Get pixel info from SDL2 texture

Question

I'm currently writing a simple program using SDL2 where you can drag some shapes (square, circle, triangle, etc) into a canvas and rotate them and move them around. Each shape is represented visually by a SDL texture that is created from a PNG file (using the IMG_LoadTexture function from the SDL_image library).

The thing is that I would like to know whether a certain pixel from the texture is transparent, so that when someone clicks on the image I could determine if I have to do some action (because the click is on the non transparent area) or not. Because this is some school assignment I'm facing some restrictions, that is, only use SDL2 libraries and I can't have some map where I can look up if the pixel in question is transparent because the images are dinamically selected. Furthermore I thought about using a SDL surface for this task creating them from the original images but due to the fact that the shapes are being rotated through the texture that wouldn't work.

Answer 1

You can accomplish this by using Render Targets.

SDL_SetRenderTarget(renderer, target);
... render your textures rotated, flipped, translated using SDL_RenderCopyEx
SDL_RenderReadPixels(renderer, rect, format, pixels, pitch);

With the last step you read the pixels from the render target using SDL_RenderReadPixels and then you have to figure out if the alpha channel of the desired pixel is zero (transparent) or not. You can read just the one pixel you want from the render target, or the whole texture, which option you take depends on the number of hit tests you have to perform, how often the texture is rotated/moved around, etc.

Answer 2

You need to create your texture using the SDL_TEXTUREACCESS_STREAMING flag and lock your texture before being able to manipulate pixel data. To tell if a certain pixel is transparent in a texture make sure that you call

SDL_SetTextureBlendMode(t, SDL_BLENDMODE_BLEND);

this allows the texture to recognize an alpha channel.

Try something like this:

SDL_Texture *t;

int main()
{
    // initialize SDL, window, renderer, texture
    int pitch, w, h;
    void *pixels;

    SDL_SetTextureBlendMode(t, SDL_BLENDMODE_BLEND);

    SDL_QueryTexture(t, NULL, &aw, &h);
    SDL_LockTexture(t, NULL, &pixels, &pitch);
    Uint32 *upixels = (Uint32*) pixels;

    // you will need to know the color of the pixel even if it's transparent
    Uint32 transparent = SDL_MapRGBA(SDL_GetWindowSurface(window)->format, r, g, b, 0x00);

    // manipulate pixels
    for (int i = 0; i < w * h; i++)
    {
        if (upixels[i] == transparent)
            // do stuff
    }

    // replace the old pixels with the new ones
    memcpy(pixels, upixels, (pitch / 4) * h);

    SDL_UnlockTexture(t);

    return 0;
}

If you have any questions please feel free to ask. Although I am no expert on this topic.

For further reading and tutorials, check out http://lazyfoo.net/tutorials/SDL/index.php. Tutorial 40 deals with pixel manipulation specifically.

I apologize if there are any errors in method names (I wrote this off the top of my head).

Hope this helped.