Draw a Triangle

Gtk Invaders

Published Oct 8, 2017

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Next we draw a triangle to make sure our OpenGL context is working.

main.c

#include <epoxy/gl.h>
#include <epoxy/glx.h>
#include <gtk/gtk.h>

static void on_realize(GtkGLArea *area);
static void on_render(GtkGLArea *area, GdkGLContext *context);

GLuint program;
GLuint vao, vbo_triangle;
GLint attribute_coord2d;

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), "Invaders 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
");

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

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

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

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

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

	GLfloat triangle_vertices[] = {
		 0.0,  0.8,
		-0.8, -0.8,
		 0.8, -0.8
	};
	
	glGenBuffers(1, &vbo_triangle);
	glBindBuffer(GL_ARRAY_BUFFER, vbo_triangle);
	glBufferData(
		GL_ARRAY_BUFFER,
		sizeof(triangle_vertices),
		triangle_vertices,
		GL_STATIC_DRAW
	);
	
	GLint compile_ok = GL_FALSE;
	GLint link_ok = GL_FALSE;

	const char *vs_source = 
	"#version 120
"
	"attribute vec2 coord2d; 
"
	"void main (void) {
"
	"	gl_Position = vec4(coord2d, 0.0, 1.0);
"
	"}";

	const char *fs_source =
	"#version 120
"
	"void main (void) {
"
	"	gl_FragColor[0] = 0.0;
"
	"	gl_FragColor[1] = 0.0;
"
	"	gl_FragColor[2] = 1.0;
"
	"}";

	GLuint fs = glCreateShader(GL_FRAGMENT_SHADER);
	glShaderSource(fs, 1, &fs_source, NULL);
	glCompileShader(fs);
	glGetShaderiv(fs, GL_COMPILE_STATUS, &compile_ok);
	if(!compile_ok) {
		fprintf(stderr, "Error in fragment shader
");
		return;
	}

	GLuint vs = glCreateShader(GL_VERTEX_SHADER);
	glShaderSource(vs, 1, &vs_source, NULL);
	glCompileShader(vs);
	glGetShaderiv(vs, GL_COMPILE_STATUS, &compile_ok);
	if(!compile_ok) {
		fprintf(stderr, "Error in vertex shader
");
		return;
	}

	program = glCreateProgram();
	glAttachShader(program, vs);
	glAttachShader(program, fs);
	glLinkProgram(program);
	glGetProgramiv(program, GL_LINK_STATUS, &link_ok);
	if(!link_ok) {
		fprintf(stderr, "Error when linking program
");
		return;
	}

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

static void on_render(GtkGLArea *area, GdkGLContext *context) {

	g_print("on render
");

	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

	glUseProgram(program);

	glBindVertexArray(vao);
	glEnableVertexAttribArray(attribute_coord2d);

	glBindBuffer(GL_ARRAY_BUFFER, vbo_triangle);
	glVertexAttribPointer(
		attribute_coord2d,
		2,
		GL_FLOAT,
		GL_FALSE,
		0,
		0
	);

	glDrawArrays(GL_TRIANGLES, 0, 3);
	glDisableVertexAttribArray(attribute_coord2d);

}

Compile

$ gcc `pkg-config --cflags gtk+-3.0` main.c `pkg-config --libs gtk+-3.0` -lepoxy

Run

$ ./a.out