SDL Guide 中文译版(二)

第二章 图像和视频

初始化SDL Video显示

视频是最常用的部分，也是SDL最完整的子系统。下面的初始化过程是每个SDL程序都要做的，即使可能有些不同。

例2-1 初始化视频

    SDL_Surface *screen;

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

    /* Clean up on exit */
    atexit(SDL_Quit);
    
    /*
     * Initialize the display in a 640x480 8-bit palettized mode,
     * requesting a software surface
     */
    screen = SDL_SetVideoMode(640, 480, 8, SDL_SWSURFACE);
    if ( screen == NULL ) {
        fprintf(stderr, "Couldn''''t set 640x480x8 video mode: %s\n",
                        SDL_GetError());
        exit(1);
    }

初始化最佳视频模式

如果你希望某种色深（颜色数）但如果用户的显示器不支持也可以接受其他色深，使用加SDL_ANYFORMAT参数的SDL_SetVideoMode。您还可以用SDL_VideoModeOK来找到与请求模式最接近的模式。

例2-2 初始化最佳视频模式

    /* Have a preference for 8-bit, but accept any depth */
    screen = SDL_SetVideoMode(640, 480, 8, SDL_SWSURFACE|SDL_ANYFORMAT);
    if ( screen == NULL ) {
        fprintf(stderr, "Couldn''''t set 640x480x8 video mode: %s\n",
                        SDL_GetError());
        exit(1);
    }
    printf("Set 640x480 at %d bits-per-pixel mode\n",
           screen->format->BitsPerPixel);

读取并显示BMP文件

当SDL已经初始化，视频模式已经选择，下面的函数将读取并显示指定的BMP文件。

例2-3 读取并显示BMP文件

void display_bmp(char *file_name)
{
    SDL_Surface *image;

    /* Load the BMP file into a surface */
    image = SDL_LoadBMP(file_name);
    if (image == NULL) {
        fprintf(stderr, "Couldn''''t load %s: %s\n", file_name, SDL_GetError());
        return;
    }

    /*
     * Palettized screen modes will have a default palette (a standard
     * 8*8*4 colour cube), but if the image is palettized as well we can
     * use that palette for a nicer colour matching
     */
    if (image->format->palette && screen->format->palette) {
    SDL_SetColors(screen, image->format->palette->colors, 0,
                  image->format->palette->ncolors);
    }

    /* Blit onto the screen surface */
    if(SDL_BlitSurface(image, NULL, screen, NULL) < 0)
        fprintf(stderr, "BlitSurface error: %s\n", SDL_GetError());

    SDL_UpdateRect(screen, 0, 0, image->w, image->h);

    /* Free the allocated BMP surface */
    SDL_FreeSurface(image);
}

直接在显示上绘图

下面两个函数实现在图像平面的像素读写。它们被细心设计成可以用于所有色深。记住在使用前要先锁定图像平面，之后要解锁。

在像素值和其红、绿、蓝值间转换，使用SDL_GetRGB()和SDL_MapRGB()。

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;

			   
			   

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