用 SDL2 处理精灵图

上面就是一个精灵图，由多个固定间隔的图标组成。利用精灵图的好处就是不必将图标逐个读入内存进行操作。我们可以将精灵图中需要的部分用一个个矩形截取下来，然后再输出到渲染器上。

环境：SDL2 + VC++2015

下面的代码将打开sprite.png，并对相应的操作做出响应。

sprite.png

 #include <stdexcept>
 #include <string>
 #include <iostream>
 #include "SDL.h"
 #include "SDL_image.h"

 //屏幕宽度
 const int SCREEN_WIDTH = ;
 const int SCREEN_HEIGHT = ;

 //全局窗口和渲染器
 SDL_Window *window = nullptr;
 SDL_Renderer *renderer = nullptr;

 //记录SDL错误
 void logSDLError(std::ostream &os, const std::string &msg)
 {
     os << msg << " error: " << SDL_GetError() << std::endl;
 }

 //加载图像材质
 SDL_Texture* loadTexture(const std::string &file, SDL_Renderer *ren)
 {
     SDL_Texture *texture = IMG_LoadTexture(ren, file.c_str());
     if (texture == nullptr) logSDLError(std::cout, "LoadTexture");
     return texture;
 }

 //根据坐标生成截取区域并复制输出到渲染器
 void renderTexture(SDL_Texture *tex, SDL_Renderer *ren, int x, int y, SDL_Rect *clip = nullptr)
 {
     //目标截面区域初始化，提供目标在渲染器的坐标
     SDL_Rect dst; dst.x = x; dst.y = y;
     //源截面区域不为空，将其值赋给目标截面
     if (clip != nullptr) { dst.w = clip->w; dst.h = clip->h; }
     //将截面输出到渲染器
     SDL_RenderCopy(ren, tex, clip, &dst);
 }

 int main(int argc, char** argv)
 {
     //初始化SDL
     if (SDL_Init(SDL_INIT_EVERYTHING) == -)
     {
         std::cout << SDL_GetError() << std::endl;
         return ;
     }

     //创建窗口
     window = SDL_CreateWindow("Lesson 5",
         SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
         SCREEN_WIDTH, SCREEN_HEIGHT, SDL_WINDOW_SHOWN);
     if (window == nullptr)
     {
         std::cout << SDL_GetError() << std::endl;
         return ;
     }

     //创建渲染器
     renderer = SDL_CreateRenderer(window, -, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);
     if (renderer == nullptr)
     {
         std::cout << SDL_GetError() << std::endl;
         return ;
     }

     //创建纹理
     SDL_Texture *image = nullptr;
     try
     {
         image = loadTexture("sprite.png", renderer);
     }
     catch (const std::runtime_error &e)
     {
         std::cout << e.what() << std::endl;
         return ;
     }

     //iW，iH 表示截面宽高
     int iW = , iH = ;
     //x，y 分别为截面在渲染器输出的坐标
     int x = SCREEN_WIDTH /  - iW / ;
     int y = SCREEN_HEIGHT /  - iH / ;

     //为精灵图设置截面
     SDL_Rect clips[];

     //初始化截面信息
     for (int i = ; i < ; ++i)
     {
         clips[i].x = i /  * iW;
         clips[i].y = i %  * iH;
         clips[i].w = iW;
         clips[i].h = iH;
     }

     //表示将要输出的截面
     int useClip = ;

     SDL_Event e;
     bool quit = false;
     //主循环
     while (!quit)
     {
         //事件轮询
         while (SDL_PollEvent(&e))
         {
             //按右上角的X退出
             if (e.type == SDL_QUIT) quit = true;

             //点击鼠标随机输出截面
             if (e.type == SDL_MOUSEBUTTONDOWN) useClip = rand() % ;

             //使用数字键决定输出截面，分别有1，2，3，4
             if (e.type == SDL_KEYDOWN)
             {
                 switch (e.key.keysym.sym)
                 {
                 case SDLK_1:
                 case SDLK_KP_1:
                     useClip = ;
                     break;
                 case SDLK_2:
                 case SDLK_KP_2:
                     useClip = ;
                     break;
                 case SDLK_3:
                 case SDLK_KP_3:
                     useClip = ;
                     break;
                 case SDLK_4:
                 case SDLK_KP_4:
                     useClip = ;
                     break;
                 case SDLK_ESCAPE:
                     quit = true;
                     break;
                 default:
                     break;
                 }
             }
         }
         //清空渲染器
         SDL_RenderClear(renderer);
         //绘制材质
         renderTexture(image, renderer, x, y, &clips[useClip]);
         //呈现渲染器
         SDL_RenderPresent(renderer);
     }

     //释放资源
     SDL_DestroyTexture(image);
     SDL_DestroyRenderer(renderer);
     SDL_DestroyWindow(window);

     SDL_Quit();

     return ;
 }