gl_stride.c

/*******************************************************************
 *                                                                 *
 *                        Using SDL With OpenGL                    *
 *                                                                 *
 *                    Tutorial by Kyle Foley (sdw)                 *
 *                                                                 *
 * http://gpwiki.org/index.php/SDL:Tutorials:Using_SDL_with_OpenGL *
 *                                                                 *
 *******************************************************************/

/*
THIS WORK, INCLUDING THE SOURCE CODE, DOCUMENTATION
AND RELATED MEDIA AND DATA, IS PLACED INTO THE PUBLIC DOMAIN.

THE ORIGINAL AUTHOR IS KYLE FOLEY.

THIS SOFTWARE IS PROVIDED AS-IS WITHOUT WARRANTY
OF ANY KIND, NOT EVEN THE IMPLIED WARRANTY OF
MERCHANTABILITY. THE AUTHOR OF THIS SOFTWARE,
ASSUMES _NO_ RESPONSIBILITY FOR ANY CONSEQUENCE
RESULTING FROM THE USE, MODIFICATION, OR
REDISTRIBUTION OF THIS SOFTWARE.
*/

#ifndef __EMSCRIPTEN__
#define USE_GLEW 0
#endif

#if USE_GLEW
#include "GL/glew.h"
#endif

#include "SDL/SDL.h"
#include "SDL/SDL_image.h"
#if !USE_GLEW
#include "SDL/SDL_opengl.h"
#endif

#include <stdio.h>
#include <string.h>
#include <assert.h>

int main(int argc, char *argv[]) {
    SDL_Surface *screen;

    // Slightly different SDL initialization
    if ( SDL_Init(SDL_INIT_VIDEO) != 0 ) {
        printf("Unable to initialize SDL: %s\n", SDL_GetError());
        return 1;
    }

    SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 ); // *new*

    screen = SDL_SetVideoMode( 640, 480, 16, SDL_OPENGL ); // *changed*
    if ( !screen ) {
        printf("Unable to set video mode: %s\n", SDL_GetError());
        return 1;
    }

    // Set the OpenGL state after creating the context with SDL_SetVideoMode

    glClearColor( 0, 0, 0, 0 );

#ifndef __EMSCRIPTEN__
    glEnable( GL_TEXTURE_2D ); // Need this to display a texture XXX unnecessary in OpenGL ES 2.0/WebGL
#endif

    glViewport( 0, 0, 640, 480 );

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();

    // Clear the screen before drawing
    glClear( GL_COLOR_BUFFER_BIT );

    typedef struct Vertex {
        GLfloat x;
        GLfloat y;
    } Vertex;

    typedef struct Color {
        GLubyte r;
        GLubyte g;
        GLubyte b;
        GLubyte a;
    } Color;

    Vertex vertices[3] = {
        {-1.0, 0.0},
        { 0.0, 1.0},
        { 1.0, 0.0}
    };

    Color colors[3] = {
        {0xFF, 0x00, 0x00, 0xFF},
        {0x00, 0xFF, 0x00, 0xFF},
        {0x00, 0x00, 0xFF, 0xFF}
    };

    Vertex vertices2[3] = {
        {-1.0,  0.0},
        { 1.0,  0.0},
        { 0.0, -1.0}
    };

    Color colors2[3] = {
        {0xFF, 0x00, 0x00, 0xFF},
        {0x00, 0x00, 0xFF, 0xFF},
        {0x00, 0xFF, 0x00, 0xFF}
    };

    // DRAW

    // Clear the screen before drawing
    glClear( GL_COLOR_BUFFER_BIT );

    // This test ensures that we can use two separate arrays in memory for different
    // attributes, and that they each can have different stride.
    // The first test shows implicit striding (the zero indicates tightly packed)
    // The second test shows explicit striding where the stride is passed in
    //   even though it also is tightly packed

    glEnableClientState(GL_VERTEX_ARRAY);
    glEnableClientState(GL_COLOR_ARRAY);

    // TEST 1

    glVertexPointer(2, GL_FLOAT, 0, vertices);
    glColorPointer(4, GL_UNSIGNED_BYTE, 0, colors);
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 3);

    // TEST 2

    glVertexPointer(2, GL_FLOAT, 8, vertices2);
    glColorPointer(4, GL_UNSIGNED_BYTE, 4, colors2);
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 3);

    glDisableClientState(GL_COLOR_ARRAY);
    glDisableClientState(GL_VERTEX_ARRAY);

    SDL_GL_SwapBuffers();

#ifndef __EMSCRIPTEN__
    // Wait for 3 seconds to give us a chance to see the image
    SDL_Delay(3000);
#endif

    SDL_Quit();

    return 0;
}