Tutorials

Tutorials for getting started with programming on Linux

Draw Player Sprite

Simple Astroids clone

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Next we’ll draw a sprite for the player character in the middle of the window. We do a lot in one step, namely loading the sprite, and adding in an MVP matrix to position the sprite accordingly.

main.c
#include <epoxy/gl.h> #include <epoxy/glx.h> #include <gtk/gtk.h> #include "DashGL/dashgl.h" static void on_realize(GtkGLArea *area); static void on_render(GtkGLArea *area, GdkGLContext *context); #define WIDTH 640.0f #define HEIGHT 480.0f GLuint program; GLuint vao; GLint attribute_texcoord, attribute_coord2d; GLint uniform_mytexture, uniform_mvp; struct { GLuint vbo; GLuint tex; vec3 pos; mat4 mvp; } player; 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); // 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 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); glBindBuffer(GL_ARRAY_BUFFER, player.vbo); glBufferData( GL_ARRAY_BUFFER, sizeof(player_vertices), player_vertices, GL_STATIC_DRAW ); player.pos[0] = WIDTH / 2.0f; player.pos[1] = HEIGHT / 2.0f; player.pos[2] = 0.0; mat4_translate(player.pos, player.mvp); player.tex = shader_load_texture("sprites/player.png"); 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); } static void on_render(GtkGLArea *area, GdkGLContext *context) { g_print("on render\n"); glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glUseProgram(program); glBindVertexArray(vao); glEnableVertexAttribArray(attribute_coord2d); glEnableVertexAttribArray(attribute_texcoord); glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_2D, player.tex); glUniform1i(uniform_mytexture, 0); glUniformMatrix4fv(uniform_mvp, 1, GL_FALSE, player.mvp); glBindBuffer(GL_ARRAY_BUFFER, player.vbo); 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); glDisableVertexAttribArray(attribute_coord2d); glDisableVertexAttribArray(attribute_texcoord); }

Vertex Shader

shader/vertex.glsl
#version 120 uniform mat4 orthograph; uniform mat4 mvp; attribute vec2 uv_coord; attribute vec2 coord2d; varying vec2 texcoord; void main (void) { gl_Position = orthograph * mvp *vec4(coord2d, 0.0, 1.0); texcoord = uv_coord; }

Fragment Shader

shader/fragment.glsl
#version 120 varying vec2 texcoord; uniform sampler2D mytexture; void main(void) { vec2 flipped_texcoord = vec2(texcoord.x, 1.0 - texcoord.y); gl_FragColor = texture2D(mytexture, flipped_texcoord); }

Compile

$ 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

$ ./a.out