Skip to content

Simplicity Itself

I just wanted to share what it took to draw a simple yellow triangle on a black background in OpenGL|ES. I hope it will give my Ruby friends and Haskell friends an aneurysm.

To show this:
Hello Triangle

I had to do this:

//The headers
#include <SDL/SDL.h>
#include <SDL/SDL_opengles.h>

//Screen attributes
const int SCREEN_WIDTH = 480;
const int SCREEN_HEIGHT = 320;
const int SCREEN_BPP = 32;

SDL_Event event;
GLuint programObject;

GLuint LoadShader ( GLenum type, const char *shaderSrc )
   GLuint shader;
   GLint compiled;

   // Create the shader object
   shader = glCreateShader ( type );

   if ( shader == 0 )
   	return 0;

   // Load the shader source
   glShaderSource ( shader, 1, &amp;amp;amp;amp;amp;shaderSrc, NULL );

   // Compile the shader
   glCompileShader ( shader );

   // Check the compile status
   glGetShaderiv ( shader, GL_COMPILE_STATUS, &amp;amp;amp;amp;amp;compiled );

   return shader;


bool init_GL() {

	const char* vShaderStr = "attribute vec4 vPosition;    \n"
		"void main()                  \n"
		"{                            \n"
		"   gl_Position = vPosition;  \n"
		"}                            \n";

	const char* fShaderStr = "precision mediump float;\n"
		"void main()                                  \n"
		"{                                            \n"
		"  gl_FragColor = vec4 ( 1.0, 1.0, 0.0, 1.0 );\n"
		"}                                            \n";

	GLuint vertexShader;
	GLuint fragmentShader;
	GLint linked;

	// Load the vertex/fragment shaders
	vertexShader = LoadShader(GL_VERTEX_SHADER, vShaderStr);
	fragmentShader = LoadShader(GL_FRAGMENT_SHADER, fShaderStr);

	// Create the program object
	programObject = glCreateProgram();

	if (programObject == 0)
		return 0;

	glAttachShader(programObject, vertexShader);
	glAttachShader(programObject, fragmentShader);

	// Bind vPosition to attribute 0
	glBindAttribLocation(programObject, 0, "vPosition");

	// Link the program

	// Check the link status
	glGetProgramiv(programObject, GL_LINK_STATUS, &amp;amp;amp;amp;amp;linked);

	glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
	return true;

bool init() {
	//Initialize SDL
//		return false;

	//Create Window
			== NULL) {
		return false;

	//Initialize OpenGL
	if (init_GL() == false) {
		return false;

	//Set caption
	SDL_WM_SetCaption("OpenGL Test", NULL);

	return true;

void clean_up() {
	//Quit SDL

void Draw() {
	GLfloat vVertices[] = { 0.0f, 0.5f, 0.0f, -0.5f, -0.5f, 0.0f, 0.5f, -0.5f,
			0.0f };

	// Set the viewport

	// Clear the color buffer

	// Use the program object

	// Load the vertex data
	glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, vVertices);

	glDrawArrays(GL_TRIANGLES, 0, 3);

int main(int argc, char *argv[]) {
	//Quit flag
	bool quit = false;

	if (init() == false) {
		return 1;

	//Wait for user exit
	while (quit == false) {

		//While there are events to handle
		while (SDL_PollEvent(&amp;amp;amp;amp;amp;event)) {
			//Handle key presses

			if (event.type == SDL_QUIT) {
				quit = true;

	//Clean up

	return 0;

You will all be happy to know that one of the first things I did (after re-learning make) was dive into google test and get a testing framework working. My last several projects have been with Ruby and Java, and in my spare time I have been building Palm Pre apps in Javascript and messing around with Erlang.

So I thought it was funny that many of my friends have been going to more and more abstract languages, while I, oddly, have decided to go lower. I have been playing around with doing OpenGL|ES and C++ lately in my spare time, hopefully in prep of the release of the Native SDK (PDK) for the Palm Pre.

One Comment

  1. Amos wrote:

    Gaffo, some of your line numbers are indenting into and on top of the code. It makes it really hard to read. We’ll have to sit and talk C sometime. It has been a while, not withstanding the git patch. I really like playing in C.

    Friday, February 19, 2010 at 6:42 am | Permalink

Post a Comment

Your email is never published nor shared.