OpenGL Triangle of Doom

The demo app

bpaladino ported this demo to SDL from a demo that is part of the maemo project. (ported from original X11 OpenGL code to SDL)

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;
}