How to get a part of texture in SDL2? - c++

Question

I try to do it by using SDL_RenderCopy()
But I just get a black box.
This is my code.
static SDL_Texture* GetAreaTextrue(SDL_Rect rect, SDL_Renderer* renderer, SDL_Texture* source)
{
SDL_Texture* result = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_TARGET, rect.w, rect.h);
SDL_SetRenderTarget(renderer, result);
SDL_RenderCopy(renderer, source, &rect, NULL);
SDL_RenderPresent(renderer);
return result;
}
What is the proper operation?

Answer 1

EDIT:
What you want to do here is render a part of the texture to the screen.
There is a way to do this by using SDL_RenderCopy, but by doing this you just "take" the part you want from your texture and "slap" it onto your screen.
What you want (by my understanding) is to take a part of a texture and save it into another texture variable that afterwards can be rendered to the screen.
The first solution to the problem goes like this:
// load your image in a SDL_Texture variable (myTexture for example)
// if the image is (256,128) and you want to render only the first half
// you need a rectangle of the same size as that part of the image
SDL_Rect imgPartRect;
imgPartRect.x = 0;
imgPartRect.y = 0;
imgPartRect.w = 32;
imgPartRect.h = 32;
// and the program loop should have a draw block looking like this:
SDL_SetRenderDrawColor( renderer, 0x00, 0x00, 0x00, 0xFF );
SDL_RenderClear( renderer );
SDL_RenderCopy( renderer, myTexture, &imgPartRect, NULL );
SDL_RenderPresent( renderer );
The approach you are trying to use has an intermediate texture that you render on and afterwards you render that texture to the screen. The problem here is that you set the renderer to draw on the texture you just created but you never reset your renderer to use the default target (the screen).
As you can see in the SDL documentation here the second parameter receives the texture that you want your renerer to draw on, or NULL if you want to reset it to the default target (the screen).
int SDL_SetRenderTarget(SDL_Renderer* renderer, SDL_Texture* texture)
Where:
renderer
the rendering context
texture
the targeted texture, which must be created with the SDL_TEXTUREACCESS_TARGET flag, or NULL for the default render target
Lets use the same example as before:
// first off let's build the function that you speak of
SDL_Texture* GetAreaTextrue(SDL_Rect rect, SDL_Renderer* renderer, SDL_Texture* source)
{
SDL_Texture* result = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_TARGET, rect.w, rect.h);
SDL_SetRenderTarget(renderer, result);
SDL_RenderCopy(renderer, source, &rect, NULL);
// the folowing line should reset the target to default(the screen)
SDL_SetRenderTarget(renderer, NULL);
// I also removed the RenderPresent funcion as it is not needed here
return result;
}
// load your image in a SDL_Texture variable (myTexture for example)
// if the image is (256,128) and you want to render only the first half
// you need a rectangle of the same size as that part of the image
SDL_Rect imgPartRect;
imgPartRect.x = 0;
imgPartRect.y = 0;
imgPartRect.w = 32;
imgPartRect.h = 32;
// now we use the function from above to build another texture
SDL_Texture* myTexturePart = GetAreaTextrue( imgPartRect, renderer, myTexture );
// and the program loop should have a draw block looking like this:
SDL_SetRenderDrawColor( renderer, 0x00, 0x00, 0x00, 0xFF );
SDL_RenderClear( renderer );
// here we render the whole newly created texture as it contains only a part of myTexture
SDL_RenderCopy( renderer, myTexturePart, NULL, NULL );
SDL_RenderPresent( renderer );
I don't know what you want to do, but I highly recommend the first way though.