Is SDL_CreateSurfaceFrom (SDL3) reading my data wrong?

Question

I'm trying to create a SDL texture from Perlin noise. I have created an array of uint8_t and filled it with noise values times 255. When I'm printing out the values they seem to be ok:

127, 137, 134, 136, 127, 127, 127, 118, 120, 117, 137, 148, 144, 
146, 137, 137, 137, 128, 130, 127, 134, 144, 141, 143, 134, 134, 
134, 125, 127, 124, 136, 146, 143, 145, 136, 136, 136, 127, 129, 
126, 127, 137, 134, 136, 127, 127, 127, 118, 120, 117, 127, 137, 
134, 136, 127, 127, 127, 118, 120, 117, 127, 137, 134, 136, 127, 
127, 127, 118, 120, 117, 118, 128, 125, 127, 118, 118, 118, 109, 
111, 108, 120, 130, 127, 129, 120, 120, 120, 111, 113, 110, 117, 
127, 124, 126, 117, 117, 117, 108, 110, 106

(e.g. 10x10 Perlin noise)

After that I call SDL's CreateSurfaceFrom with valid width, height, pitch (I think) and pixel format as SDL_PIXELFORMAT_INDEX8 and then I create texture from this surface and try to render it. Unfortunately, the output is not what I want, mostly black or white screen.

I've also tried those pixel formats, but only the last one gave some output, but still not exactly correct:

SDL_PIXELFORMAT_UNKNOWN
SDL_PIXELFORMAT_INDEX4LSB
SDL_PIXELFORMAT_INDEX4MSB
SDL_PIXELFORMAT_INDEX1LSB
SDL_PIXELFORMAT_INDEX1MSB

And here is the output: SDL_PIXELFORMAT_INDEX1MSB output

Also here is the code:

uint8_t sdlPixels[_perlinWidth*_perlinHeight];
for (int i = 0; i < _perlinWidth*_perlinHeight; i++) {
    sdlPixels[i] = static_cast<uint8_t>(_pixels[i]*255);
    std::cout << (int)sdlPixels[i] << ", ";
}

SDL_Surface* surface = Renderer::GetInstance()->CreateSurfaceFrom(
    sdlPixels, _perlinWidth, _perlinHeight, _perlinWidth, SDL_PIXELFORMAT_INDEX8);
_perlinTexture = Renderer::GetInstance()->CreateTextureFromSurface(surface);
delete [] _pixels; _pixels = nullptr; // clean the _pixel data

And here is the Renderer functions (simply mapped SDL functions):

SDL_Surface* Renderer::CreateSurfaceFrom(void* pixels, int width, int height,
        int pitch, Uint32 format) {

    return SDL_CreateSurfaceFrom(pixels, width, height, pitch, format);
}

SDL_Texture* Renderer::CreateTextureFromSurface(SDL_Surface* surface) {
    SDL_Texture* out = SDL_CreateTextureFromSurface(_renderer, surface);
    SDL_DestroySurface(surface); // free surface (we already have a texture)
    return out;
}

Is the pixel data invalid in format that I give as a argument or have I missed something while creating the texture from SDL_Surface in SDL3?

Answer 1

The pixel format you want is SDL_INDEX8. This means that your input sdlPixels points to an array of 8-bit indices into a palette. In order to create a texture from that, the palette needs to contain meaningful values.

You can do that with SDL_SetPaletteColors and SDL_SetSurfacePalette as follows:

SDL_Palette* pal = SDL_CreatePalette(256 /* colors */);
std::vector<SDL_Color> colors(256);
for (int i = 0; i < 256; i++) {
  colors[i] = { i, i, i, 0xFF };
}
SDL_SetPaletteColors(pal, colors.data(), 0, 256);

SDL_Surface* surface = Renderer::GetInstance()->CreateSurfaceFrom(
    sdlPixels, _perlinWidth, _perlinHeight, _perlinWidth, SDL_PIXELFORMAT_INDEX8);
SDL_SetSurfacePalette(surface, pal); // <-- use the palette
_perlinTexture = Renderer::GetInstance()->CreateTextureFromSurface(surface);

Although from reading the code this does modify the pixelformat's palette ... 🤔