OpenGL Triangle of Doom

The demo app

This app is loosely based on a demo that is part of the maemo project. (ported from original X11 OpenGL code to SDL) [ http://wiki.maemo.org/SimpleGL_Example ]

Compile it natively on the Pre in debian chroot (you'll need to extract palm's patched SDL library to /usr/local/include) with:

gcc -o doomtriangle doomtriangle.c  -I/usr/local/include/SDL -lSDL -lGLESv2

doomtriangle.c

#include <stdio.h>
#include <stdlib.h>
#include "SDL.h"
#include "SDL_opengles.h"
#include "SDL_video.h"
#include <math.h>
#include <sys/time.h>
#include <unistd.h>
#include <string.h>

const char* vertexSrc = "attribute vec4 position; varying mediump vec2 pos; void main() { gl_Position = position; pos = position.xy; }";
const char* fragmentSrc = "varying mediump vec2 pos; uniform mediump float phase; void main() { gl_FragColor = vec4(1, 1, 1, 1) * sin((pos.x * pos.x + pos.y * pos.y) * 40.0 + phase); }";
//const char* fragmentSrc = "varying mediump vec2 pos; uniform mediump float phase; void main() { gl_FragColor = vec4(1, 1, 1, 1) * step(pos.x * pos.x + pos.y * pos.y, phase * 0.2); }";

void printShaderInfoLog(GLuint shader) {
    GLint length;
    glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &length);
    if(length) {
        char* buffer = (char *)malloc( sizeof(char) * length ) ;
        glGetShaderInfoLog(shader, length, NULL, buffer);
        printf("%s", buffer);
        free( buffer ) ;
        GLint success;
        glGetShaderiv(shader, GL_COMPILE_STATUS, &success);
        if(success != GL_TRUE) {
            exit(1);
        }
    }
}

GLuint createShader(GLenum type, const char* pSource) {
    GLuint shader = glCreateShader(type);
    glShaderSource(shader, 1, &pSource, NULL);
    glCompileShader(shader);
    printShaderInfoLog(shader);
    return shader;
}

int phaseLocation ;

const float vertexArray[] = {
    0, -1, 0, 1,
    1, 1, 0, 1,
    -1, 1, 0, 1
};

#define false 0
#define true 1
#define bool int

void render() {
        static float offset = 0;

        glViewport(0, 0, 320, 480 ) ;
        glClearColor(0, 1, 0, 1);
        glClear(GL_COLOR_BUFFER_BIT);
        
        glUniform1f(phaseLocation, offset);
        
        glVertexAttribPointer(0, 4, GL_FLOAT, false, 0, vertexArray);
        glEnableVertexAttribArray(0);
        glDrawArrays(GL_TRIANGLE_STRIP, 0, 3);
        
        SDL_GL_SwapBuffers( ) ;
        
        offset = fmodf(offset + 0.2, 2*3.141f);
}

int main() {

    if( SDL_Init( SDL_INIT_VIDEO ) < 0 ) {
        fprintf( stderr, "Couldn't initialize SDL: %s\n", SDL_GetError() ) ;
        exit( 1 ) ;
    }

    if ( SDL_SetVideoMode( 320, 480, 32, SDL_OPENGLES ) == NULL ) {
        fprintf(stderr, "Couldn't set GL mode: %s\n", SDL_GetError());
        SDL_Quit();
        exit(1);
    }

    printf("Screen BPP: %d\n", SDL_GetVideoSurface()->format->BitsPerPixel);
    printf("\n");
    printf( "Vendor     : %s\n", glGetString( GL_VENDOR ) );
    printf( "Renderer   : %s\n", glGetString( GL_RENDERER ) );
    printf( "Version    : %s\n", glGetString( GL_VERSION ) );
    printf( "Extensions : %s\n", glGetString( GL_EXTENSIONS ) );
    printf("\n");

    GLuint shaderProgram = glCreateProgram();
    GLuint vertexShader = createShader(GL_VERTEX_SHADER, vertexSrc);
    GLuint fragmentShader = createShader(GL_FRAGMENT_SHADER, fragmentSrc);
    
    glAttachShader(shaderProgram, vertexShader);
    glAttachShader(shaderProgram, fragmentShader);
    
    glLinkProgram(shaderProgram);
    
    glUseProgram(shaderProgram);
    
    phaseLocation = glGetUniformLocation(shaderProgram, "phase");
    if(phaseLocation < 0) {
        printf("Unable to get uniform location\n");
        return 1;
    }
    
    bool quit = false;
    
    //timeval startTime;
    //timezone tz;
    //gettimeofday(&startTime, &tz);
    int numFrames = 0;
    
    while(!quit) {
        SDL_Event event ;
        while( SDL_PollEvent( &event ) ) {
            if( event.type == SDL_KEYDOWN && event.key.keysym.sym == SDLK_ESCAPE ) quit = 1 ;
        }
                render();
                numFrames++;
                if(numFrames % 100 == 0) {
                    //timeval now;
                    //gettimeofday(&now, &tz);
                    //float delta = now.tv_sec - startTime.tv_sec + (now.tv_usec - startTime.tv_usec) * 0.000001f;
                    //printf("fps: %f\n", numFrames / delta);
                }
    }
    return 0;
}