SDL2 C++ - 多图像渲染错误

Question

在开始之前，我使用的 IDE 是 Code::Blocks。

我打算在与我的学习项目不同的另一个项目中练习我在网上学到的关于 SDL 的知识。我想在背景上加载一个球的图像。它在我的学习项目中运行良好。我将代码复制到我的测试项目中，球消失了。只有背景是可见的。

我删除了涉及背景的代码，它工作正常。

我的代码：

#include <iostream>
#include <cstdlib>
#include <SDL.h>
#include <SDL_image.h>
#undef main

using namespace std;

SDL_Texture *LoadTexture (string filepath, SDL_Renderer *renderTarget){
    SDL_Texture *texture = 0;
    SDL_Surface *surface = IMG_Load(filepath.c_str());

    texture = SDL_CreateTextureFromSurface(renderTarget, surface);

    SDL_FreeSurface(surface);

    return texture;
}

int main () {

    SDL_Window *window = 0;
    SDL_Texture *ball = 0;
    SDL_Texture *background_sky = 0;

    SDL_Renderer *renderTarget = 0;

    int frameW, frameH;
    int textureW, textureH;

    //--Crop Instructions--//
    frameW = textureW / 1; //row division
    frameH = textureH / 1; //column division

    SDL_Rect ballcrop;
    ballcrop.x = ballcrop.y = 0;
    ballcrop.w = frameW; //length of selected area
    ballcrop.h = frameH; //height of selected area

    SDL_Rect ballpos;
    ballpos.x = ballpos.y = 10; //ball position
    ballpos.w = ballpos.h = 64; //size of crop
    //---------------------//

    SDL_Init(SDL_INIT_EVERYTHING);
    IMG_Init(IMG_INIT_PNG);

    window = SDL_CreateWindow("Ball", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 500, 500, SDL_WINDOW_SHOWN);
    renderTarget = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);

    ball = LoadTexture("ball.png", renderTarget);
    background_sky = LoadTexture("bg_sky.png", renderTarget);


    SDL_QueryTexture(ball, NULL, NULL, &textureW, &textureH); //crop action

    bool Active = true;
    SDL_Event ev;

    while (Active) {
        while (SDL_PollEvent(&ev)!=0) {
            if (ev.type == SDL_QUIT)
                Active = false;

            else if (ev.type == SDL_KEYDOWN) {
                switch (ev.key.keysym.sym) {
                case SDLK_UP:
                    ballpos.y -= 5;
                    break;
                case SDLK_DOWN:
                    ballpos.y += 5;
                    break;
                }
            }
        }
        SDL_RenderClear(renderTarget);

        SDL_RenderCopy(renderTarget, background_sky, NULL, NULL);
        SDL_RenderCopy(renderTarget, ball, &ballcrop, &ballpos);

        SDL_RenderPresent(renderTarget);
    }

    SDL_DestroyWindow(window);
    SDL_DestroyTexture(background_sky);
    SDL_DestroyTexture(ball);

    SDL_DestroyRenderer(renderTarget);

    window = 0;
    ball = background_sky = 0;
    renderTarget = 0;

    IMG_Quit();
    SDL_Quit();

    return 0;
}

我不想展示我的研究项目的源代码，因为它非常很乱。

任何帮助将不胜感激:)

Answer 1

您对frameW 和frameH 的计算有误；计算代码

//--Crop Instructions--//
frameW = textureW / 1; //row division
frameH = textureH / 1; //column division

在初始化之前使用textureW 和textureH。移动整个“裁剪指令”块，直到您调用 SDL_QueryTexture。