Downsizing Large Images with SDL2 and SDL_Image

Game Development
1

Hi everyone,

I’m currently facing an issue with downscaling very large images using SDL2 and SDL_Image. I’ve set SDL_HINT_RENDER_SCALE_QUALITY to linear, but the results are not satisfactory. I’m wondering if it’s possible to achieve better downscaling purely with SDL_Image and SDL2. Could someone please guide me on the correct approach and provide some code snippets if possible? Your assistance is highly appreciated!

comparation
comparation744×400 68.8 KB

The image rendered using SDL appears excessively sharp and contains noticeable noise.

2

I think this is an inherent problem with bilinear interpolation. It’s simply not good at downscaling to much smaller sizes because there are a lot of pixels that gets ignored and doesn’t contribute to the result. You might find this discussion interesting.

2 Likes
3

To address this issue, I’ve implemented a solution using the stb_image_resize. Prior to creating the texture, I utilized the stbir_resize_uint8_linear function. This involved passing the original surface pixels as the source and the resized surface as the destination, resulting in a more refined image quality.

pseudo code:

SDL_Surface *surface = IMG_Load(path);
SDL_Surface *resized_surface = SDL_CreateRGBSurface(0, {{width}}, {height}},
                                                      surface->format->BitsPerPixel,
                                                      surface->format->Rmask,
                                                      surface->format->Gmask,
                                                      surface->format->Bmask,
                                                      surface->format->Amask);

stbir_resize_uint8_linear(surface->pixels, surface->w, surface->h, 0,
                          resized_surface->pixels, {{width}}, {{height}}, 0,
                          surface->format->BytesPerPixel);

SDL_Texture *texture = SDL_CreateTextureFromSurface(renderer, resized_surface);