第四课：多个Texture的绘制

 上一课我们讲解了关于如何用SDL显示一个BMP的图片，其实就是涉及SDL_Window,SDL_Surface,SDL_Renderer,SDL_Texture，如果在SDL上面显示几个图片呢，也很简单，也就是用多个SDL_Texture绘制在一个SDL_Renderer上，在指定位置以及大小就可以了，下面看一下代码的实现：

讲解：

const int SCREEN_WIDTH  = 640;//Window窗口宽度const int SCREEN_HEIGHT = 480;//Window窗口的高度//SDL_window窗口的销毁void Clesson03::cleanup(SDL_Window* window){    SDL_DestroyWindow(window);    //SDL_QUIT();}//销毁Renderer,window,Texure，原作者写的这个函数比这个要好，这里仅仅是为了实现功能。void Clesson03::cleanup(SDL_Texture* background, SDL_Texture* image, SDL_Renderer* renderer, SDL_Window* window){    if(background)    {        SDL_DestroyTexture(background);    }    if(image)    {        SDL_DestroyTexture(image);    }    if(renderer)    {        SDL_DestroyRenderer(renderer);    }    if(window)    {        SDL_DestroyWindow(window);    }}/*    这个就是标准的C++输出功能，SDL_GetError()获取错误码  */void Clesson03::logSDLError(std::ostream &os, const std::string &msg){os << msg << " error: " << SDL_GetError() << std::endl;}/* * LOAD一个BMP文件，并且返回一个SDL_Texture * @param file BMP文件 * @param ren 需要渲染的renderer * @return 一个SDL_Texture指针或者NULL. */SDL_Texture* Clesson03::loadTexture(const std::string &file, SDL_Renderer *ren){SDL_Texture *texture = nullptr;//Load the imageSDL_Surface *loadedImage = SDL_LoadBMP(file.c_str());//If the loading went ok, convert to texture and return the textureif (loadedImage != nullptr){texture = SDL_CreateTextureFromSurface(ren, loadedImage);SDL_FreeSurface(loadedImage);//Make sure converting went ok tooif (texture == nullptr){logSDLError(std::cout, "CreateTextureFromSurface");}}else {logSDLError(std::cout, "LoadBMP");}return texture;}/* * 绘制Texure在Renderer指定的位置。 * the texture's width and height * @param tex 需要在Renderer绘制的Texture; * @param ren Texture绘制的Renderer * @param x X位置 * @param y Y位置 感觉写的意思很别扭，文笔不是很好（只可意会，不可言传） */void Clesson03::renderTexture(SDL_Texture *tex, SDL_Renderer *ren, int x, int y){//Setup the destination rectangle to be at the position we wantSDL_Rect dst;dst.x = x;dst.y = y;//Query the texture to get its width and height to useSDL_QueryTexture(tex, NULL, NULL, &dst.w, &dst.h);SDL_RenderCopy(ren, tex, NULL, &dst);}int Clesson03::Run(int argc, char** argv){//SDL的初始化if (SDL_Init(SDL_INIT_EVERYTHING) != 0){logSDLError(std::cout, "SDL_Init");return 1;}//创建SDL_WindowSDL_Window *window = SDL_CreateWindow("OAD工作室－多个BMP显示", 100, 100, SCREEN_WIDTH, SCREEN_HEIGHT, SDL_WINDOW_SHOWN);if (window == nullptr){logSDLError(std::cout, "CreateWindow");SDL_Quit();return 1;}    //创建SDL_RendererSDL_Renderer *renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);if (renderer == nullptr){logSDLError(std::cout, "CreateRenderer");cleanup(window);SDL_Quit();return 1;}//创建两个Texture,一个背景，一个前景const std::string resPath(argv[1]);SDL_Texture *background = loadTexture(resPath + "background.bmp", renderer);SDL_Texture *image = loadTexture(resPath + "image.bmp", renderer);//Make sure they both loaded okif (background == nullptr || image == nullptr){cleanup(background, image, renderer, window);SDL_Quit();return 1;}//Clear SDL_Window，准备开始绘制SDL_RenderClear(renderer);//这个函数就是获取Texture的信息，如果不能理解（你可以认为是BMP的一些信息，其实这里已经没有sureface什么事了）    int bW, bH;SDL_QueryTexture(background, NULL, NULL, &bW, &bH);//在renderer上绘制4个图，这个算法肯定是最笨，就简单的，知道怎么回事就可以了，必定就是一个DemorenderTexture(background, renderer, 0, 0);renderTexture(background, renderer, bW, 0);renderTexture(background, renderer, 0, bH);renderTexture(background, renderer, bW, bH);    //这里绘制前景在中间显示，代码很简单。int iW, iH;SDL_QueryTexture(image, NULL, NULL, &iW, &iH);int x = SCREEN_WIDTH / 2 - iW / 2;int y = SCREEN_HEIGHT / 2 - iH / 2;renderTexture(image, renderer, x, y);//更新一下rendererSDL_RenderPresent(renderer);        bool bRun = true;//程序是否允许    SDL_Event AppEven;//SDL事件    while (bRun) {        if(SDL_WaitEvent(&AppEven))        {            switch (AppEven.type) {                case SDL_KEYDOWN:                    bRun = false;                    break;                case SDL_QUIT:                    bRun = false;                    break;                default:                    break;            }        }    }        cleanup(background, image, renderer, window);SDL_Quit();return 0;}

上面的实现都很简单，关于这些理论知识我们都基本上没有写，因为如果写也是到网站到处去搜索，如果注解有什么不对的地方，请指出。文章中的代码可以到第二课给出的GIT地址下载第二课：SDL Window的创建与显示，我们修改的代码教程完以后会上传到CSDN上。