: https://github.com/learnopengles/airhockey/tree/article-1-clearing-the-screen.

For a “friendlier” introduction to OpenGL ES 2 using Java as the development language of choice, you can also check out Android Lesson One: Getting Started or OpenGL ES 2 for Android: A Quick-Start Guide.

Updating the platform-independent code

In this section, we’ll cover all of the new changes to the platform-independent core code that we’ll be making to support the new features. The first thing that we’ll do is move things around, so that they follow this new structure:

/src/common => rename to /src/core

/src/android => rename to /src/platform/android

/src/ios => rename to /src/platform/ios

/src/emscripten => rename to /src/platform/emscripten

We’ll also rename glwrapper.h to platform_gl.h for all platforms. This will help to keep our source code more organized as we add more features and source files.

To start off, let’s cover all of the source files that go into /src/core.

Loading vertex buffer objects

Let’s begin with buffer.h:

#include "platform_gl.h"

#define BUFFER_OFFSET(i) ((void*)(i))

GLuint create_vbo(const GLsizeiptr size, const GLvoid* data, const GLenum usage);

We’ll use create_vbo to upload data into a vertex buffer object. BUFFER_OFFSET() is a helper macro that we’ll use to pass the right offsets to glVertexAttribPointer().

Let’s follow up with the implementation in buffer.c:

#include "buffer.h"
#include "platform_gl.h"
#include <assert.h>
#include <stdlib.h>

GLuint create_vbo(const GLsizeiptr size, const GLvoid* data, const GLenum usage) {
	assert(data != NULL);
	GLuint vbo_object;
	glGenBuffers(1, &vbo_object);
	assert(vbo_object != 0);

	glBindBuffer(GL_ARRAY_BUFFER, vbo_object);
	glBufferData(GL_ARRAY_BUFFER, size, data, usage);
	glBindBuffer(GL_ARRAY_BUFFER, 0);

	return vbo_object;
}

First, we generate a new OpenGL vertex buffer object, and then we bind to it and upload the data from data into the VBO. We also assert that the data is not null and that we successfully created a new vertex buffer object. Why do we assert instead of returning an error code? There are a couple of reasons for that:

  1. In the context of a game, there isn’t really a reasonable course of action that we can take in the event that creating a new VBO fails. Something is going to fail to display properly, so our game experience isn’t going to be as intended. We would also never expect this to fail, unless we’re abusing the platform and trying to do too much for the target hardware.
  2. Returning an error means that we now have to expand our code by handling the error and checking for the error at the other end, perhaps cascading that across several function calls. This adds a lot of maintenance burden with little gain.

I have been greatly influenced by this excellent series over at the Bitsquid blog:

assert() is only compiled into the program in debug mode by default, so in release mode, the application will just continue to run and might end up crashing on bad data. To avoid this, when going into production, you may want to create a special assert() that works in release mode and does a little bit more, perhaps showing a dialog box to the user before crashing and writing out a log to a file, so that it can be sent off to the developers.

Loading and compiling shaders:

Let’s add the following shader.h:

#include "platform_gl.h"

GLuint compile_shader(const GLenum type, const GLchar* source, const GLint length);
GLuint link_program(const GLuint vertex_shader, const GLuint fragment_shader);
GLuint build_program(
	const GLchar * vertex_shader_source, const GLint vertex_shader_source_length,
	const GLchar * fragment_shader_source, const GLint fragment_shader_source_length);

/* Should be called just before using a program to draw, if validation is needed. */
GLint validate_program(const GLuint program);

Here, we have methods to compile a shader and to link two shaders into an OpenGL shader program. We also have a helper method here for validating a program, if we want to do that for debugging reasons.

Let’s begin the implementation for shader.c:

#include "shader.h"
#include "platform_gl.h"
#include "platform_log.h"
#include <assert.h>
#include <stdlib.h>
#include <string.h>

#define TAG "shaders"

static void log_v_fixed_length(const GLchar* source, const GLint length) {
	if (LOGGING_ON) {
		char log_buffer[length + 1];
		memcpy(log_buffer, source, length);
		log_buffer[length] = '\0';

		DEBUG_LOG_WRITE_V(TAG, log_buffer);
	}
}

static void log_shader_info_log(GLuint shader_object_id) {
	if (LOGGING_ON) {
		GLint log_length;
		glGetShaderiv(shader_object_id, GL_INFO_LOG_LENGTH, &log_length);
		GLchar log_buffer[log_length];
		glGetShaderInfoLog(shader_object_id, log_length, NULL, log_buffer);

		DEBUG_LOG_WRITE_V(TAG, log_buffer);
	}
}

static void log_program_info_log(GLuint program_object_id) {
	if (LOGGING_ON) {
		GLint log_length;
		glGetProgramiv(program_object_id, GL_INFO_LOG_LENGTH, &log_length);
		GLchar log_buffer[log_length];
		glGetProgramInfoLog(program_object_id, log_length, NULL, log_buffer);

		DEBUG_LOG_WRITE_V(TAG, log_buffer);
	}
}

We’ve added some helper functions to help us log the shader and program info logs when logging is enabled. We’ll define LOGGING_ON and the other logging functions in other include files, soon. Let’s continue:

GLuint compile_shader(const GLenum type, const GLchar* source, const GLint length) {
	assert(source != NULL);
	GLuint shader_object_id = glCreateShader(type);
	GLint compile_status;

	assert(shader_object_id != 0);

	glShaderSource(shader_object_id, 1, (const GLchar **)&source, &length);
	glCompileShader(shader_object_id);
	glGetShaderiv(shader_object_id, GL_COMPILE_STATUS, &compile_status);

	if (LOGGING_ON) {
		DEBUG_LOG_WRITE_D(TAG, "Results of compiling shader source:");
		log_v_fixed_length(source, length);
		log_shader_info_log(shader_object_id);
	}

	assert(compile_status != 0);

	return shader_object_id;
}

We create a new shader object, pass in the source, compile it, and if everything was successful, we then return the shader ID. Now we need a method for linking two shaders together into an OpenGL program:

GLuint link_program(const GLuint vertex_shader, const GLuint fragment_shader) {
	GLuint program_object_id = glCreateProgram();
	GLint link_status;

	assert(program_object_id != 0);

	glAttachShader(program_object_id, vertex_shader);
	glAttachShader(program_object_id, fragment_shader);
	glLinkProgram(program_object_id);
	glGetProgramiv(program_object_id, GL_LINK_STATUS, &link_status);

	if (LOGGING_ON) {
		DEBUG_LOG_WRITE_D(TAG, "Results of linking program:");
		log_program_info_log(program_object_id);
	}

	assert(link_status != 0);

	return program_object_id;
}

To link the program, we pass in two OpenGL shader objects, one for the vertex shader and one for the fragment shader, and then we link them together. If all was successful, then we return the program object ID.

Let’s complete shader.c by adding two helper methods:

GLuint build_program(
	const GLchar * vertex_shader_source, const GLint vertex_shader_source_length, 
	const GLchar * fragment_shader_source, const GLint fragment_shader_source_length) {
	assert(vertex_shader_source != NULL);
	assert(fragment_shader_source != NULL);

	GLuint vertex_shader = compile_shader(
		GL_VERTEX_SHADER, vertex_shader_source, vertex_shader_source_length);
	GLuint fragment_shader = compile_shader(
		GL_FRAGMENT_SHADER, fragment_shader_source, fragment_shader_source_length);
	return link_program(vertex_shader, fragment_shader);
}

This helper method method takes in the source for a vertex shader and a fragment shader, and returns the linked program object. Let’s add the second helper method:

GLint validate_program(const GLuint program) {
	if (LOGGING_ON) {
		int validate_status;

		glValidateProgram(program);
		glGetProgramiv(program, GL_VALIDATE_STATUS, &validate_status);
		DEBUG_LOG_PRINT_D(TAG, "Results of validating program: %d", validate_status);
		log_program_info_log(program);
		return validate_status;
	}

	return 0;
}

We can use validate_program() for debugging purposes, if we want some extra info about a program during a specific moment in our rendering code.

Loading in textures

Now we need some code to load in raw data into a texture. Let’s add the following into a new file called texture.h:

#include "platform_gl.h"

GLuint load_texture(
	const GLsizei width, const GLsizei height,
	const GLenum type, const GLvoid* pixels);

Let’s follow that up with the implementation in texture.c:

#include "texture.h"
#include "platform_gl.h"
#include <assert.h>

GLuint load_texture(
	const GLsizei width, const GLsizei height,
	const GLenum type, const GLvoid* pixels) {
	GLuint texture_object_id;
	glGenTextures(1, &texture_object_id);
	assert(texture_object_id != 0);

	glBindTexture(GL_TEXTURE_2D, texture_object_id);

	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
	glTexImage2D(
		GL_TEXTURE_2D, 0, type, width, height, 0, type, GL_UNSIGNED_BYTE, pixels);
	glGenerateMipmap(GL_TEXTURE_2D);

	glBindTexture(GL_TEXTURE_2D, 0);
	return texture_object_id;
}

This is pretty straightforward and not currently customized for special cases: it just loads in the raw data in pixels into the texture, assuming that each component is 8-bit. It then sets up the texture for trilinear mipmapping.

Loading in PNG files

For this post, we’ll package our texture asset as a PNG file, and use libpng to decode the file into raw data. For that we’ll need to add some wrapper code around libpng so that we can decode a PNG file into raw data suitable for upload into an OpenGL texture.

Let’s create a new file called image.h, with the following contents:

#include "platform_gl.h"

typedef struct {
	const int width;
	const int height;
	const int size;
	const GLenum gl_color_format;
	const void* data;
} RawImageData;

/* Returns the decoded image data, or aborts if there's an error during decoding. */
RawImageData get_raw_image_data_from_png(const void* png_data, const int png_data_size);
void release_raw_image_data(const RawImageData* data);

We’ll use get_raw_image_data_from_png() to read in the PNG data from png_data and return the raw data in a struct. When we no longer need to keep that raw data around, we can call release_raw_image_data() to release the associated resources.

Let’s start writing the implementation in image.c:

#include "image.h"
#include "platform_log.h"
#include <assert.h>
#include <png.h>
#include <string.h>
#include <stdlib.h>

typedef struct {
	const png_byte* data;
	const png_size_t size;
} DataHandle;

typedef struct {
	const DataHandle data;
	png_size_t offset;
} ReadDataHandle;

typedef struct {
	const png_uint_32 width;
	const png_uint_32 height;
	const int color_type;
} PngInfo;

We’ve started off with the includes and a few structs that we’ll be using locally. Let’s continue with a few function prototypes:

static void read_png_data_callback(
	png_structp png_ptr, png_byte* png_data, png_size_t read_length);
static PngInfo read_and_update_info(const png_structp png_ptr, const png_infop info_ptr);
static DataHandle read_entire_png_image(
	const png_structp png_ptr, const png_infop info_ptr, const png_uint_32 height);
static GLenum get_gl_color_format(const int png_color_format);

We’ll be using these as local helper functions. Now we can add the implementation for get_raw_image_data_from_png():

RawImageData get_raw_image_data_from_png(const void* png_data, const int png_data_size) {
	assert(png_data != NULL && png_data_size > 8);
	assert(png_check_sig((void*)png_data, 8));

	png_structp png_ptr = png_create_read_struct(
		PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
	assert(png_ptr != NULL);
	png_infop info_ptr = png_create_info_struct(png_ptr);
	assert(info_ptr != NULL);

	ReadDataHandle png_data_handle = (ReadDataHandle) {{png_data, png_data_size}, 0};
	png_set_read_fn(png_ptr, &png_data_handle, read_png_data_callback);

	if (setjmp(png_jmpbuf(png_ptr))) {
		CRASH("Error reading PNG file!");
	}

	const PngInfo png_info = read_and_update_info(png_ptr, info_ptr);
	const DataHandle raw_image = read_entire_png_image(
		png_ptr, info_ptr, png_info.height);

	png_read_end(png_ptr, info_ptr);
	png_destroy_read_struct(&png_ptr, &info_ptr, NULL);

	return (RawImageData) {
		png_info.width,
		png_info.height,
		raw_image.size,
		get_gl_color_format(png_info.color_type),
		raw_image.data};
}

There’s a lot going on here, so let’s explain each part in turn:

	assert(png_data != NULL && png_data_size > 8);
	assert(png_check_sig((void*)png_data, 8));

This checks that the PNG data is present and has a valid header.

	png_structp png_ptr = png_create_read_struct(
		PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
	assert(png_ptr != NULL);
	png_infop info_ptr = png_create_info_struct(png_ptr);
	assert(info_ptr != NULL);

This initializes the PNG structures that we’ll use to read in the rest of the data.

	ReadDataHandle png_data_handle = (ReadDataHandle) {{png_data, png_data_size}, 0};
	png_set_read_fn(png_ptr, &png_data_handle, read_png_data_callback);

As the PNG data is parsed, libpng will call read_png_data_callback() for each part of the PNG file. Since we’re reading in the PNG file from memory, we’ll use ReadDataHandle to wrap this memory buffer so that we can read from it as if it were a file.

	if (setjmp(png_jmpbuf(png_ptr))) {
		CRASH("Error reading PNG file!");
	}

This is how libpng does its error handling. If something goes wrong, then setjmp will return true and we’ll enter the body of the if statement. We want to handle this like an assert, so we just crash the program. We’ll define the CRASH macro later on.

	const PngInfo png_info = read_and_update_info(png_ptr, info_ptr);

We’ll use one of our helper functions here to parse the PNG information, such as the color format, and convert the PNG into a format that we want.

	const DataHandle raw_image = read_entire_png_image(
		png_ptr, info_ptr, png_info.height);

We’ll use another helper function here to read in and decode the PNG image data.

	png_read_end(png_ptr, info_ptr);
	png_destroy_read_struct(&png_ptr, &info_ptr, NULL);

	return (RawImageData) {
		png_info.width,
		png_info.height,
		raw_image.size,
		get_gl_color_format(png_info.color_type),
		raw_image.data};

Once reading is complete, we clean up the PNG structures and then we return the data inside of a RawImageData struct.

Let’s define our helper methods now:

static void read_png_data_callback(
	png_structp png_ptr, png_byte* raw_data, png_size_t read_length) {
	ReadDataHandle* handle = png_get_io_ptr(png_ptr);
	const png_byte* png_src = handle->data.data + handle->offset;

	memcpy(raw_data, png_src, read_length);
	handle->offset += read_length;
}

read_png_data_callback() will be called by libpng to read from the memory buffer. To read from the right place in the memory buffer, we store an offset and we increase that offset every time that read_png_data_callback() is called.

static PngInfo read_and_update_info(const png_structp png_ptr, const png_infop info_ptr)
{
	png_uint_32 width, height;
	int bit_depth, color_type;

	png_read_info(png_ptr, info_ptr);
	png_get_IHDR(
		png_ptr, info_ptr, &width, &height, &bit_depth, &color_type, NULL, NULL, NULL);

	// Convert transparency to full alpha
	if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS))
		png_set_tRNS_to_alpha(png_ptr);

	// Convert grayscale, if needed.
	if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8)
		png_set_expand_gray_1_2_4_to_8(png_ptr);

	// Convert paletted images, if needed.
	if (color_type == PNG_COLOR_TYPE_PALETTE)
		png_set_palette_to_rgb(png_ptr);

	// Add alpha channel, if there is none.
	// Rationale: GL_RGBA is faster than GL_RGB on many GPUs)
	if (color_type == PNG_COLOR_TYPE_PALETTE || color_type == PNG_COLOR_TYPE_RGB)
	   png_set_add_alpha(png_ptr, 0xFF, PNG_FILLER_AFTER);

	// Ensure 8-bit packing
	if (bit_depth < 8)
	   png_set_packing(png_ptr);
	else if (bit_depth == 16)
		png_set_scale_16(png_ptr);

	png_read_update_info(png_ptr, info_ptr);

	// Read the new color type after updates have been made.
	color_type = png_get_color_type(png_ptr, info_ptr);

	return (PngInfo) {width, height, color_type};
}

This helper function reads in the PNG data, and then it asks libpng to perform several transformations based on the PNG type:

  • Transparency information is converted into a full alpha channel.
  • Grayscale images are converted to 8-bit.
  • Paletted images are converted to full RGB.
  • RGB images get an alpha channel added, if none is present.
  • Color channels are converted to 8-bit, if less than 8-bit or 16-bit.

The PNG is then updated with the new transformations and the new color type is stored into color_type.

For the next step, we’ll add a helper function to decode the PNG image data into raw image data:

static DataHandle read_entire_png_image(
	const png_structp png_ptr, 
	const png_infop info_ptr, 
	const png_uint_32 height) 
{
	const png_size_t row_size = png_get_rowbytes(png_ptr, info_ptr);
	const int data_length = row_size * height;
	assert(row_size > 0);

	png_byte* raw_image = malloc(data_length);
	assert(raw_image != NULL);

	png_byte* row_ptrs[height];

	png_uint_32 i;
	for (i = 0; i < height; i++) {
		row_ptrs[i] = raw_image + i * row_size;
	}

	png_read_image(png_ptr, &row_ptrs[0]);

	return (DataHandle) {raw_image, data_length};
}

First, we allocate a block of memory large enough to hold the decoded image data. Since libpng wants to decode things line by line, we also need to setup an array on the stack that contains a set of pointers into this image data, one pointer for each line. We can then call png_read_image() to decode all of the PNG data and then we return that as a DataHandle.

Let’s add the last helper method:

static GLenum get_gl_color_format(const int png_color_format) {
	assert(png_color_format == PNG_COLOR_TYPE_GRAY
	    || png_color_format == PNG_COLOR_TYPE_RGB_ALPHA
	    || png_color_format == PNG_COLOR_TYPE_GRAY_ALPHA);

	switch (png_color_format) {
		case PNG_COLOR_TYPE_GRAY:
			return GL_LUMINANCE;
		case PNG_COLOR_TYPE_RGB_ALPHA:
			return GL_RGBA;
		case PNG_COLOR_TYPE_GRAY_ALPHA:
			return GL_LUMINANCE_ALPHA;
	}

	return 0;
}

This function will read in the PNG color format and return the matching OpenGL color format. We expect that after the transformations that we did, the PNG color format will be either PNG_COLOR_TYPE_GRAY, PNG_COLOR_TYPE_GRAY_ALPHA, or PNG_COLOR_TYPE_RGB_ALPHA, so we assert against those types.

To wrap up our image loading code, we just need to add the release method:

void release_raw_image_data(const RawImageData* data) {
	assert(data != NULL);
	free((void*)data->data);
}

We’ll call this when we’re done with the raw data and can return the associated memory to the heap.

The benefits of using libpng versus platform-specific code

At this point, you might be asking why we simply didn’t use what each platform offers us, such as BitmapFactory.decode??? on Android, where ??? is one of the decode methods. Using platform specific code means that we would have to duplicate the code for each platform, so on Android we would wrap some code around BitmapFactory, and on the other platforms we would do something else. This might be a good idea if the platform-specific code was better at the job; however, in personal testing on the Nexus 7, using BitmapFactory actually seems to be a lot slower than just using libpng directly.

Here were the timings I observed for loading a single PNG file from the assets folder and uploading it into an OpenGL texture:

iPhone 5, libpng:       ~28ms
Nexus 7, libpng:        ~35ms
Nexus 7, BitmapFactory: ~93ms

 
To reduce possible sources of slowdown, I avoided JNI and had the Java code upload the data directly into a texture, and return the texture object ID to C. I also used inScaled = false and placed the image in the assets folder to avoid extra scaling; if someone has extra insight into this issue, I would definitely love to hear it! I can only surmise that there must be a lot of extra stuff going on behind the scenes, or that the overhead of doing this from Java using the Dalvik VM is just so great that it results in that much of a slowdown. The Nexus 7 is a powerful Android device, so these timings are going to be much worse on slower Android devices. Since libpng is faster than the platform-specific alternative, at least on Android, and since maintaining one set of code is easier than maintaining separate code for each platform, I’ve decided to just use libpng on all platforms for PNG image decoding.

Just for fun, here are the emscripten numbers on a MacBook Air with a 1.7 GHz Intel Core i5 and 4GB 1333 Mhz DDR3 RAM, loading an uncompressed HTML with embedded resources from the local filesystem:

Chrome 28, first time: ~318ms
Chrome 28, reload: ~67ms
Firefox 22: ~27ms

Interestingly enough, the code ran faster when it was compiled without the closure compiler and LLVM LTO.

Wrapping up the rest of the changes to the core folder

Let’s wrap up the rest of the changes to the core folder by adding the following files:

config.h:

#define LOGGING_ON 1

We’ll use this to control whether logging should be turned on or off.

macros.h:

#define UNUSED(x) (void)(x)

This will help us suppress compiler warnings related to unused parameters, which is useful for JNI methods which get called by Java.

asset_utils.h

#include "platform_gl.h"

GLuint load_png_asset_into_texture(const char* relative_path);
GLuint build_program_from_assets(
	const char* vertex_shader_path, const char* fragment_shader_path);

We’ll use these helper methods in game.c to make it easier to load in the texture and shaders.

asset_utils.c

#include "asset_utils.h"
#include "image.h"
#include "platform_asset_utils.h"
#include "shader.h"
#include "texture.h"
#include <assert.h>
#include <stdlib.h>

GLuint load_png_asset_into_texture(const char* relative_path) {
	assert(relative_path != NULL);

	const FileData png_file = get_asset_data(relative_path);
	const RawImageData raw_image_data = 
		get_raw_image_data_from_png(png_file.data, png_file.data_length);
	const GLuint texture_object_id = load_texture(
		raw_image_data.width, raw_image_data.height, 
		raw_image_data.gl_color_format, raw_image_data.data);

	release_raw_image_data(&raw_image_data);
	release_asset_data(&png_file);

	return texture_object_id;
}

GLuint build_program_from_assets(
	const char* vertex_shader_path, const char* fragment_shader_path) {
	assert(vertex_shader_path != NULL);
	assert(fragment_shader_path != NULL);

	const FileData vertex_shader_source = get_asset_data(vertex_shader_path);
	const FileData fragment_shader_source = get_asset_data(fragment_shader_path);
	const GLuint program_object_id = build_program(
		vertex_shader_source.data, vertex_shader_source.data_length,
		fragment_shader_source.data, fragment_shader_source.data_length);

	release_asset_data(&vertex_shader_source);
	release_asset_data(&fragment_shader_source);

	return program_object_id;
}

This is the implementation for asset_utils.h. We’ll use load_png_asset_into_texture() to load a PNG file from the assets folder into an OpenGL texture, and we’ll use build_program_from_assets() to load in two shaders from the assets folder and compile and link them into an OpenGL shader program.

Updating game.c

We’ll need to update game.c to use all of the new code that we’ve added. Delete everything that’s there and replace it with the following start to our new code:

#include "game.h"
#include "asset_utils.h"
#include "buffer.h"
#include "image.h"
#include "platform_gl.h"
#include "platform_asset_utils.h"
#include "shader.h"
#include "texture.h"

static GLuint texture;
static GLuint buffer;
static GLuint program;

static GLint a_position_location;
static GLint a_texture_coordinates_location;
static GLint u_texture_unit_location;

// position X, Y, texture S, T
static const float rect[] = {-1.0f, -1.0f, 0.0f, 0.0f,
		                     -1.0f,  1.0f, 0.0f, 1.0f,
		                      1.0f, -1.0f, 1.0f, 0.0f,
		                      1.0f,  1.0f, 1.0f, 1.0f};

We’ve added our includes, a few local variables to hold the OpenGL objects and shader attribute and uniform locations, and an array of floats which contains a set of positions and texture coordinates for a rectangle that will completely fill the screen. We’ll use that to draw our texture onto the screen.

Let’s continue the code:

void on_surface_created() {
	glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
}

void on_surface_changed() {
	texture = load_png_asset_into_texture("textures/air_hockey_surface.png");
	buffer = create_vbo(sizeof(rect), rect, GL_STATIC_DRAW);
	program = build_program_from_assets("shaders/shader.vsh", "shaders/shader.fsh");

	a_position_location = glGetAttribLocation(program, "a_Position");
	a_texture_coordinates_location = 
		glGetAttribLocation(program, "a_TextureCoordinates");
	u_texture_unit_location = glGetUniformLocation(program, "u_TextureUnit");
}

glClearColor() is just as we were doing it before. In on_surface_changed(), we load in a texture from textures/air_hockey_surface.png, we create a VBO from the data stored in rect, and then we build an OpenGL shader program from the shaders located at shaders/shader.vsh and shaders/shader.fsh. Once we have the program loaded, we use it to grab the attribute and uniform locations out of the shader.

We haven’t yet defined the code to load in the actual assets from the file system, since a good part of that is platform-specific. When we do, we’ll take care to set things up so that these relative paths “just work”.

Let’s complete game.c:

void on_draw_frame() {
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

	glUseProgram(program);

	glActiveTexture(GL_TEXTURE0);
	glBindTexture(GL_TEXTURE_2D, texture);
	glUniform1i(u_texture_unit_location, 0);

	glBindBuffer(GL_ARRAY_BUFFER, buffer);
	glVertexAttribPointer(a_position_location, 2, GL_FLOAT, GL_FALSE, 
		4 * sizeof(GL_FLOAT), BUFFER_OFFSET(0));
	glVertexAttribPointer(a_texture_coordinates_location, 2, GL_FLOAT, GL_FALSE, 
		4 * sizeof(GL_FLOAT), BUFFER_OFFSET(2 * sizeof(GL_FLOAT)));
	glEnableVertexAttribArray(a_position_location);
	glEnableVertexAttribArray(a_texture_coordinates_location);
	glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

	glBindBuffer(GL_ARRAY_BUFFER, 0);
}

In the draw loop, we clear the screen, set the shader program, bind the texture and VBO, setup the attributes using glVertexAttribPointer(), and then draw to the screen with glDrawArrays(). If you’ve looked at the Java tutorials before, one thing you’ll notice is that it’s a bit easier to use glVertexAttribPointer() from C than it is from Java. For one, if we were using client-side arrays, we could just pass the array without worrying about any ByteBuffers, and for two, we can use the sizeof operator to get the size of a datatype in bytes, so no need to hardcode that.

This wraps up everything for the core folder, so in the next few steps, we’re going to add in the necessary platform wrappers to get this working on Android.

Adding the common platform code

These new files should go in /airhockey/src/platform/common:

platform_file_utils.h

#pragma once
typedef struct {
	const long data_length;
	const void* data;
	const void* file_handle;
} FileData;

FileData get_file_data(const char* path);
void release_file_data(const FileData* file_data);

We’ll use this to read data from the file system on iOS and emscripten. We’ll also use FileData for our Android asset reading code. We won’t define the implementation of the functions for now since we won’t need them for Android.

platform_asset_utils.h

#include "platform_file_utils.h"

FileData get_asset_data(const char* relative_path);
void release_asset_data(const FileData* file_data);

We’ll use this to read in assets. For Android this will be specialized code since it will use the AssetManager class to read files straight from the APK file.

platform_log.h

#include "platform_macros.h"
#include "config.h"

void _debug_log_v(const char* tag, const char* text, ...) PRINTF_ATTRIBUTE(2, 3);
void _debug_log_d(const char* tag, const char* text, ...) PRINTF_ATTRIBUTE(2, 3);
void _debug_log_w(const char* tag, const char* text, ...) PRINTF_ATTRIBUTE(2, 3);
void _debug_log_e(const char* tag, const char* text, ...) PRINTF_ATTRIBUTE(2, 3);

#define DEBUG_LOG_PRINT_V(tag, fmt, ...) do { if (LOGGING_ON) _debug_log_v(tag, "%s:%d:%s(): " fmt, __FILE__, __LINE__, __func__, __VA_ARGS__); } while (0)
#define DEBUG_LOG_PRINT_D(tag, fmt, ...) do { if (LOGGING_ON) _debug_log_d(tag, "%s:%d:%s(): " fmt, __FILE__, __LINE__, __func__, __VA_ARGS__); } while (0)
#define DEBUG_LOG_PRINT_W(tag, fmt, ...) do { if (LOGGING_ON) _debug_log_w(tag, "%s:%d:%s(): " fmt, __FILE__, __LINE__, __func__, __VA_ARGS__); } while (0)
#define DEBUG_LOG_PRINT_E(tag, fmt, ...) do { if (LOGGING_ON) _debug_log_e(tag, "%s:%d:%s(): " fmt, __FILE__, __LINE__, __func__, __VA_ARGS__); } while (0)

#define DEBUG_LOG_WRITE_V(tag, text) DEBUG_LOG_PRINT_V(tag, "%s", text)
#define DEBUG_LOG_WRITE_D(tag, text) DEBUG_LOG_PRINT_D(tag, "%s", text)
#define DEBUG_LOG_WRITE_W(tag, text) DEBUG_LOG_PRINT_W(tag, "%s", text)
#define DEBUG_LOG_WRITE_E(tag, text) DEBUG_LOG_PRINT_E(tag, "%s", text)

#define CRASH(e) DEBUG_LOG_WRITE_E("Assert", #e); __builtin_trap()

This contains a bunch of macros to help us do logging from our core game code. CRASH() is a special macro that will log the message passed to it, then call __builtin_trap() to stop execution. We used this macro above when we were loading in the PNG file.

platform_macros.h

#if defined(__GNUC__)
#define PRINTF_ATTRIBUTE(format_pos, arg_pos) __attribute__((format(printf, format_pos, arg_pos)))
#else
#define PRINTF_ATTRIBUTE(format_pos, arg_pos)
#endif

This is a special macro that helps the compiler do format checking when checking the formats that we pass to our log functions.

Updating the Android code

For the Android target, we have a bit of cleanup to do first. Let’s open up the Android project in Eclipse, get rid of GameLibJNIWrapper.java and update RendererWrapper.java as follows:

package com.learnopengles.airhockey;

import javax.microedition.khronos.egl.EGLConfig;
import javax.microedition.khronos.opengles.GL10;

import android.content.Context;
import android.opengl.GLSurfaceView.Renderer;

import com.learnopengles.airhockey.platform.PlatformFileUtils;

public class RendererWrapper implements Renderer {	
	static {
		System.loadLibrary("game");		
	}
	
	private final Context context;	
	
	public RendererWrapper(Context context) {
		this.context = context;
	}
	
	@Override
	public void onSurfaceCreated(GL10 gl, EGLConfig config) {		
		PlatformFileUtils.init_asset_manager(context.getAssets());
		on_surface_created();
	}

	@Override
	public void onSurfaceChanged(GL10 gl, int width, int height) {
		on_surface_changed(width, height);
	}

	@Override
	public void onDrawFrame(GL10 gl) {
		on_draw_frame();
	}
	
	private static native void on_surface_created();

	private static native void on_surface_changed(int width, int height);

	private static native void on_draw_frame();
}

We’ve moved the native methods into RendererWrapper itself. The new RendererWrapper wants a Context passed into its contructor, so give it one by updating the constructor call in MainActivity.java as follows:

glSurfaceView.setRenderer(new RendererWrapper(this));

For Android, we’ll be using the AssetManager to read in assets that are compiled directly into the APK file. We’ll need a way to pass a reference to the AssetManager to our C code, so let’s create a new class in a new package called com.learnopengles.airhockey.platform called PlatformFileUtils, and add the following code:

package com.learnopengles.airhockey.platform;

import android.content.res.AssetManager;

public class PlatformFileUtils {
	public static native void init_asset_manager(AssetManager assetManager);	
}

We are calling init_asset_manager() from RendererWrapper.onSurfaceCreated(), which you can see just a few lines above.

Updating the JNI code

We’ll also need to add platform-specific JNI code to the jni folder in the android folder. Let’s start off with platform_asset_utils.c:

#include "platform_asset_utils.h"
#include "macros.h"
#include "platform_log.h"
#include <android/asset_manager_jni.h>
#include <assert.h>

static AAssetManager* asset_manager;

JNIEXPORT void JNICALL Java_com_learnopengles_airhockey_platform_PlatformFileUtils_init_1asset_1manager(
	JNIEnv * env, jclass jclazz, jobject java_asset_manager) {
	UNUSED(jclazz);
	asset_manager = AAssetManager_fromJava(env, java_asset_manager);
}

FileData get_asset_data(const char* relative_path) {
	assert(relative_path != NULL);
	AAsset* asset = 
		AAssetManager_open(asset_manager, relative_path, AASSET_MODE_STREAMING);
	assert(asset != NULL);

	return (FileData) { AAsset_getLength(asset), AAsset_getBuffer(asset), asset };
}

void release_asset_data(const FileData* file_data) {
	assert(file_data != NULL);
	assert(file_data->file_handle != NULL);
	AAsset_close((AAsset*)file_data->file_handle);
}

We use get_asset_data() to wrap Android’s native asset manager and return the data to the calling code, and we release the data when release_asset_data() is called. The advantage of doing things like this is that the asset manager can choose to optimize data loading by mapping the file into memory, and we can return that mapped data directly to the caller.

Let’s add the logging code:

platform_log.c

#include "platform_log.h"
#include <android/log.h>
#include <stdio.h>
#include <stdlib.h>

#define ANDROID_LOG_VPRINT(priority)	\
va_list arg_ptr; \
va_start(arg_ptr, fmt); \
__android_log_vprint(priority, tag, fmt, arg_ptr); \
va_end(arg_ptr);

void _debug_log_v(const char *tag, const char *fmt, ...) {
	ANDROID_LOG_VPRINT(ANDROID_LOG_VERBOSE);
}

void _debug_log_d(const char *tag, const char *fmt, ...) {
	ANDROID_LOG_VPRINT(ANDROID_LOG_DEBUG);
}

void _debug_log_w(const char *tag, const char *fmt, ...) {
	ANDROID_LOG_VPRINT(ANDROID_LOG_WARN);
}

void _debug_log_e(const char *tag, const char *fmt, ...) {
	ANDROID_LOG_VPRINT(ANDROID_LOG_ERROR);
}

This code wraps Android’s native logging facilities.

Finally, let’s rename jni.c to renderer_wrapper.c and update it to the following:

#include "game.h"
#include "macros.h"
#include <jni.h>

/* These functions are called from Java. */

JNIEXPORT void JNICALL Java_com_learnopengles_airhockey_RendererWrapper_on_1surface_1created(
	JNIEnv * env, jclass cls) {
	UNUSED(env);
	UNUSED(cls);
	on_surface_created();
}

JNIEXPORT void JNICALL Java_com_learnopengles_airhockey_RendererWrapper_on_1surface_1changed(
	JNIEnv * env, jclass cls, jint width, jint height) {
	UNUSED(env);
	UNUSED(cls);
	on_surface_changed();
}

JNIEXPORT void JNICALL Java_com_learnopengles_airhockey_RendererWrapper_on_1draw_1frame(
	JNIEnv* env, jclass cls) {
	UNUSED(env);
	UNUSED(cls);
	on_draw_frame();
}

Nothing has really changed here; we just use the UNUSED() macro (defined earlier in macros.h in the core folder) to suppress some unnecessary compiler warnings.

Updating the NDK build files

We’re almost ready to build & test, just a few things left to be done. Download libpng 1.6.2 from http://www.libpng.org/pub/png/libpng.html and place it in /src/3rdparty/libpng. To configure libpng, copy pnglibconf.h.prebuilt from libpng/scripts/ to libpng/ and remove the .prebuilt extension.

To compile libpng with the NDK, let’s add a build script called Android.mk to the libpng folder, as follows:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE := libpng
LOCAL_SRC_FILES = png.c \
				  pngerror.c \
				  pngget.c \
				  pngmem.c \
				  pngpread.c \
				  pngread.c \
				  pngrio.c \
				  pngrtran.c \
				  pngrutil.c \
				  pngset.c \
				  pngtrans.c \
				  pngwio.c \
				  pngwrite.c \
				  pngwtran.c \
				  pngwutil.c
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
LOCAL_EXPORT_LDLIBS := -lz

include $(BUILD_STATIC_LIBRARY)

This build script will tell the NDK tools to build a static library called libpng that is linked against zlib, which is built into Android. It also sets up the right variables so that we can easily import this library into our own projects, and we won’t even have to do anything special because the right includes and libs are already exported.

Let’s also update the Android.mk file in our jni folder:

LOCAL_PATH := $(call my-dir)
PROJECT_ROOT_PATH := $(LOCAL_PATH)/../../../
CORE_RELATIVE_PATH := ../../../core/

include $(CLEAR_VARS)

LOCAL_MODULE    := game
LOCAL_CFLAGS    := -Wall -Wextra
LOCAL_SRC_FILES := platform_asset_utils.c \
                   platform_log.c \
                   renderer_wrapper.c \
                   $(CORE_RELATIVE_PATH)/asset_utils.c \
                   $(CORE_RELATIVE_PATH)/buffer.c \
                   $(CORE_RELATIVE_PATH)/game.c \
                   $(CORE_RELATIVE_PATH)/image.c \
                   $(CORE_RELATIVE_PATH)/shader.c \
                   $(CORE_RELATIVE_PATH)/texture.c \
                  
LOCAL_C_INCLUDES := $(PROJECT_ROOT_PATH)/platform/common/
LOCAL_C_INCLUDES += $(PROJECT_ROOT_PATH)/core/
LOCAL_STATIC_LIBRARIES := libpng
LOCAL_LDLIBS := -lGLESv2 -llog -landroid

include $(BUILD_SHARED_LIBRARY)

$(call import-add-path,$(PROJECT_ROOT_PATH)/3rdparty)
$(call import-module,libpng)

Our new build script links in the new files that we’ve created in core, and it also imports libpng from the 3rdparty folder and builds it as a static library that is then linked into our Android application.

Adding in the assets

The last step is to add in the assets into /airhockey/assets, which includes the textures and the shaders. To do this, download the assets from https://github.com/learnopengles/airhockey/tree/article-2-loading-png-file/assets and place them in your airhockey folder. To have them automatically included in the APK, follow these steps:

  1. Delete the existing assets folder from the project.
  2. Right-click the project and select Properties. In the window that appears, select Resource->Linked Resources and click New….
  3. Enter ‘ASSETS_LOC’ as the name, and ‘${PROJECT_LOC}/../../../assets’ as the location. Once that’s done, click OK until the Properties window is closed.
  4. Right-click the project again and select New->Folder, enter ‘assets’ as the name, select Advanced, select Link to alternate location (Linked Folder), select Variables…, select ASSETS_LOC, and select OK, then Finish.

You should now have a new assets folder that is linked to the assets folder that we created in the airhockey root. More information can be found on Stack Overflow: How to link assets/www folder in Eclipse / Phonegap / Android project?

Running the app

We should be able to check out the new code now. If you run the app on your Android emulator or device, it should look similar to the following image:

Texture on the Nexus 7

The texture looks a bit stretched/squashed, because we are currently asking OpenGL to fill the screen with that texture. With a basic framework in place, we can start adding some more detail in future lessons and start turning this into an actual game.

Debugging NDK code

While developing this project, I had to hook up a debugger as something was going bad in the PNG loading code, and I just wasn’t sure what. It turns out that I had confused a png_bytep* with a png_byte* — the ‘p’ in the first one means that it’s already a pointer, so I didn’t have to put another star there. I had some issues using the debugging at first, so here are some tips that might help you out if you want to hook up the debugger:

  1. Your project absolutely cannot have any spaces in its path. Otherwise, the debugger will inexplicably fail to connect.
  2. The native code needs to be built with NDK_DEBUG=1; see “Debugging native applications” on this page: Using the NDK plugin.
  3. Android will not wait for gdb to connect before executing the code. Add SystemClock.sleep(10000); to RendererWrapper’s onSurfaceCreated() method to add a sufficient delay to hit your breakpoints.

Once that’s done, you can start debugging from Eclipse by right-clicking the project and selecting Debug As->Android Native Application.

Exploring further

The full source code for this lesson can be found at the GitHub project. For a “friendlier” introduction to OpenGL ES 2 that is focused on Java and Android, see Android Lesson One: Getting Started or OpenGL ES 2 for Android: A Quick-Start Guide.

What could we do to further streamline the code? If we were using C++, we could take advantage of destructors to create, for example, a FileData that cleans itself up when it goes out of scope. I’d also like to make the structs private somehow, as their internals don’t really need to be exposed to clients. What else would you do?

Further reading

In the next two posts, we’ll look at adding support for iOS and emscripten. Now that we’ve built up this base, it actually won’t take too much work!

Calling OpenGL from C on Android, Using the NDK

For this first post in the Developing a Simple Game of Air Hockey Using C++ and OpenGL ES 2 for Android, iOS, and the Web series, we’ll create a simple Android program that initializes OpenGL, then renders simple frames from native code.

Prerequisites

  • The Android SDK & NDK installed, along with a suitable IDE.
  • An emulator or a device supporting OpenGL ES 2.0.

We’ll be using Eclipse in this lesson.

To prepare and test the code for this article, I used revision 22.0.1 of the ADT plugin and SDK tools, and revision 17 of the platform and build tools, along with revision 8e of the NDK and Eclipse Juno Service Pack 2.

Getting started

The first thing to do is create a new Android project in Eclipse, with support for the NDK. You can follow along all of the code at the GitHub project.

Before creating the new project, create a new folder called airhockey, and then create a new Git repository in that folder. Git is a source version control system that will help you keep track of changes to the source and to roll back changes if anything goes wrong. To learn more about how to use Git, see the Git documentation.

To create a new project, select File->New->Android Application Project, and then create a new project called ‘AirHockey’, with the application name set to ‘Air Hockey’ and the package name set to ‘com.learnopengles.airhockey’. Leaving the rest as defaults or filling out as you prefer, save this new project in a new folder called android, inside of the airhockey folder that we created in the previous step.

Once the project has been created, right-click on the project in the Package Explorer, select Android Tools from the drop-down menu, then select Add Native Support…. When asked for the Library Name, enter ‘game’ and hit Finish, so that the library will be called libgame.so. This will create a new folder called jni in the project tree.

Initializing OpenGL

With our project created, we can now edit the default activity and configure it to initialize OpenGL. We’ll first add two member variables to the top of our activity class:

	private GLSurfaceView glSurfaceView;
	private boolean rendererSet;

Now we can set the body of onCreate() as follows:

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);

		ActivityManager activityManager
			= (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
		ConfigurationInfo configurationInfo = activityManager.getDeviceConfigurationInfo();

		final boolean supportsEs2 =
			configurationInfo.reqGlEsVersion >= 0x20000 || isProbablyEmulator();

		if (supportsEs2) {
			glSurfaceView = new GLSurfaceView(this);

			if (isProbablyEmulator()) {
				// Avoids crashes on startup with some emulator images.
				glSurfaceView.setEGLConfigChooser(8, 8, 8, 8, 16, 0);
			}

			glSurfaceView.setEGLContextClientVersion(2);
			glSurfaceView.setRenderer(new RendererWrapper());
			rendererSet = true;
			setContentView(glSurfaceView);
		} else {
			// Should never be seen in production, since the manifest filters
			// unsupported devices.
			Toast.makeText(this, "This device does not support OpenGL ES 2.0.",
					Toast.LENGTH_LONG).show();
			return;
		}
	}

First we check if the device supports OpenGL ES 2.0, and then if it does, we initialize a new GLSurfaceView and configure it to use OpenGL ES 2.0.

The check for configurationInfo.reqGlEsVersion >= 0x20000 doesn’t work on the emulator, so we also call isProbablyEmulator() to see if we’re running on an emulator. Let’s define that method as follows:

	private boolean isProbablyEmulator() {
		return Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH_MR1
				&& (Build.FINGERPRINT.startsWith("generic")
						|| Build.FINGERPRINT.startsWith("unknown")
						|| Build.MODEL.contains("google_sdk")
						|| Build.MODEL.contains("Emulator")
						|| Build.MODEL.contains("Android SDK built for x86"));
	}

OpenGL ES 2.0 will only work in the emulator if it’s been configured to use the host GPU. For more info, read Android Emulator Now Supports Native OpenGL ES2.0!

Let’s complete the activity by adding the following methods:

	@Override
	protected void onPause() {
		super.onPause();

		if (rendererSet) {
			glSurfaceView.onPause();
		}
	}

	@Override
	protected void onResume() {
		super.onResume();

		if (rendererSet) {
			glSurfaceView.onResume();
		}
	}

We need to handle the Android lifecycle, so we also pause & resume the GLSurfaceView as needed. We only do this if we’ve also called glSurfaceView.setRenderer(); otherwise, calling these methods will cause the application to crash.

For a more detailed introduction to OpenGL ES 2, see Android Lesson One: Getting Started or OpenGL ES 2 for Android: A Quick-Start Guide.

Adding a default renderer

Create a new class called RendererWrapper, and add the following code:

public class RendererWrapper implements Renderer {
	@Override
	public void onSurfaceCreated(GL10 gl, EGLConfig config) {
		glClearColor(0.0f, 0.0f, 1.0f, 0.0f);
	}

	@Override
	public void onSurfaceChanged(GL10 gl, int width, int height) {
		// No-op
	}

	@Override
	public void onDrawFrame(GL10 gl) {
		glClear(GL_COLOR_BUFFER_BIT);
	}
}

This simple renderer will set the clear color to blue and clear the screen on every frame. Later on, we’ll change these methods to call into native code. To call methods like glClearColor() without prefixing them with GLES20, add import static android.opengl.GLES20.*; to the top of the class file, then select Source->Organize Imports.

If you have any issues in getting the code to compile, ensure that you’ve organized all imports, and that you’ve included the following imports in RendererWrapper:

import javax.microedition.khronos.egl.EGLConfig;
import javax.microedition.khronos.opengles.GL10;

import android.opengl.GLSurfaceView.Renderer;

Updating the manifest to exclude unsupported devices

We should also update the manifest to make sure that we exclude devices that don’t support OpenGL ES 2.0. Add the following somewhere inside AndroidManifest.xml:

    <uses-feature
        android:glEsVersion="0x00020000"
        android:required="true" />

Since OpenGL ES 2.0 is only fully supported from Android Gingerbread 2.3.3 (API 10), replace any existing <uses-sdk /> tag with the following:

    <uses-sdk
        android:minSdkVersion="10"
        android:targetSdkVersion="17" />

If we run the app now, we should see a blue screen as follows:

First pass
First pass

Adding native code

We’ve verified that things work from Java, but what we really want to do is to be using OpenGL from native code! In the next few steps, we’ll move the OpenGL code to a set of C files and setup an NDK build for these files.

We’ll be sharing this native code with our future projects for iOS and the web, so let’s create a folder called common located one level above the Android project. What this means is that in your airhockey folder, you should have one folder called android, containing the Android project, and a second folder called common which will contain the common code.

Linking a relative folder that lies outside of the project’s base folder is unfortunately not the easiest thing to do in Eclipse. To accomplish this, we’ll have to follow these steps:

  1. Right-click the project and select Properties. In the window that appears, select Resource->Linked Resources and click New….
  2. Enter ‘COMMON_SRC_LOC’ as the name, and ‘${PROJECT_LOC}\..\common’ as the location. Once that’s done, click OK until the Properties window is closed.
  3. Right-click the project again and select Build Path->Link Source…, select Variables…, select COMMON_SRC_LOC, and select OK. Enter ‘common’ as the folder name and select Finish, then close the Properties window.

You should now see a new folder in your project called common, linked to the folder that we created.

Let’s create two new files in the common folder, game.c and game.h. You can create these files by right-clicking on the folder and selecting New->File. Add the following to game.h:

void on_surface_created();
void on_surface_changed();
void on_draw_frame();

In C, a .h file is known as a header file, and can be considered as an interface for a given .c source file. This header file defines three functions that we’ll be calling from Java.

Let’s add the following implementation to game.c:

#include "game.h"
#include "glwrapper.h"

void on_surface_created() {
	glClearColor(1.0f, 0.0f, 0.0f, 0.0f);
}

void on_surface_changed() {
	// No-op
}

void on_draw_frame() {
	glClear(GL_COLOR_BUFFER_BIT);
}

This code will set the clear color to red, and will clear the screen every time on_draw_frame() is called. We’ll use a special header file called glwrapper.h to wrap the platform-specific OpenGL libraries, as they are often located at a different place for each platform.

Adding platform-specific code and JNI code

To use this code, we still need to add two things: a definition for glwrapper.h, and some JNI glue code so that we can call our C code from Java. JNI stands for Java Native Interface, and it’s how C and Java can talk to each other on Android.

Inside your project, create a new file called glwrapper.h in the jni folder, with the following contents:

#include <GLES2/gl2.h>

That wraps Android’s OpenGL headers. To create the JNI glue, we’ll first need to create a Java class that exposes the native interface that we want. To do this, let’s create a new class called GameLibJNIWrapper, with the following code:

public class GameLibJNIWrapper {
	static {
		System.loadLibrary("game");
	}

	public static native void on_surface_created();

	public static native void on_surface_changed(int width, int height);

	public static native void on_draw_frame();
}

This class will load the native library called libgame.so, which is what we’ll be calling our native library later on when we create the build scripts for it. To create the matching C file for this class, build the project, open up a command prompt, change to the bin/classes folder of your project, and run the following command:

javah -o ../../jni/jni.c com.learnopengles.airhockey.GameLibJNIWrapper

The javah command should be located in your JDKs bin directory. This command will create a jni.c file that will look very messy, with a bunch of stuff that we don’t need. Let’s simplify the file and replace it with the following contents:

#include "../../common/game.h"
#include <jni.h>

JNIEXPORT void JNICALL Java_com_learnopengles_airhockey_GameLibJNIWrapper_on_1surface_1created
	(JNIEnv * env, jclass cls) {
	on_surface_created();
}

JNIEXPORT void JNICALL Java_com_learnopengles_airhockey_GameLibJNIWrapper_on_1surface_1changed
	(JNIEnv * env, jclass cls, jint width, jint height) {
	on_surface_changed();
}

JNIEXPORT void JNICALL Java_com_learnopengles_airhockey_GameLibJNIWrapper_on_1draw_1frame
	(JNIEnv * env, jclass cls) {
	on_draw_frame();
}

We’ve simplified the file greatly, and we’ve also added a reference to game.h so that we can call our game methods. Here’s how it works:

  1. GameLibJNIWrapper defines the native C functions that we want to be able to call from Java.
  2. To be able to call these C functions from Java, they have to be named in a very specific way, and each function also has to have at least two parameters, with a pointer to a JNIEnv as the first parameter, and a jclass as the second parameter. To make life easier, we can use javah to create the appropriate function signatures for us in a file called jni.c.
  3. From jni.c, we call the functions that we declared in game.h and defined in game.c. That completes the connections and allows us to call our native functions from Java.

Compiling the native code

To compile and run the native code, we need to describe our native sources to the NDK build system. We’ll do this with two files that should go in the jni folder: Android.mk and Application.mk. When we added native support to our project, a file called game.cpp was automatically created in the jni folder. We won’t be needing this file, so you can go ahead and delete it.

Let’s set Android.mk to the following contents:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE    := game
LOCAL_CFLAGS    := -Wall -Wextra
LOCAL_SRC_FILES := ../../common/game.c jni.c
LOCAL_LDLIBS := -lGLESv2

include $(BUILD_SHARED_LIBRARY)

This file describes our sources, and tells the NDK that it should compile game.c and jni.c and build them into a shared library called libgame.so. This shared library will be dynamically linked with libGLESv2.so at runtime.

When specifying this file, be careful not to leave any trailing spaces after any of the commands, as this may cause the build to fail.

The next file, Application.mk, should have the following contents:

APP_PLATFORM := android-10
APP_ABI := armeabi-v7a

This tells the NDK build system to build for Android API 10, so that it doesn’t complain about us using unsupported features not present in earlier versions of Android, and it also tells the build system to generate a library for the ARMv7-A architecture, which supports hardware floating point and which most newer Android devices use.

Updating RendererWrapper

Before we can see our new changes, we have to update RendererWrapper to call into our native code. We can do that by updating it as follows:

	@Override
	public void onSurfaceCreated(GL10 gl, EGLConfig config) {
		GameLibJNIWrapper.on_surface_created();
	}

	@Override
	public void onSurfaceChanged(GL10 gl, int width, int height) {
		GameLibJNIWrapper.on_surface_changed(width, height);
	}

	@Override
	public void onDrawFrame(GL10 gl) {
		GameLibJNIWrapper.on_draw_frame();
	}

The renderer now calls our GameLibJNIWrapper class, which calls the native functions in jni.c, which calls our game functions defined in game.c.

Building and running the application

You should now be able to build and run the application. When you build the application, a new shared library called libgame.so should be created in your project’s /libs/armeabi-v7a/ folder. When you run the application, it should look as follows:

Second pass
Second pass

We know that our native code is being called with the color changing from blue to red.

Exploring further

The full source code for this lesson can be found at the GitHub project. For a more detailed introduction to OpenGL ES 2, see Android Lesson One: Getting Started or OpenGL ES 2 for Android: A Quick-Start Guide.

In the next part of this series, we’ll create an iOS project and we’ll see how easy it is to reuse our code from the common folder and wrap it up in Objective-C. Please let me know if you have any questions or feedback!

OpenGL Roundup, June 24, 2013

Here are some interesting links I’ve come across recently:

Cross-platform

Giveaways

OpenGL

NDK

SDKs

OpenGL ES 2 for Android, Printed in Full Color

OpenGL ES 2 for Android: A Quick-Start GuideOpenGL ES 2 for Android is now in full color print!

Have you ever wanted to learn more about OpenGL and graphics programming? With OpenGL ES 2 for Android: A Quick-Start Guide, you’ll learn about modern OpenGL graphics programming from the ground up. You’ll find out all about shaders and the OpenGL pipeline, and discover the power of OpenGL ES 2.0, which is much more feature-rich than its predecessor.

OpenGL can be somewhat of a dark art to the uninitiated. As you read this book, you’ll learn each new concept from first principles. You won’t just learn about a feature; you’ll also understand how it works, and why it works the way it does. Everything you learn is forward-compatible with the just-released OpenGL ES 3, and you can even apply these techniques to other platforms, such as iOS or HTML5 WebGL.

Android is now on top of the market, with millions of devices shipping every day. It’s never been a better time to learn how to create your own 3D games and live wallpaper for Android.  If you can program in Java and you have a creative vision that you’d like to share with the world, then this is the book for you.

I am again grateful to all of my reviewers, readers, commentators, family and friends, and especially my managing editor, Susannah Davidson Pfalzer, and the rest of the team at the Pragmatic Bookshelf for taking on my book and being so supportive and helpful along the way, and Mario Zechner for his great feedback and foreword. Mario’s also co-authored a book, Beginning Android Games, with Robert Green, which is a a great compliment to the book as it covers additional topics specific to game development and Android.

Learn more about OpenGL ES 2 for Android: A Quick-Start Guide:

This has been a long journey, challenging at times, but very rewarding in the end. Thank you for being there with me along the way. 🙂

OpenGL ES Resources and Best Practices

The first place to start for OpenGL documentation on the various mobile platforms is straight at the source:

  • Google’s Android documentation has a small amount of info about OpenGL, though not too much. You’ll get an overview of the APIs and learn how to exclude your application from unsupported devices. According to the OpenGL Dashboard, most devices out there now support OpenGL ES 2.0.
  • Apple’s OpenGL documentation is much better and goes into a lot more depth and detail. At their OpenGL ES for iOS website, you can learn more about best practices and the specifics of using OpenGL on their platform, and they also have videos and sample projects to download.

It’s also worth checking out what the various GPU vendors have to say about best practices and guidelines:

Each GPU vendor also often provides their own SDKs, tools, and IDEs for developing on their GPUs, which can help a great deal with tracing and finding performance issues.

Hope this helps out on your journey ahead!

Beginning Android Games, to Learn More About Game Development for Android

I’m happy to announce that my book, OpenGL ES 2 for Android: A Quick-Start Guide, is now being readied to be sent off to the printers! I owe a special thanks to the publishers, to you guys, my readers and reviewers, and I also owe a special thanks to Mario Zechner, the creator of libgdx, for writing a great foreword and generously helping to promote the book on his end!

Mario has also co-authored “Beginning Android Games” with Robert Green;  I think that his book can be the perfect complement to my own, as you’ll also learn about many of the additional aspects of game development that I didn’t get the chance to cover in my own book, such as:

  • How to develop 2D games, from beginning to end.
  • How to publish to the market, support your users, and deal with crash reports.
  • Using the Native Development Kit (NDK) to support C and C++ code.

If you’re looking to hit additional platforms, libgdx also has you covered. You can port your Java-based Android game to the desktop, to the web via WebGL, and even to iOS with a few nifty tricks. I plan to cover cross-platform development using libgdx in some subsequent posts, as well as going by the C / C++ route which I will also be covering in future posts.

If you use Reddit, you can also visit our respective Reddit threads here:

I just completed my first book: “OpenGL ES 2 for Android: A Quick-Start Guide” for beginners (EDIT: It seems someone removed my Reddit thread! Oh well :()

My book “Beginning Android Games, 2nd Edition” is out, and i’m super happy

I’m glad that the book is finally starting to head out the door; it feels like the end of a journey. It was a journey that was well worth it. 🙂