SDL2_image渲染无法正常工作

Question

I am trying to render an image using SDL_image v2.0.0, in SDL2.

I have an image called Red.png in my res/img/ folder. When I try to load the texture, and use SDL_QueryTexture() it gets the size and everything just fine. But when it comes to rendering the actual image, Ive put a rectangle outline to know where the image is, but there is no image in the box.

The class I use to load and render the texture:

class LTexture
{
public:

    ~LTexture()
    {
        SDL_DestroyTexture(image_);
        renderer_ = nullptr;
        image_ = nullptr;
    }

    void init(SDL_Renderer* renderer)
    {
        printf("init texture\n");
        renderer_ = renderer;
    }

    void loadBMP(std::string filename)
    {
        printf("load texture\n");
        image_ = IMG_LoadTexture(renderer_, ("res/img/"+filename).c_str());
        SDL_QueryTexture(image_, NULL, NULL, &imgrect.w, &imgrect.h);
    }

    void render(int x, int y)
    {
        imgrect.x = x;
        imgrect.y = y;
        SDL_SetRenderDrawColor(renderer_, 128, 128, 128, 255);
        if (image_ != nullptr && renderer_ != nullptr)
        {
            printf("%i,  %i\n", imgrect.x, imgrect.y);
            SDL_RenderDrawRect(renderer_, &imgrect);
            SDL_RenderCopy(renderer_, image_, &imgrect, &imgrect);
        }
    }

    bool isLoaded()
    {
        return image_ != nullptr;
    }

private:
    SDL_Renderer* renderer_ = nullptr;
    SDL_Texture* image_ = nullptr;
    SDL_Rect imgrect;

};

I know it correctly gets the renderer and loads the image because the DrawRect function works, and if you didn't guess by the name, Red.png is a red rectangle.

Answer 1

Pass nullptr in for srcrect in your SDL_RenderCopy() call:

SDL_RenderCopy(renderer_, image_, nullptr, &imgrect);

Right now if x and/or y are larger than image_ SDL will clip srcrect to the extents of image_ , end up with an empty rect, and do nothing.

Example:

#include <SDL2/SDL.h>
#include <SDL2/SDL_image.h>

#include <cstdlib>
#include <string>

class LTexture
{
public:

    LTexture()
        : renderer_( nullptr )
        , image_( nullptr )
    { }

    ~LTexture()
    {
        SDL_DestroyTexture(image_);
        renderer_ = nullptr;
        image_ = nullptr;
    }

    void init(SDL_Renderer* renderer)
    {
        printf("init texture\n");
        renderer_ = renderer;
    }

    void loadBMP(std::string filename)
    {
        printf("load texture\n");
        image_ = IMG_LoadTexture(renderer_, filename.c_str());
        SDL_QueryTexture(image_, NULL, NULL, &imgrect.w, &imgrect.h);
    }

    void render(int x, int y)
    {
        imgrect.x = x;
        imgrect.y = y;
        SDL_SetRenderDrawColor(renderer_, 128, 128, 128, 255);
        if (image_ != nullptr && renderer_ != nullptr)
        {
            printf("%i,  %i\n", imgrect.x, imgrect.y);
            SDL_RenderDrawRect(renderer_, &imgrect);
            SDL_RenderCopy(renderer_, image_, nullptr, &imgrect);
        }
    }

    bool isLoaded()
    {
        return image_ != nullptr;
    }

private:
    SDL_Renderer* renderer_;
    SDL_Texture* image_;
    SDL_Rect imgrect;
};


int main( int argc, char** argv )
{
    SDL_Init( SDL_INIT_EVERYTHING );
    IMG_Init( IMG_INIT_PNG );

    SDL_Window * window = SDL_CreateWindow
        (
        "SDL2", 
        SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 
        300, 300, 
        SDL_WINDOW_SHOWN
        );

    SDL_Renderer* renderer = SDL_CreateRenderer
        (
        window,
        0,
        SDL_RENDERER_ACCELERATED
        );

    LTexture tex;
    tex.init( renderer );
    tex.loadBMP( "red.png" );

    bool running = true;
    while( running )
    {
        SDL_Event ev;
        while( SDL_PollEvent( &ev ) )
        {
            if ( ev.type == SDL_QUIT )
                running = false;
        }

        SDL_SetRenderDrawColor( renderer, 0, 0, 0, 255 );
        SDL_RenderFillRect( renderer, NULL );

        tex.render( 50, 50 );

        SDL_RenderPresent( renderer );
        SDL_Delay( 33 );
    }

    SDL_DestroyRenderer( renderer );
    SDL_DestroyWindow( window );

    IMG_Quit();
    SDL_Quit();

    return 0;
}

red.png for reference:

Answer 2

我不知道这是否仍然可行，但是当我使用SW加速时，使用SDL_RENDERER_ACCELERATED标志（在Linuix Mint下）会遇到问题，SDL_Render *可以工作。