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I'm trying to make a brightness/contrast filter using SDL3 and ImGui but for some reason when I try to get the rgb values of each pixel using SDL_GetRGB the values are always 0,0,0. This is my function:

`void brightness_contrast_effect(SDL_Surface* surface, int brightness, int contrast)
{
    if(SDL_MUSTLOCK(surface))
    {
        SDL_LockSurface(surface);
    }

    Uint8 r,g,b;
    Uint32* pixels = (Uint32*) surface->pixels;
    const SDL_PixelFormatDetails* pixelDetails = SDL_GetPixelFormatDetails(surface->format);

    float constrast_factor = (259.0f * (contrast + 255)) / (255.0f * (255 - contrast));
    std::cout << "contrast_value: " << std::fixed << std::setprecision(4) << constrast_factor << std::endl; 

    for(int y = 0; y < surface->h; y++)
    {
        for (int x = 0; x < surface->w ; x ++)
        {
            Uint32* pixel = pixels + (y * surface->w + x);

            SDL_GetRGB(*pixel, pixelDetails, NULL, &r, &g, &b);

            // std::cout << "R: " << static_cast<int>(r)
            // << " G: " << static_cast<int>(g)
            // << " B: " << static_cast<int>(b)
            // << std::endl;

            r = SDL_clamp(r + brightness, 0 , 255);
            g = SDL_clamp(g + brightness, 0 , 255);
            b = SDL_clamp(b + brightness, 0 , 255);

            r = SDL_clamp((constrast_factor * (r - 128)) + 128.0f, 0 , 255);
            g = SDL_clamp((constrast_factor * (g - 128)) + 128.0f, 0 , 255);
            b = SDL_clamp((constrast_factor * (b - 128)) + 128.0f, 0 , 255);

            *pixel = SDL_MapRGB(pixelDetails, NULL, r,g,b);
        }
    }

    if(SDL_MUSTLOCK(surface))
    {
        SDL_UnlockSurface(surface);
    }`

Since the rgb values are always 0 when I move my slider the image disappears and I just get a black to grey gradient.

I'm trying to make a brightness/contrast filter using SDL3 and ImGui but for some reason when I try to get the rgb values of each pixel using SDL_GetRGB the values are always 0,0,0. This is my function:

`void brightness_contrast_effect(SDL_Surface* surface, int brightness, int contrast)
{
    if(SDL_MUSTLOCK(surface))
    {
        SDL_LockSurface(surface);
    }

    Uint8 r,g,b;
    Uint32* pixels = (Uint32*) surface->pixels;
    const SDL_PixelFormatDetails* pixelDetails = SDL_GetPixelFormatDetails(surface->format);

    float constrast_factor = (259.0f * (contrast + 255)) / (255.0f * (255 - contrast));
    std::cout << "contrast_value: " << std::fixed << std::setprecision(4) << constrast_factor << std::endl; 

    for(int y = 0; y < surface->h; y++)
    {
        for (int x = 0; x < surface->w ; x ++)
        {
            Uint32* pixel = pixels + (y * surface->w + x);

            SDL_GetRGB(*pixel, pixelDetails, NULL, &r, &g, &b);

            // std::cout << "R: " << static_cast<int>(r)
            // << " G: " << static_cast<int>(g)
            // << " B: " << static_cast<int>(b)
            // << std::endl;

            r = SDL_clamp(r + brightness, 0 , 255);
            g = SDL_clamp(g + brightness, 0 , 255);
            b = SDL_clamp(b + brightness, 0 , 255);

            r = SDL_clamp((constrast_factor * (r - 128)) + 128.0f, 0 , 255);
            g = SDL_clamp((constrast_factor * (g - 128)) + 128.0f, 0 , 255);
            b = SDL_clamp((constrast_factor * (b - 128)) + 128.0f, 0 , 255);

            *pixel = SDL_MapRGB(pixelDetails, NULL, r,g,b);
        }
    }

    if(SDL_MUSTLOCK(surface))
    {
        SDL_UnlockSurface(surface);
    }`

Since the rgb values are always 0 when I move my slider the image disappears and I just get a black to grey gradient.

• c++
• sdl-3

Share Improve this question Follow asked Mar 18 at 6:46 Oscar RamosOscar Ramos 2311 silver badge22 bronze badges 0

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1 Answer 1

Sorted by: Reset to default 4

There are multiple issues with your code that I will address in the following.

Wrong pixel access

Your pixel access is not portable and does not work for any 3-bytes-per-pixel image, which would be the standard for a bitmap (.bmp) file. Use an 8bit-pointer to the pixel address and the pitch member to correctly extract the pixel data you pass to SDL_GetRGB.

Uint8 r = 0u, g = 0u, b = 0u;
const SDL_PixelFormatDetails* pixelDetails = SDL_GetPixelFormatDetails(surface->format);
const Uint8 bpp = SDL_BYTESPERPIXEL(surface->format);

for (int y = 0; y < surface->h; y++)
{
    for (int x = 0; x < surface->w; x++)
    {
        Uint8* pixel = static_cast<Uint8*>(surface->pixels) + y * surface->pitch + x * bpp;

        SDL_GetRGB(*reinterpret_cast<Uint32*>(pixel), pixelDetails, NULL, &r, &g, &b);

Also upon writing back the pixel data make sure not to overwrite the next pixel. Writing a full Uint32 onto a 3-bytes-per-pixel image thus does not work.

Uint32 res = SDL_MapRGB(pixelDetails, NULL, r, g, b);
memcpy(pixel, &res, bpp);

Implicit type conversion

Converting between Uint8 and float may have undesirable effects, e.g. over- or underflow. Make sure you cast to a wide-enough datatype prior to your operations.

r = static_cast<Uint8>(SDL_clamp(static_cast<int>(r) + brightness, 0, 255)); 
g = static_cast<Uint8>(SDL_clamp(static_cast<int>(g) + brightness, 0, 255)); 
b = static_cast<Uint8>(SDL_clamp(static_cast<int>(b) + brightness, 0, 255)); 
 
r = static_cast<Uint8>(SDL_clamp((constrast_factor * (static_cast<float>(r) - 128.0f)) + 128.0f, 0.0f, 255.0f));
g = static_cast<Uint8>(SDL_clamp((constrast_factor * (static_cast<float>(g) - 128.0f)) + 128.0f, 0.0f, 255.0f));
b = static_cast<Uint8>(SDL_clamp((constrast_factor * (static_cast<float>(b) - 128.0f)) + 128.0f, 0.0f, 255.0f));

Brightness and contrast perception

Changing brightness and/or contrast of an image is beyond simply adding the same offset to the R,G,B channels. There are physical models of human perception and corresponding color-space transformations. If you want to dive into these, start with CIE LAB color space.

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