Finishing Up Our Native Air Hockey Project With Touch Events and Basic Collision Detection

In this post in the air hockey series, we’re going to wrap up our air hockey project and add touch event handling and basic collision detection with support for Android, iOS, and emscripten.

Prerequisites

This lesson continues the air hockey project series, building upon the code from GitHub for ‘article-3-matrices-and-objects’. Here are the previous posts in this series:

Setting up a simple build system

Adding support for PNG loading into a texture

Adding a 3d perspective, mallets, and a puck

Updating our game code for touch interaction

The first thing we’ll do is update the core to add touch interaction to the game. We’ll first need to add some helper functions to a new core file called geometry.h.

geometry.h

Let’s start off with the following code:

#include "linmath.h"
#include

typedef struct {
	vec3 point;
	vec3 vector;
} Ray;

typedef struct {
	vec3 point;
	vec3 normal;
} Plane;

typedef struct {
	vec3 center;
	float radius;
} Sphere;

These are a few typedefs that build upon linmath.h to add a few basic types that we’ll use in our code. Let’s wrap up geometry.h:

static inline int sphere_intersects_ray(Sphere sphere, Ray ray);
static inline float distance_between(vec3 point, Ray ray);
static inline void ray_intersection_point(vec3 result, Ray ray, Plane plane);

static inline int sphere_intersects_ray(Sphere sphere, Ray ray) {
	if (distance_between(sphere.center, ray) < sphere.radius)
		return 1;
	return 0;
}

static inline float distance_between(vec3 point, Ray ray) {
	vec3 p1_to_point;
	vec3_sub(p1_to_point, point, ray.point);
	vec3 p2_to_point;
	vec3 translated_ray_point;
	vec3_add(translated_ray_point, ray.point, ray.vector);
	vec3_sub(p2_to_point, point, translated_ray_point);

	// The length of the cross product gives the area of an imaginary
	// parallelogram having the two vectors as sides. A parallelogram can be
	// thought of as consisting of two triangles, so this is the same as
	// twice the area of the triangle defined by the two vectors.
	// http://en.wikipedia.org/wiki/Cross_product#Geometric_meaning
	vec3 cross_product;
	vec3_mul_cross(cross_product, p1_to_point, p2_to_point);
	float area_of_triangle_times_two = vec3_len(cross_product);
	float length_of_base = vec3_len(ray.vector);

	// The area of a triangle is also equal to (base * height) / 2. In
	// other words, the height is equal to (area * 2) / base. The height
	// of this triangle is the distance from the point to the ray.
	float distance_from_point_to_ray = area_of_triangle_times_two / length_of_base;
	return distance_from_point_to_ray;
}

// http://en.wikipedia.org/wiki/Line-plane_intersection
// This also treats rays as if they were infinite. It will return a
// point full of NaNs if there is no intersection point.
static inline void ray_intersection_point(vec3 result, Ray ray, Plane plane) {
	vec3 ray_to_plane_vector;
	vec3_sub(ray_to_plane_vector, plane.point, ray.point);

	float scale_factor = vec3_mul_inner(ray_to_plane_vector, plane.normal)
					   / vec3_mul_inner(ray.vector, plane.normal);

	vec3 intersection_point;
	vec3 scaled_ray_vector;
	vec3_scale(scaled_ray_vector, ray.vector, scale_factor);
	vec3_add(intersection_point, ray.point, scaled_ray_vector);
	memcpy(result, intersection_point, sizeof(intersection_point));
}

We’ll do a line-sphere intersection test to see if we’ve touched the mallet using our fingers or a mouse. Once we’ve grabbed the mallet, we’ll do a line-plane intersection test to determine where to place the mallet on the board.

game.h

We’ll need two new function prototypes in game.h:

void on_touch_press(float normalized_x, float normalized_y);
void on_touch_drag(float normalized_x, float normalized_y);

game.c

Now we can begin the implementation in game.c. Add the following in the appropriate places to the top of the file:

#include "geometry.h"
// ...
static const float puck_radius = 0.06f;
static const float mallet_radius = 0.08f;

static const float left_bound = -0.5f;
static const float right_bound = 0.5f;
static const float far_bound = -0.8f;
static const float near_bound = 0.8f;
// ...
static mat4x4 inverted_view_projection_matrix;

static int mallet_pressed;
static vec3 blue_mallet_position;
static vec3 previous_blue_mallet_position;
static vec3 puck_position;
static vec3 puck_vector;

static Ray convert_normalized_2D_point_to_ray(float normalized_x, float normalized_y);
static void divide_by_w(vec4 vector);
static float clamp(float value, float min, float max);

We’ll now begin with the code for handling a touch press:

void on_touch_press(float normalized_x, float normalized_y) {
	Ray ray = convert_normalized_2D_point_to_ray(normalized_x, normalized_y);

	// Now test if this ray intersects with the mallet by creating a
	// bounding sphere that wraps the mallet.
	Sphere mallet_bounding_sphere = (Sphere) {
	   {blue_mallet_position[0],
		blue_mallet_position[1],
		blue_mallet_position[2]},
	mallet_height / 2.0f};

	// If the ray intersects (if the user touched a part of the screen that
	// intersects the mallet's bounding sphere), then set malletPressed =
	// true.
	mallet_pressed = sphere_intersects_ray(mallet_bounding_sphere, ray);
}

static Ray convert_normalized_2D_point_to_ray(float normalized_x, float normalized_y) {
	// We'll convert these normalized device coordinates into world-space
	// coordinates. We'll pick a point on the near and far planes, and draw a
	// line between them. To do this transform, we need to first multiply by
	// the inverse matrix, and then we need to undo the perspective divide.
	vec4 near_point_ndc = {normalized_x, normalized_y, -1, 1};
	vec4 far_point_ndc = {normalized_x, normalized_y,  1, 1};

    vec4 near_point_world, far_point_world;
    mat4x4_mul_vec4(near_point_world, inverted_view_projection_matrix, near_point_ndc);
    mat4x4_mul_vec4(far_point_world, inverted_view_projection_matrix, far_point_ndc);

	// Why are we dividing by W? We multiplied our vector by an inverse
	// matrix, so the W value that we end up is actually the *inverse* of
	// what the projection matrix would create. By dividing all 3 components
	// by W, we effectively undo the hardware perspective divide.
    divide_by_w(near_point_world);
    divide_by_w(far_point_world);

	// We don't care about the W value anymore, because our points are now
	// in world coordinates.
	vec3 near_point_ray = {near_point_world[0], near_point_world[1], near_point_world[2]};
	vec3 far_point_ray = {far_point_world[0], far_point_world[1], far_point_world[2]};
	vec3 vector_between;
	vec3_sub(vector_between, far_point_ray, near_point_ray);
	return (Ray) {
		{near_point_ray[0], near_point_ray[1], near_point_ray[2]},
		{vector_between[0], vector_between[1], vector_between[2]}};
}

static void divide_by_w(vec4 vector) {
	vector[0] /= vector[3];
	vector[1] /= vector[3];
	vector[2] /= vector[3];
}

This code first takes normalized touch coordinates which it receives from the Android, iOS or emscripten front ends, and then turns those touch coordinates into a 3D ray in world space. It then intersects the 3D ray with a bounding sphere for the mallet to see if we’ve touched the mallet.

Let’s continue with the code for handling a touch drag:

void on_touch_drag(float normalized_x, float normalized_y) {
	if (mallet_pressed == 0)
		return;

	Ray ray = convert_normalized_2D_point_to_ray(normalized_x, normalized_y);
	// Define a plane representing our air hockey table.
	Plane plane = (Plane) {{0, 0, 0}, {0, 1, 0}};

	// Find out where the touched point intersects the plane
	// representing our table. We'll move the mallet along this plane.
	vec3 touched_point;
	ray_intersection_point(touched_point, ray, plane);

	memcpy(previous_blue_mallet_position, blue_mallet_position,
		sizeof(blue_mallet_position));

	// Clamp to bounds
	blue_mallet_position[0] =
		clamp(touched_point[0], left_bound + mallet_radius, right_bound - mallet_radius);
	blue_mallet_position[1] = mallet_height / 2.0f;
	blue_mallet_position[2] =
		clamp(touched_point[2], 0.0f + mallet_radius, near_bound - mallet_radius);

	// Now test if mallet has struck the puck.
	vec3 mallet_to_puck;
	vec3_sub(mallet_to_puck, puck_position, blue_mallet_position);
	float distance = vec3_len(mallet_to_puck);

	if (distance < (puck_radius + mallet_radius)) {
		// The mallet has struck the puck. Now send the puck flying
		// based on the mallet velocity.
		vec3_sub(puck_vector, blue_mallet_position, previous_blue_mallet_position);
	}
}

static float clamp(float value, float min, float max) {
	return fmin(max, fmax(value, min));
}

Once we’ve grabbed the mallet, we move it across the air hockey table by intersecting the new touch point with the table to determine the new position on the table. We then move the mallet to that new position. We also check if the mallet has struck the puck, and if so, we use the movement distance to calculate the puck’s new velocity.

We next need to update the lines that initialize our objects inside on_surface_created() as follows:

puck = create_puck(puck_radius, puck_height, 32, puck_color);
	red_mallet = create_mallet(mallet_radius, mallet_height, 32, red);
	blue_mallet = create_mallet(mallet_radius, mallet_height, 32, blue);

	blue_mallet_position[0] = 0;
	blue_mallet_position[1] = mallet_height / 2.0f;
	blue_mallet_position[2] = 0.4f;
	puck_position[0] = 0;
	puck_position[1] = puck_height / 2.0f;
	puck_position[2] = 0;
	puck_vector[0] = 0;
	puck_vector[1] = 0;
	puck_vector[2] = 0;

The new linmath.h has merged in the custom code we added to our matrix_helper.h, so we no longer need that file. As part of those changes, our perspective method call in on_surface_changed() now needs the angle entered in radians, so let’s update that method call as follows:

mat4x4_perspective(projection_matrix, deg_to_radf(45),
	(float) width / (float) height, 1.0f, 10.0f);

We can then update on_draw_frame() to add the new movement code. Let’s first add the following to the top, right after the call to glClear():

// Translate the puck by its vector
	vec3_add(puck_position, puck_position, puck_vector);

	// If the puck struck a side, reflect it off that side.
	if (puck_position[0] < left_bound + puck_radius 
	 || puck_position[0] > right_bound - puck_radius) {
		puck_vector[0] = -puck_vector[0];
		vec3_scale(puck_vector, puck_vector, 0.9f);
	}
	if (puck_position[2] < far_bound + puck_radius
	 || puck_position[2] > near_bound - puck_radius) {
		puck_vector[2] = -puck_vector[2];
		vec3_scale(puck_vector, puck_vector, 0.9f);
	}

	// Clamp the puck position.
	puck_position[0] = 
		clamp(puck_position[0], left_bound + puck_radius, right_bound - puck_radius);
	puck_position[2] = 
		clamp(puck_position[2], far_bound + puck_radius, near_bound - puck_radius);

	// Friction factor
	vec3_scale(puck_vector, puck_vector, 0.99f);

This code will update the puck’s position and cause it to go bouncing around the table. We’ll also need to add the following after the call to mat4x4_mul(view_projection_matrix, projection_matrix, view_matrix);:

mat4x4_invert(inverted_view_projection_matrix, view_projection_matrix);

This sets up the inverted view projection matrix, which we need for turning the normalized touch coordinates back into world space coordinates.

Let’s finish up the changes to game.c by updating the following calls to position_object_in_scene():

position_object_in_scene(blue_mallet_position[0], blue_mallet_position[1],
	blue_mallet_position[2]);
// ...
position_object_in_scene(puck_position[0], puck_position[1], puck_position[2]);

Adding touch events to Android

With these changes in place, we now need to link in the touch events from each platform. We’ll start off with Android:

MainActivity.java

In MainActivity.java, we first need to update the way that we create the renderer in onCreate():

final RendererWrapper rendererWrapper = new RendererWrapper(this);
// ...
glSurfaceView.setRenderer(rendererWrapper);

Let’s add the touch listener:

glSurfaceView.setOnTouchListener(new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
	if (event != null) {
		// Convert touch coordinates into normalized device
		// coordinates, keeping in mind that Android's Y
		// coordinates are inverted.
		final float normalizedX = (event.getX() / (float) v.getWidth()) * 2 - 1;
		final float normalizedY = -((event.getY() / (float) v.getHeight()) * 2 - 1);

		if (event.getAction() == MotionEvent.ACTION_DOWN) {
			glSurfaceView.queueEvent(new Runnable() {
			@Override
			public void run() {
				rendererWrapper.handleTouchPress(normalizedX, normalizedY);
			}});
		} else if (event.getAction() == MotionEvent.ACTION_MOVE) {
			glSurfaceView.queueEvent(new Runnable() {
			@Override
			public void run() {
				rendererWrapper.handleTouchDrag(normalizedX, normalizedY);
			}});
		}

		return true;
	} else {
		return false;
	}
}});

This touch listener takes the incoming touch events from the user, converts them into normalized coordinates in OpenGL’s normalized device coordinate space, and then calls the renderer wrapper which will pass the event on into our native code.

RendererWrapper.java

We’ll need to add the following to RendererWrapper.java:

public void handleTouchPress(float normalizedX, float normalizedY) {
		on_touch_press(normalizedX, normalizedY);
	}

	public void handleTouchDrag(float normalizedX, float normalizedY) {
		on_touch_drag(normalizedX, normalizedY);
	}

	private static native void on_touch_press(float normalized_x, float normalized_y);

	private static native void on_touch_drag(float normalized_x, float normalized_y);

renderer_wrapper.c

We’ll also need to add the following to renderer_wrapper.c in our jni folder:

JNIEXPORT void JNICALL Java_com_learnopengles_airhockey_RendererWrapper_on_1touch_1press(
	JNIEnv* env, jclass cls, jfloat normalized_x, jfloat normalized_y) {
	UNUSED(env);
	UNUSED(cls);
	on_touch_press(normalized_x, normalized_y);
}

JNIEXPORT void JNICALL Java_com_learnopengles_airhockey_RendererWrapper_on_1touch_1drag(
	JNIEnv* env, jclass cls, jfloat normalized_x, jfloat normalized_y) {
	UNUSED(env);
	UNUSED(cls);
	on_touch_drag(normalized_x, normalized_y);
}

We now have everything in place for Android, and if we run the app, it should look similar to as seen below:

Air Hockey with touch, running on a Galaxy Nexus
Air Hockey with touch, running on a Galaxy Nexus

Adding support for iOS

To add support for iOS, we need to update ViewController.m and add support for touch events. To do that and update the frame rate at the same time, let’s add the following to viewDidLoad: before the call to [self setupGL]:

view.userInteractionEnabled = YES;
self.preferredFramesPerSecond = 60;

To listen to the touch events, we need to override a few methods. Let’s add the following methods before - (void)glkView:(GLKView *)view drawInRect:(CGRect)rect:

static CGPoint getNormalizedPoint(UIView* view, CGPoint locationInView)
{
    const float normalizedX = (locationInView.x / view.bounds.size.width) * 2.f - 1.f;
    const float normalizedY = -((locationInView.y / view.bounds.size.height) * 2.f - 1.f);
    return CGPointMake(normalizedX, normalizedY);
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesBegan:touches withEvent:event];
    UITouch* touchEvent = [touches anyObject];
    CGPoint locationInView = [touchEvent locationInView:self.view];
    CGPoint normalizedPoint = getNormalizedPoint(self.view, locationInView);
    on_touch_press(normalizedPoint.x, normalizedPoint.y);
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesMoved:touches withEvent:event];
    UITouch* touchEvent = [touches anyObject];
    CGPoint locationInView = [touchEvent locationInView:self.view];
    CGPoint normalizedPoint = getNormalizedPoint(self.view, locationInView);
    on_touch_drag(normalizedPoint.x, normalizedPoint.y);
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesEnded:touches withEvent:event];
}

- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesCancelled:touches withEvent:event];
}

This is similar to the Android code in that it takes the input touch event, converts it to OpenGL’s normalized device coordinate space, and then sends it on to our game code.

Our iOS app should look similar to the following image:

Air Hockey with touch, on iOS
Air Hockey with touch, on iOS

Adding support for emscripten

Adding support for emscripten is just as easy. Let’s first add the following to the top of main.c:

static void handle_input();
// ...
int is_dragging;

At the beginning of do_frame(), add a call to handle_input();:

static void do_frame()
{
	handle_input();
	// ...

Add the following for handle_input:

static void handle_input()
{
	glfwPollEvents();
	const int left_mouse_button_state = glfwGetMouseButton(GLFW_MOUSE_BUTTON_1);
	if (left_mouse_button_state == GLFW_PRESS) {
		int x_pos, y_pos;
		glfwGetMousePos(&x_pos, &y_pos);
		const float normalized_x = ((float)x_pos / (float) width) * 2.f - 1.f;
	    const float normalized_y = -(((float)y_pos / (float) height) * 2.f - 1.f);

		if (is_dragging == 0) {
			is_dragging = 1;
			on_touch_press(normalized_x, normalized_y);
		} else {
			on_touch_drag(normalized_x, normalized_y);
		}
	} else {
		is_dragging = 0;
	}
}

This code sets is_dragging depending on whether we just clicked the primary mouse button or if we’re currently dragging the mouse. Depending on the case, we’ll call either on_touch_press or on_touch_drag. The code to normalize the coordinates is the same as in Android and iOS, and indeed a case could be made to abstract out into the common game code, and just pass in the raw coordinates relative to the view size to that game code.

After compiling with emcc make, we should get output similar to the below:

Exploring further

That concludes our air hockey project! The full source code for this lesson can be found at the GitHub project. You can find a more in-depth look at the concepts behind the project from the perspective of Java Android in OpenGL ES 2 for Android: A Quick-Start Guide. For exploring further, there are many things you could add, like improved graphics, support for sound, a simple AI, multiplayer (on the same device), scoring, or a menu system.

Whether you end up using a commercial cross-platform solution like Unity or Corona, or whether you decide to go the independent route, I hope this series was helpful to you and most importantly, that you enjoy your future projects ahead and have a lot of fun with them. 🙂

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.