SDL Guide 中文译版(二)[2]

例2-4 getpixel()

/*
 * return the pixel value at (x, y)
 * note: the surface must be locked before calling this!
 */
uint32 getpixel(sdl_surface *surface, int x, int y)
{
    int bpp = surface->format->bytesperpixel;
    /* here p is the address to the pixel we want to retrieve */
    uint8 *p = (uint8 *)surface->pixels + y * surface->pitch + x * bpp;

    switch(bpp) {
    case 1:
        return *p;

    case 2:
        return *(uint16 *)p;

    case 3:
        if(sdl_byteorder == sdl_big_endian)
            return p[0] << 16 | p[1] << 8 | p[2];
        else
            return p[0] | p[1] << 8 | p[2] << 16;

    case 4:
        return *(uint32 *)p;

    default:
        return 0;       /* shouldn't happen, but avoids warnings */
    }
}

例2-5 putpixel()

/*
 * set the pixel at (x, y) to the given value
 * note: the surface must be locked before calling this!
 */
void putpixel(sdl_surface *surface, int x, int y, uint32 pixel)
{
    int bpp = surface->format->bytesperpixel;
    /* here p is the address to the pixel we want to set */
    uint8 *p = (uint8 *)surface->pixels + y * surface->pitch + x * bpp;

    switch(bpp) {
    case 1:
        *p = pixel;
        break;

    case 2:
        *(uint16 *)p = pixel;
        break;

    case 3:
        if(sdl_byteorder == sdl_big_endian) {
            p[0] = (pixel >> 16) & 0xff;
            p[1] = (pixel >> 8) & 0xff;
            p[2] = pixel & 0xff;
        } else {
            p[0] = pixel & 0xff;
            p[1] = (pixel >> 8) & 0xff;
            p[2] = (pixel >> 16) & 0xff;
        }
        break;

    case 4:
        *(uint32 *)p = pixel;
        break;
    }
}

例2-6 使用上面的putpixel()在屏幕中心画一个黄点

    /* code to set a yellow pixel at the center of the screen */

    int x, y;
    uint32 yellow;

    /* map the color yellow to this display (r=0xff, g=0xff, b=0x00)
       note:  if the display is palettized, you must set the palette first.
    */
    yellow = sdl_maprgb(screen->format, 0xff, 0xff, 0x00);

    x = screen->w / 2;
    y = screen->h / 2;

    /* lock the screen for direct access to the pixels */
    if ( sdl_mustlock(screen) ) {
        if ( sdl_locksurface(screen) < 0 ) {
            fprintf(stderr, "can't lock screen: %s\n", sdl_geterror());
            return;
        }
    }

    putpixel(screen, x, y, yellow);

    if ( sdl_mustlock(screen) ) {
        sdl_unlocksurface(screen);
    }
    /* update just the part of the display that we've changed */
    sdl_updaterect(screen, x, y, 1, 1);

    return;

并用sdl和opengl

sdl可以在多种平台（linux/x11, win32, beos, macos classic/toolbox, macos x, freebsd/x11 and solaris/x11）上创建和使用opengl上下文。这允许你在opengl程序中使用sdl的音频、事件、线程和记时器，而这些通常是glut的任务。

和普通的初始化类似，但有三点不同：必须传sdl_opengl参数给sdl_setvideomode；必须使用sdl_gl_setattribute指定一些gl属性（深度缓冲区位宽，帧缓冲位宽等）；如果您想使用双缓冲，必须作为gl属性指定