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 = 500;
const int SCREEN_HEIGHT = 500;

//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) == -1)
    {
        std::cout << SDL_GetError() << std::endl;
        return 1;
    }

    //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 2;
    }

    //创建渲染器
    renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);
    if (renderer == nullptr)
    {
        std::cout << SDL_GetError() << std::endl;
        return 3;
    }

    //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 4;
    }

    //iW，iH 表示截面宽高
    int iW = 100, iH = 100;
    //x，y 分别为截面在渲染器输出的坐标
    int x = SCREEN_WIDTH / 2 - iW / 2;
    int y = SCREEN_HEIGHT / 2 - iH / 2;

    //为精灵图设置截面
    SDL_Rect clips[4];

    //初始化截面信息
    for (int i = 0; i < 4; ++i)
    {
        clips[i].x = i / 2 * iW;
        clips[i].y = i % 2 * iH;
        clips[i].w = iW;
        clips[i].h = iH;
    }

    //表示将要输出的截面
    int useClip = 0;

    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() % 4;

            //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 = 0;
                    break;
                case SDLK_2:
                case SDLK_KP_2:
                    useClip = 1;
                    break;
                case SDLK_3:
                case SDLK_KP_3:
                    useClip = 2;
                    break;
                case SDLK_4:
                case SDLK_KP_4:
                    useClip = 3;
                    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 0;
}