Loading a PNG into Memory and Displaying It as a Texture with OpenGL ES 2: Adding Support for iOS

In the previous post, we looked at loading in texture data from a PNG file and uploading it to an OpenGL texture, and then displaying that on the screen in Android. To do that, we used libpng and loaded in the data from our platform-independent C code.

In this post, we’ll add supporting files to our iOS project so we can do the same from there.

Prerequisites

To complete this lesson, you’ll need to have completed 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. The previous iOS post, Calling OpenGL from C on iOS, Sharing Common Code with Android, covers setup of the Xcode project and environment.

You can also just download the completed project for this part of the series from GitHub and check out the code from there.

Adding the common platform code

The first thing we’ll do is add new supporting files to the common platform code, as we’ll need them for both iOS and emscripten. These new files should go in /airhockey/src/platform/common:

platform_file_utils.c

#include "platform_file_utils.h"
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

FileData get_file_data(const char* path) {
	assert(path != NULL);
		
	FILE* stream = fopen(path, "r");
	assert (stream != NULL);

	fseek(stream, 0, SEEK_END);	
	long stream_size = ftell(stream);
	fseek(stream, 0, SEEK_SET);

	void* buffer = malloc(stream_size);
	fread(buffer, stream_size, 1, stream);

	assert(ferror(stream) == 0);
	fclose(stream);

	return (FileData) {stream_size, buffer, NULL};
}

void release_file_data(const FileData* file_data) {
	assert(file_data != NULL);	
	assert(file_data->data != NULL);

	free((void*)file_data->data);
}

We’ll use these two functions to read data from a file and return it in a memory buffer, and release that buffer when we no longer need to keep it around. For iOS & emscripten, our asset loading code will wrap these file loading functions.

platform_log.c

#include "platform_log.h"
#include <stdarg.h>
#include <stdlib.h>
#include <stdio.h>

#define LOG_VPRINTF(priority)	printf("(" priority ") %s: ", tag); \
								va_list arg_ptr; \
								va_start(arg_ptr, fmt); \
								vprintf(fmt, arg_ptr); \
								va_end(arg_ptr); \
								printf("\n");

void _debug_log_v(const char *tag, const char *fmt, ...) {
	LOG_VPRINTF("VERBOSE");
}

void _debug_log_d(const char *tag, const char *fmt, ...) {
	LOG_VPRINTF("DEBUG");
}

void _debug_log_w(const char *tag, const char *fmt, ...) {
	LOG_VPRINTF("WARN");
}

void _debug_log_e(const char *tag, const char *fmt, ...) {
	LOG_VPRINTF("ERROR");
}

For iOS and emscripten, our platform logging code just wraps around printf.

Updating the iOS code

There’s just one new file that we we need to add to the ios group in our Xcode project, platform_asset_utils.m:

#include "platform_asset_utils.h"
#include "platform_file_utils.h"
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

FileData get_asset_data(const char* relative_path) {
	assert(relative_path != NULL);
    
    NSMutableString* adjusted_relative_path = 
        [[NSMutableString alloc] initWithString:@"/assets/"];
    [adjusted_relative_path appendString:
        [[NSString alloc] initWithCString:relative_path encoding:NSASCIIStringEncoding]];
    
    return get_file_data(
        [[[NSBundle mainBundle] pathForResource:adjusted_relative_path ofType:nil]
         cStringUsingEncoding:NSASCIIStringEncoding]);
}

void release_asset_data(const FileData* file_data) {
    assert(file_data != NULL);
	release_file_data(file_data);
}

To load in an asset that’s been bundled with the application, we first prefix the path with ‘/assets/’, and then we use the mainBundle of the application to get the path for the resource. Once we’ve done that, we can use the regular file reading code that we’ve defined in platform_file_utils.c.

iOS experts: When I was researching how to do this, I wasn’t sure if this was the best way or even the right way, but it does seem to work. I’d love to know if there’s another way to do this that is more appropriate, perhaps just by grabbing the path of the application and concatenating that with the relative path?

Aside from adding this new file, we just need to add some references to the project and then we’ll be able to compile & run the app.

Adding the libpng files

Right-click the project and select Add Files to “Air Hockey”…. Add the following C files from the libpng folder, and add them as a new folder group:

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

Remove the common folder group that may be left there from the last lesson, and then add all of the files from the core folder as a new folder group. Do the same for all of the files in /platform/common. Finally, add the assets folder as a folder reference, not as a folder group. That will link the assets folder directly into the project and package those files with the application.

We’ll also need to link to libz.dylib. To do this, click on the ‘airhockey’ target, select Build Phases, expand Link Binary With Libraries, and add a reference to ‘libz.dylib’.

The Xcode Project Navigator should look similar to the below:

Xcode Project Navigator
Xcode Project Navigator

It might make more sense to link in the libpng sources as a static library somehow, but I found that this compiled very fast even from a clean build. Once you run the application in the simulator, it should look similar to the following image:

iOS Simulator, showing the texture on the screen
iOS Simulator, showing the texture on the screen

Now the same code that we used in Android is running on iOS to load in a texture, with very little work required to customize it for iOS! One of the advantages of this approach is that we can also take advantage of the vastly superior debugging and profiling capabilities of Xcode (as compared to what you get in Eclipse with the NDK!), and Xcode can also build the project far faster than the Android tools can, leading to quicker iteration times.

Exploring further

The full source code for this lesson can be found at the GitHub project. In the next post, we’ll also cover an Emscripten target, and we’ll see that it also won’t take much work to support. As always, let me know your feedback. 🙂

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 iOS, Sharing Common Code with Android

In the last post, we covered how to call OpenGL from C on Android by using the NDK; in this post, we’ll call into the same common code from an Objective-C codebase which will run on an iOS device.

Prerequisites

  • A Mac with a suitable IDE installed.
  • An iOS emulator, or a provisioned iPhone or iPad.

You’ll also need to have completed the first post in this series. If not, then you can also download the code from GitHub so that you can follow along.

We’ll be using Xcode in this lesson. There are other IDEs available, such as AppCode. If you don’t have a Mac available for development, there is more info on alternatives available here:

For this article, I used Xcode 4.6.3 with the iOS 6.1 Simulator.

Getting started

We’ll create a new project from an Xcode template with support for OpenGL already configured. You can follow along all of the code at the GitHub project.

To create the new project, open Xcode, and select File->New->Project…. When asked to choose a template, select Application under iOS, and then select OpenGL Game and select Next. Enter ‘Air Hockey’ as the Product Name, enter whatever you prefer for the Organization Name and Company Identifier, select Universal next to Devices, and make sure that Use Storyboards and Use Automatic Reference Counting are both checked, then select Next.

Place the project in a new folder called ios inside of the airhockey folder that we worked with from the previous post. This means that we should now have three folders inside of airhockeyandroidcommon, and ios. Don’t check Create local git repository for this project, as we already setup a git repository in the previous post.

Once the project’s been created, you should see a new folder called Air Hockey inside of the ios folder, and inside of Air Hockey, there’ll be another folder called Air Hockey, as well as a project folder called Air Hockey.xcodeproj.

Flattening the Xcode project

I personally prefer to flatten this all out and put everything in the ios folder, and the following steps will show you how to do this; please feel free to skip this section if you don’t mind having the nested folders:

  1. Close the project in Xcode, and then move all of the files inside of the second Air Hockey folder so that they are directly inside of the ios folder.
  2. Move Air Hockey.xcodeproj so that it’s also inside of the ios folder. The extra Air Hockey folders can now be deleted.
  3. Right-click Air Hockey.xcodeproj in the Finder and select Show Package Contents.
  4. Edit project.pbxproj in a text editor, and delete all occurrences of ‘Air Hockey/’.
  5. Go back to the ios folder and open up Air Hockey.xcodeproj in Xcode.
  6. Select View->Navigator->Show Project Navigator and View->Utilities->Show File Inspector.
  7. Select Air Hockey in the Project Navigator on the left. On the right in the File Inspector, click the button under Relative to Group, to the right of Air Hockey, select some random folder (this is to work around a bug) and select Choose, then click it again and select the ios folder this time.

The project should now be able to build OK, with all files in the ios folder. More information can be found here: How do you move an Xcode 4.2 project file to another folder?

Adding a reference to the common code

Here’s how we add our common code to the project:

  1. Right-click the project root in the Project Navigator (the item that looks like “Air Hockey, 1 target, iOS SDK 6.1”).
  2. Select Add Files to “Air Hockey”….
  3. Select the common folder, which will be located next to the ios folder, make sure that Copy items into destination group’s folder (if needed) is not checked, that Create groups for any added folders is selected, and that Air Hockey is selected next to Add to targets, then select Add.

You should now see the common folder appear in the Project Navigator, with game.h and game.c inside.

Understanding how iOS manages OpenGL through the GLKit framework

When we created a new project with the OpenGL Game template, Xcode set things up so that when the application is launched, it displays an OpenGL view on the screen, and drives that view with a special view controller. A view controller in iOS manages a set of views, and can be thought of as being sort of like the iOS counterpart of an Android Activity.

When the application is launched, the OS reads the storyboard file, which tells it to create a new view controller that is subclassed from GLKViewController and add it to the application’s window. This view controller is part of the GLKit framework and provides an OpenGL ES rendering loop. It can be configured with a preferred frame rate, and it can also automatically handle application-level events, such as pausing the rendering loop when the application is about to go to the background.

That GLKViewController contains a GLKView as its root view, which is what creates and manages the actual OpenGL framebuffer. This GLKView is automatically linked to the GLKViewController, so that when it’s time to draw a new frame, it will call a method called drawInRect: in our GLKViewController.

Before moving on to the next step, you may want to check out the default project by running it in the simulator, just to see what it looks like.

Calling our common code from the view controller

The default code in the view controller does a lot more than we need, since it creates an entire demo scene. We want to keep things simple for now and just see that we can call OpenGL from C and wrap that with the view controller, so let’s open up ViewController.m, delete everything, and start off by adding the following code:

#import "ViewController.h"
#include "game.h"

@interface ViewController () {
}

@property (strong, nonatomic) EAGLContext *context;

- (void)setupGL;

@end

This includes game.h so that we can call our common functions, defines a new property to hold the EAGL context, and declares a method called setupGL: which we’ll define soon. Let’s continue the code:

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];

    if (!self.context) {
        NSLog(@"Failed to create ES context");
    }

    GLKView *view = (GLKView *)self.view;
    view.context = self.context;
    view.drawableDepthFormat = GLKViewDrawableDepthFormat24;

    [self setupGL];
}

- (void)dealloc
{
    if ([EAGLContext currentContext] == self.context) {
        [EAGLContext setCurrentContext:nil];
    }
}

Once the GLKView has been loaded into memory, viewDidLoad: will be called so that we can initialize our OpenGL context. We initialize an OpenGL ES 2 context here and assign it to the context property by calling:

self.context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2]

This allocates a new instance of an EAGLContext, which manages all of the state and resources required to render using OpenGL ES. We then initialize that instance by calling initWithAPI:, passing in a special token which tells it to initialize the context for OpenGL ES 2 rendering.

For those of you not used to Objective-C syntax, here’s what this could look like if it were using Java syntax:

this.context = new EAGLContext().initWithAPI(kEAGLRenderingAPIOpenGLES2);

Once we have an EAGLContext, we assign it to the view, we configure the view’s depth buffer format, and then we call the following to do further OpenGL setup:

[self setupGL]

We’ll define this method further below. dealloc: will be called when the view controller is destroyed, so there we release the EAGLContext if needed by calling the following:

[EAGLContext setCurrentContext:nil]

Let’s complete the code for ViewController.m:


- (void)setupGL
{
[EAGLContext setCurrentContext:self.context];
on_surface_created();
on_surface_changed();
}

- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect
{
on_draw_frame();
}

@end

Here is where we call our game code to do the actual rendering. In setupGL:, we set the EAGLContext so that we have a valid context to use for drawing, and then we call on_surface_created() and on_surface_changed() from our common code. Every time a new frame needs to be drawn, drawInRect: will be called, so there we call on_draw_frame().

Why don’t we also need to set the context from drawInRect:? This method is actually a delegate method which is declared in GLKViewDelegate and called by the GLKView, and the view takes care of setting the context and framebuffer target for us before it calls our delegate. For those of you from the Java world, this is like having our class implement an interface and passing ourselves as a listener to another class, so that it can call us back via that interface. Our view controller automatically set itself as the delegate when it was linked to the GLKView by the storyboard.

We don’t have to do things this way — we could also subclass GLKView instead and override its drawRect: method. Delegation is simply a preferred pattern in iOS when subclassing isn’t required.

As a quick reminder, here’s what we had defined in our three functions from game.c:

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);
}

So, when we actually run our project, we should expect the screen to get cleared to red.

Before we can build and run the project, we’ll need to add a glwrapper.h header to the project, like we did for the Android project in the previous post. In the same folder as ViewController.m, add a new header file called glwrapper.h, and add the following contents:

#include <OpenGLES/ES2/gl.h>

Building and running the project

We should now be able to build and run the project in the iOS simulator. Click the play button to run the app and launch the simulator. Once it’s launched, you should see a screen similar to the following image:

Running in iOS Simulator
Running in iOS Simulator

And that’s it! By using GLKit, we can easily wrap our OpenGL code and call it from Objective-C.

To tell iOS and the App Store that our application should not be displayed to unsupported devices, we can add the key ‘opengles-2’ to Air Hockey-Info.plist, inside Required device capabilities.

Exploring further

The full source code for this lesson can be found at the GitHub project. For further reading, I recommend the following excellent intro to GLKit, which goes into more detail on using GLKView, GLKViewController and other areas of GLKit:

Beginning OpenGL ES 2.0 with GLKit Part 1

In the next part of this series, we’ll take a look at using emscripten to create a new project that also calls into our common code and compiles it for the web. I am coming to iOS from a background in Java and Android and I am new to iOS and Objective-C, so please let me know if anything doesn’t make sense here, and I’ll go and fix it up. 🙂

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!

Developing a Simple Game of Air Hockey Using C and OpenGL ES 2 for Android, iOS, and the Web

Some of you have been curious about what the air hockey game from the book would be like if we brought it over to other platforms. I would like to find out, myself. 🙂 In the spirit of my last post about cross-platform development, I want to port the air hockey project over to a native cross-platform code base that can be built for Android and iOS, and even the web by using emscripten and WebGL. Everything will be open-source and available on GitHub.

Here are some of the things that we’ll have to figure out and learn along the way:

  • Setting up a simple build system for each platform.
  • Initializing OpenGL.
  • Adding support for basic touch and collision detection.

In the next post, we’ll take a look at setting up a simple build system to initialize OpenGL across these different platforms. Here are all of the posts for the series so far:

Setting up a simple build system

Adding support for PNG loading into a texture

Adding a 3d perspective, mallets, and a puck

Adding touch events and basic collision detection

The code is available on Github, with each section organized by tags.

Site Updates, and Thoughts on Native Development for the Web

I’ve recently been spending time travelling overseas, taking a bit of a break after reaching an important milestone with the book, and also taking a bit of a rest from working for myself! The trip has been good so far, and I’ve even been keeping up to date with items from the RSS feed. Here is some of the news that I wanted to share with y’all, as well as to get your thoughts:

Book nearing production

OpenGL ES for Android: A Quick-Start Guide reached its final beta a couple of weeks ago, and is now being readied to be sent off to the printers. I would like to thank everyone again for their feedback and support; I am so grateful for it, and happy that the book is now going out the door. I’d also like to give a special thanks to Mario Zechner, the creator behind libgdx and Beginning Android Games, for generously contributing his foreword and a lot of valuable feedback!

Site news

Not too long ago, I decided to add a new forums section to the site to hopefully build up some more community involvement and get a two-way dialogue going; unfortunately, things didn’t quite take off. The forums have also suffered from spam and some technical issues, and recently I was even locked out of the forum administration. I have no idea what happened or how to fix it, so since the posting rate was low, I am just putting the forums on ice for now.

I’d still love to find a way to have some more discussions happening on the site. In which other ways do you believe that I could improve the site so that I could encourage this? I’d love to hear your thoughts.

Topics to explore further

I’ve also been thinking about new topics to explore and write about, as a lot of exciting things are happening with 3D on the mobile and web. One big trend that seems to be taking place: Native is making a comeback.

For many years,  C and C++ were proclaimed to be dead languages, lingering around only for legacy reasons, and soon to be replaced by the glorious world of managed languages. Having started out my own development career in Java, I can agree that the Java world does have a lot of advantages. The language is easier to learn than a behemoth like C++, and, at least on the desktop, the performance on the JVM can even come close to rivalling native languages.

So, why the resurgence in C and C++? Here are some of my thoughts:

  • The world is not just limited to the desktop anymore, and there are more important platforms to target than ever before. C and C++ excel at cross-platform portability, as just about every platform has a C/C++ compiler. By contrast, the JVM and .NET runtimes are limited to certain platforms, and Android’s Dalvik VM is not as good as the JVM in producing fast, efficient JIT compiled code. Yes, there are bytecode translators and commercial alternatives such as Xamarin’s Mono platforms for mobile, but this comes with its own set of disadvantages.
  • Resource usage can be more important than programmer productivity. This is true in big, expensive data centers, and it’s also true on mobile, where smaller downloads and lower battery usage can lead to happier customers.
  • C and C++ are still king when it comes to fast, efficient compiled code that can be compiled almost anywhere. Other native would-be competitors lose out because they are either not as fast or not as widely available on the different platforms. When productivity becomes more important than performance, these alternatives also get squeezed out by the managed and scripting languages.

As much as C and C++ excel at the things they’re good at, they also come with a lot of legacy cruft. C++ is a huge language, and it gets larger with each new standard. On the other hand, at least the compilers give you some freedom. Don’t want to use the STL? Roll out your own custom containers. Don’t want the cost/limitations of exception handling and RTTI? Compile with -fno-exceptions and -fno-rtti. Undefined behavior is another nasty issue which can rear its head, though compilers like Clang now feature additional tools to help catch and fix these errors. With data-oriented design and sensible error handling, C++ code can be both fast and maintainable.

Compiling C and C++ to the web

With tools like emscripten, you can now compile your C/C++ code to JavaScript and run it in a browser, and if you use the asm.js subset, it can actually run with very good performance, enough to run a modern 3D game using JavaScript and WebGL. I’ve always been skeptical of the whole “JavaScript everywhere” meme, because how can the web truly become an open computing platform by forcing the use of one language for everything? There’s no way a single language can be equally suitable for all tasks, and why would I want to develop a second code base just for the web? For this reason, I used to believe that Google’s Native Client held more promise, since it can run native code with almost no speed loss. Why use JavaScript when you can just execute directly on the CPU and bring your existing code over?

Now I see things a bit differently and I think that the asm.js approach has a lot of merit to it. NaCl has been around for years now, and it still only runs in Google Chrome, and then only on certain platforms and only if the software is distributed through the Chrome store, or if the user enables a developer flag. The asm.js approach, on the other end, can run on every browser that supports modern JavaScript. This approach is also portable, meaning it will work into the foreseeable future, even on new device architectures. NaCl, on the other hand, is limited to what was compiled. Portable NaCl is supposed to fix this, but it’s been a work-in-progress for years now, and given the experience with NaCl, it may never find its way to another browser besides Google Chrome. Combined with WebGL, compiling to JavaScript really opens up the web to a lot of new possibilities, one where you can deploy across the web without being tied to a single browser or plugin. The BananaBread demo shows just some of what is possible.

I’d like to learn more about writing OpenGL apps that can run on Android, iOS, and the web, all with a single code base in C++. I know that this is also possible with Java by using Google’s Web Toolkit and bytecode translators (after all, this is how libgdx does it), but I’d like to learn something different, outside of the Java sphere. Is this something that you guys would be interested in reading more of? This is all relatively new to me and I’m currently exploring, so as always, looking forward to your feedback. 🙂

Update: I am now developing an air hockey project here: Developing a Simple Game of Air Hockey Using C++ and OpenGL ES 2 for Android, iOS, and the Web