在屏幕上绘制像素

通过直接写入图形帧缓冲（framebuffer）和调用屏幕更新函数来绘制屏幕。

提示： 如果你需要进行大量绘制，最好在绘制前锁住屏幕（如果有必要），绘制时保持一个需要更新的区域列表，并且在更新显示前对屏幕解锁。

例程：

在任意格式的屏幕上绘制像素

void DrawPixel(SDL_Surface *screen, Uint8 R, Uint8 G, Uint8 B)
{
    Uint32 color = SDL_MapRGB(screen->format, R, G, B);

    if ( SDL_MUSTLOCK(screen) ) {
        if ( SDL_LockSurface(screen) < 0 ) {
            return;
        }
    }
    switch (screen->format->BytesPerPixel) {
        case 1: { /* 假定是8-bpp */
            Uint8 *bufp;

            bufp = (Uint8 *)screen->pixels + y*screen->pitch + x;
            *bufp = color;
        }
        break;

        case 2: { /* 可能是15-bpp 或者 16-bpp */
            Uint16 *bufp;

            bufp = (Uint16 *)screen->pixels + y*screen->pitch/2 + x;
            *bufp = color;
        }
        break;

        case 3: { /* 慢速的24-bpp模式，通常不用 */
            Uint8 *bufp;

            bufp = (Uint8 *)screen->pixels + y*screen->pitch + x;
            *(bufp+screen->format->Rshift/8) = R;
            *(bufp+screen->format->Gshift/8) = G;
            *(bufp+screen->format->Bshift/8) = B;
        }
        break;

        case 4: { /* 可能是32-bpp */
            Uint32 *bufp;

            bufp = (Uint32 *)screen->pixels + y*screen->pitch/4 + x;
            *bufp = color;
        }
        break;
    }
    if ( SDL_MUSTLOCK(screen) ) {
        SDL_UnlockSurface(screen);
    }
    SDL_UpdateRect(screen, x, y, 1, 1);
}