DashGL

We "split" an astroid after it has been hit. The way we accomplish this is to first toggle the hit astroid to "off" to hide it. Then we decrement the size and create two new astroids at the position of prior astroid. To keep the new astroids from over-lapping we multiply the original velocity of the astroid by +x, -y for one, and -x, +y for the other. It's simple and creates a convincing effect.

#include <epoxy/gl.h>
#include <epoxy/glx.h>
#include <gtk/gtk.h>
#include <math.h>
#include <stdlib.h>
#include "DashGL/dashgl.h"

static void on_realize(GtkGLArea *area);
static void on_render(GtkGLArea *area, GdkGLContext *context);
static gboolean on_idle(gpointer data);
static gboolean on_keydown(GtkWidget *widget, GdkEventKey *event);
static gboolean on_keyup(GtkWidget *widget, GdkEventKey *event);

#define NUM_BULLETS 10
#define WIDTH 640.0f
#define HEIGHT 480.0f
#define NUM_ROCKS 16

GLuint program;
GLuint vao;
GLint attribute_texcoord, attribute_coord2d;
GLint uniform_mytexture, uniform_mvp;

struct bullet {
	vec3 pos;
	GLfloat rot;
	gboolean active;
	int ticks;
};

struct {
	GLuint vbo[2];
	GLuint tex[2];
	vec3 pos;
	mat4 mvp;
	gboolean left;
	gboolean right;
	gboolean up;
	gboolean space;
	GLfloat rot;
	GLfloat dx, dy, max;
	float bullet_speed;
	int num_ticks;
	struct bullet bullets[NUM_BULLETS];
} player;

struct rock {
	vec3 pos;
	float rot;
	float dx, dy;
	int size;
	gboolean active;
};

struct {
	GLuint vbo;
	GLuint tex;
	mat4 mvp;
	float radius;
	struct rock rocks[NUM_ROCKS];
} astroid;

int main(int argc, char *argv[]) {

	GtkWidget *window;
	GtkWidget *glArea;

	gtk_init(&argc, &argv);

	// Initialize Window

	window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
	gtk_window_set_title(GTK_WINDOW(window), "Astroids Tutorial");
	gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
	gtk_window_set_default_size(GTK_WINDOW(window), 640, 480);
	gtk_window_set_type_hint(GTK_WINDOW(window), GDK_WINDOW_TYPE_HINT_UTILITY);
	g_signal_connect(window, "destroy", G_CALLBACK(gtk_main_quit), NULL);
	g_signal_connect(window, "key-press-event", G_CALLBACK(on_keydown), NULL);
	g_signal_connect(window, "key-release-event", G_CALLBACK(on_keyup), NULL);

	// Initialize GTK GL Area

	glArea = gtk_gl_area_new();
	gtk_widget_set_vexpand(glArea, TRUE);
	gtk_widget_set_hexpand(glArea, TRUE);
	g_signal_connect(glArea, "realize", G_CALLBACK(on_realize), NULL);
	g_signal_connect(glArea, "render", G_CALLBACK(on_render), NULL);
	gtk_container_add(GTK_CONTAINER(window), glArea);

	// Show widgets

	gtk_widget_show_all(window);
	gtk_main();

	return 0;

}

static void on_realize(GtkGLArea *area) {

	// Debug Message
	
	int i;
	g_print("on realize\n");

	gtk_gl_area_make_current(area);
	if(gtk_gl_area_get_error(area) != NULL) {
		fprintf(stderr, "Unknown error\n");
		return;
	}

	const GLubyte *renderer = glGetString(GL_RENDER);
	const GLubyte *version = glGetString(GL_VERSION);

	printf("Renderer: %s\n", renderer);
	printf("OpenGL version supported %s\n", version);

	glClearColor(0.0f, 0.0f, 0.0f, 1.0f);

	glGenVertexArrays(1, &vao);
	glBindVertexArray(vao);

	GLfloat player_vertices[] = {
		-24.0, -24.0, 0.0, 0.0, 
		-24.0,  24.0, 0.0, 1.0,
		 24.0,  24.0, 1.0, 1.0,
		
		-24.0, -24.0, 0.0, 0.0,
		 24.0,  24.0, 1.0, 1.0,
		 24.0, -24.0, 1.0, 0.0
	};
	
	glGenBuffers(1, &player.vbo[0]);
	glBindBuffer(GL_ARRAY_BUFFER, player.vbo[0]);
	glBufferData(
		GL_ARRAY_BUFFER,
		sizeof(player_vertices),
		player_vertices,
		GL_STATIC_DRAW
	);

	GLfloat bullet_vertices[] = {
		-8.0, -8.0, 0.0, 0.0, 
		-8.0,  8.0, 0.0, 1.0,
		 8.0,  8.0, 1.0, 1.0,
		
		-8.0, -8.0, 0.0, 0.0,
		 8.0,  8.0, 1.0, 1.0,
		 8.0, -8.0, 1.0, 0.0
	};
	
	glGenBuffers(1, &player.vbo[1]);
	glBindBuffer(GL_ARRAY_BUFFER, player.vbo[1]);
	glBufferData(
		GL_ARRAY_BUFFER,
		sizeof(bullet_vertices),
		bullet_vertices,
		GL_STATIC_DRAW
	);
	
	player.pos[0] = WIDTH / 2.0f;
	player.pos[1] = HEIGHT / 2.0f;
	player.pos[2] = 0.0;
	player.left = FALSE;
	player.right = FALSE;
	player.up = FALSE;
	player.space = FALSE;
	mat4_translate(player.pos, player.mvp);
	player.tex[0] = shader_load_texture("sprites/player.png");
	player.tex[1] = shader_load_texture("sprites/bullet.png");

	player.dx = 0.0f;
	player.dy = 0.0f;
	player.max = 8.0f;
	player.bullet_speed = 6.0f;
	player.num_ticks = 70;

	for(i = 0; i astroid.rocks[1].active = TRUE;
	astroid.rocks[1].pos[0] = WIDTH*3 / 4.0f;
	astroid.rocks[1].pos[1] = HEIGHT*3 / 4.0f;
	astroid.rocks[1].pos[2] = 0.0f;
	astroid.rocks[1].rot = M_PI / 2.0f;
	astroid.rocks[1].dx = 0.25f;
	astroid.rocks[1].dy = -0.5f;
	astroid.rocks[1].size = 2;

	astroid.rocks[2].active = TRUE;
	astroid.rocks[2].pos[0] = WIDTH*3 / 4.0f;
	astroid.rocks[2].pos[1] = HEIGHT / 4.0f;
	astroid.rocks[2].pos[2] = 0.0f;
	astroid.rocks[2].rot = M_PI / 2.0f;
	astroid.rocks[2].dx = -0.35f;
	astroid.rocks[2].dy = 0.5f;
	astroid.rocks[2].size = 2;

	const char *vs = "shader/vertex.glsl";
	const char *fs = "shader/fragment.glsl";

	program = shader_load_program(vs, fs);

	const char *attribute_name = "coord2d";
	attribute_coord2d = glGetAttribLocation(program, attribute_name);
	if(attribute_coord2d == -1) {
		fprintf(stderr, "Could not bind attribute %s\n", attribute_name);
		return;
	}

	const char *uniform_name = "orthograph";
	GLint uniform_ortho = glGetUniformLocation(program, uniform_name);
	if(uniform_ortho == -1) {
		fprintf(stderr, "Could not bind uniform %s\n", uniform_name);
		return;
	}

	uniform_name = "mvp";
	uniform_mvp = glGetUniformLocation(program, uniform_name);
	if(uniform_mvp == -1) {
		fprintf(stderr, "Could not bind uniform %s\n", uniform_name);
		return;
	}

	glUseProgram(program);
	mat4 orthograph;
	mat4_orthagonal(WIDTH, HEIGHT, orthograph);
	glUniformMatrix4fv(uniform_ortho, 1, GL_FALSE, orthograph);

	g_timeout_add(20, on_idle, (void*)area);

}

static void on_render(GtkGLArea *area, GdkGLContext *context) {
	
	int i;
	vec3 scale;
	mat4 scl, pos, rot;
	gboolean draw_mirror = FALSE;
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

	glUseProgram(program);

	glBindVertexArray(vao);
	glEnableVertexAttribArray(attribute_coord2d);
	glEnableVertexAttribArray(attribute_texcoord);

	// Draw bullet sprites

	glActiveTexture(GL_TEXTURE0);
	glBindTexture(GL_TEXTURE_2D, player.tex[1]);
	glUniform1i(uniform_mytexture, 0);

	for(i = 0; i  WIDTH - 24.0f) {
			player.bullets[i].pos[0] -= WIDTH;
			draw_mirror = TRUE;
		} else if(player.bullets[i].pos[0] <24.0f) {
			player.bullets[i].pos[0] += WIDTH;
			draw_mirror = TRUE;
		}

		if(player.bullets[i].pos[1] > HEIGHT - 24.0f) {
			player.bullets[i].pos[1] -= HEIGHT;
			draw_mirror = TRUE;
		} else if(player.bullets[i].pos[1] <24.0f) {
			player.bullets[i].pos[1] += HEIGHT;
			draw_mirror = TRUE;
		}

		if(!draw_mirror) {
			continue;
		}

		mat4_translate(player.bullets[i].pos, pos);
		mat4_rotate_z(player.bullets[i].rot, rot);
		mat4_multiply(pos, rot, player.mvp);
	
		glUniformMatrix4fv(uniform_mvp, 1, GL_FALSE, player.mvp);
		glDrawArrays(GL_TRIANGLES, 0, 6);
		draw_mirror = FALSE;

	}

	// Draw Player Sprite
	
	glActiveTexture(GL_TEXTURE0);
	glBindTexture(GL_TEXTURE_2D, player.tex[0]);
	glUniform1i(uniform_mytexture, 0);

	mat4_translate(player.pos, pos);
	mat4_rotate_z(player.rot, rot);
	mat4_multiply(pos, rot, player.mvp);
	
	glUniformMatrix4fv(uniform_mvp, 1, GL_FALSE, player.mvp);
	glBindBuffer(GL_ARRAY_BUFFER, player.vbo[0]);
	glVertexAttribPointer(
		attribute_coord2d,
		2,
		GL_FLOAT,
		GL_FALSE,
		sizeof(float) * 4,
		0
	);

	glVertexAttribPointer(
		attribute_texcoord,
		2,
		GL_FLOAT,
		GL_FALSE,
		sizeof(float) * 4,
		(void*)(sizeof(float) * 2)
	);
	glDrawArrays(GL_TRIANGLES, 0, 6);

	if(player.pos[0] > WIDTH - 24.0f) {
		player.pos[0] -= WIDTH;
		draw_mirror = TRUE;
	} else if(player.pos[0] <24.0f) {
		player.pos[0] += WIDTH;
		draw_mirror = TRUE;
	}

	if(player.pos[1] > HEIGHT - 24.0f) {
		player.pos[1] -= HEIGHT;
		draw_mirror = TRUE;
	} else if(player.pos[1] <24.0f) {
		player.pos[1] += HEIGHT;
		draw_mirror = TRUE;
	}

	if(draw_mirror) {

		draw_mirror = FALSE;
		mat4_translate(player.pos, pos);
		mat4_rotate_z(player.rot, rot);
		mat4_multiply(pos, rot, player.mvp);
	
		glUniformMatrix4fv(uniform_mvp, 1, GL_FALSE, player.mvp);
		glDrawArrays(GL_TRIANGLES, 0, 6);
		
	}
	
	// Draw Astroid Sprites

	glActiveTexture(GL_TEXTURE0);
	glBindTexture(GL_TEXTURE_2D, astroid.tex);
	glUniform1i(uniform_mytexture, 0);

	for(i = 0; i  WIDTH - astroid.radius*scale[0]) {
			astroid.rocks[i].pos[0] -= WIDTH;
			draw_mirror = TRUE;
		} else if(astroid.rocks[i].pos[0]  HEIGHT - astroid.radius*scale[0]) {
			astroid.rocks[i].pos[1] -= HEIGHT;
			draw_mirror = TRUE;
		} else if(astroid.rocks[i].pos[1] split, index;
	float scale, dx, dy, x_dif, y_dif;

	split = 0;

	if(player.up) {
		player.dx -= sin(player.rot) * 0.05;
		player.dy += cos(player.rot) * 0.05;
	}
	
	if(player.dx <-player.max) {
		player.dx = -player.max;
	} else if(player.dx > player.max) {
		player.dx = player.max;
	}

	if(player.dy <-player.max) {
		player.dy = -player.max;
	} else if(player.dy > player.max) {
		player.dy = player.max;
	}

	player.pos[0] += player.dx;
	player.pos[1] += player.dy;
	
	for(i = 0; i  astroid.radius * scale) {
				continue;
			}
			
			astroid.rocks[k].active = FALSE;
			player.bullets[i].active = FALSE;
			
			if(astroid.rocks[k].size) {
				split = 2;
				index = k;
			}

			break;

		}
		
		if(!split) {
			continue;
		}

		for(k = 0; k 

	}

	for(i = 0; i keyval) {
		case GDK_KEY_Left:
			player.left = TRUE;
		break;
		case GDK_KEY_Right:
			player.right = TRUE;
		break;
		case GDK_KEY_Up:
			player.up = TRUE;
		break;
		case GDK_KEY_space:
			
			if(!player.space) {
				int i;
				for(i = 0; i keyval) {
		case GDK_KEY_Left:
			player.left = FALSE;
		break;
		case GDK_KEY_Right:
			player.right = FALSE;
		break;
		case GDK_KEY_Up:
			player.up = FALSE;
		break;
		case GDK_KEY_space:
			player.space = FALSE;
		break;
	}

}

Compile with:

$ gcc -c -o DashGL/dashgl.o DashGL/dashgl.c -lepoxy -lpng -lm
$ gcc `pkg-config --cflags gtk+-3.0` main.c DashGL/dashgl.o `pkg-config --libs gtk+-3.0` \
-lepoxy -lm -lpng

Run with:

$ ./a.out