SDL2 read PNG24 pixel by pixel

Question

I have a getpixel function that, given a surface, reads the r, g, b and alpha value of a given pixel:

void getpixel(SDL_Surface *surface, int x, int y) {
    int bpp = surface->format->BytesPerPixel;
    Uint8 *p = (Uint8 *)surface->pixels + y * surface->pitch + x * bpp;

    Uint8 red, green, blue, alpha;

    SDL_GetRGBA(*p, surface->format, &red, &green, &blue, &alpha);

    cout << (int)red << " " << (int)green << " " << (int)blue << " " << (int)alpha  << endl;

}

I saved a PNG image with Phosothop "Save for Web" and I choose PNG24 as format. The problem is that the function only reads the red value and always read alpha as 0. I tried to force the format like so:

SDL_Surface* temp  = IMG_Load(png_file_path.c_str()); 
SDL_Surface* image =  SDL_ConvertSurfaceFormat(temp, SDL_PIXELFORMAT_RGBA8888, 0);
SDL_FreeSurface(temp);

By doing so it only read the alpha value, instead. How to read a PNG pixel by pixel in SDL2?

Answer 1

SDL_GetRGBA(*p, surface->format, &red, &green, &blue, &alpha); attempts to extract values from *p , which have type Uint8 . It is only one byte, so yes - it would be only red or alpha depending on pixel format. SDL_GetRGBA expects Uint32 , so call should be eg SDL_GetRGBA(*(Uint32*)p, surface->format, &red, &green, &blue, &alpha);

(it would only be valid for 32bpp formats - if it isn't the case, you should either convert surface to 32 bit, or or memcpy BytesPerPixel amount of pixel data, otherwise results will be incorrect)