Support sRGB color space in RenderTexture

[Sakarah]

• Has this change been discussed on the forum or in an issue before?
• Does the code follow the SFML Code Style Guide?
• Have you provided some example/test code for your changes?
• Does the fix fully solve the issue

---

Description

This PR tries to fix #1092.

When the sf::ContextSettings asks for an sRGB capable buffer, set the rendered image to sRGB mode and convert pixels when drawing.

This is the choice of simplicity compared to actually support textures with more than 8 bit color depth: see my post in forum (https://en.sfml-dev.org/forums/index.php?topic=20358.msg175496#msg175496).

It differs from #1093 because now we try to both convert to sRGB space when writing to the texture and when decoding from the texture. This repairs the rendering on multisampled textures, and on non-FBO implementations.
It also repairs rendering for sRGB RenderTexture used along a non-sRGB window (or no window at all).

Tasks

• Tested on Linux
• Tested on Windows
• Tested on macOS
• Tested on iOS
• Tested on Android

How to test this PR?

To test the PR, you should run the following code and check that it matches the reference output for all parameter combination.
You will need to following image: Gradient.png

GitHub's image compression makes it slightly hard to see, but it is normal if configurations lS -> sT -> lW and sS -> lT -> sW show color banding, the other configurations must not.
Enabling or disabling antialiasing should not change the output at all.

The code is adapted from the issue by @Fewes in the forum thread https://en.sfml-dev.org/forums/index.php?topic=20358.msg146448#msg146448.

#include <SFML/Graphics.hpp>

void updateWindowTitle(sf::Window& window, bool bypass_texture, bool sRgbSprite, bool sRgbTexture, bool sRgbWindow, bool antialiasing)
{
    sf::String title;
    title += sRgbSprite ? "sS" : "lS";
    title += " -> ";
    if (!bypass_texture)
    {
        title += sRgbTexture ? "sT" : "lT";
        if (antialiasing)
            title += " (MSAA x2)";
        title += " -> ";
    }
    title += sRgbWindow ? "sW" : "lW";
    window.setTitle(title);
}

int main()
{
    bool bypass_texture = false;

    sf::RenderWindow window;
    sf::VideoMode videoMode;
    videoMode.width = 800;
    videoMode.height = 600;
    sf::ContextSettings windowContextSettings;
    windowContextSettings.sRgbCapable = true;

    window.create(videoMode, "sRGB testing", sf::Style::Default, windowContextSettings);

    sf::Texture spriteTexture;
    spriteTexture.setSrgb(true);
    spriteTexture.setSmooth(true);
    spriteTexture.loadFromFile("Gradient.png");

    sf::Sprite sprite;
    sprite.setTexture(spriteTexture);

    sf::RenderTexture renderTexture;
    renderTexture.setSmooth(true);
    sf::ContextSettings textureContextSettings;
    textureContextSettings.sRgbCapable = true;
    renderTexture.create(videoMode.width, videoMode.height, textureContextSettings);

    sf::Sprite renderTextureDrawable;
    renderTextureDrawable.setTexture(renderTexture.getTexture());

    updateWindowTitle(window, bypass_texture, spriteTexture.isSrgb(), textureContextSettings.sRgbCapable, windowContextSettings.sRgbCapable, textureContextSettings.antialiasingLevel);

    while (window.isOpen())
    {
        // Poll events
        sf::Event event;
        while (window.pollEvent(event))
        {
            // Window closed
            if (event.type == sf::Event::Closed || (event.type == sf::Event::KeyPressed && event.key.code == sf::Keyboard::Key::Escape))
            {
                window.close();
            }
            else if (event.type == sf::Event::KeyPressed)
            {
                if (event.key.code == sf::Keyboard::Space)
                {
                    bypass_texture = !bypass_texture;
                }
                else if (event.key.code == sf::Keyboard::A)
                {
                    textureContextSettings.antialiasingLevel = textureContextSettings.antialiasingLevel ? 0 : 2;
                    renderTexture.create(videoMode.width, videoMode.height, textureContextSettings);
                }
                else if (event.key.code == sf::Keyboard::T)
                {
                    textureContextSettings.sRgbCapable = !textureContextSettings.sRgbCapable;
                    renderTexture.create(videoMode.width, videoMode.height, textureContextSettings);
                }
                else if (event.key.code == sf::Keyboard::S)
                {
                    spriteTexture.setSrgb(!spriteTexture.isSrgb());
                    spriteTexture.loadFromFile("Gradient.png");
                }
                else if (event.key.code == sf::Keyboard::W)
                {
                    windowContextSettings.sRgbCapable = !windowContextSettings.sRgbCapable;
                    window.create(videoMode, "sRGB testing", sf::Style::Default, windowContextSettings);
                }
                updateWindowTitle(window, bypass_texture, spriteTexture.isSrgb(), textureContextSettings.sRgbCapable, windowContextSettings.sRgbCapable, textureContextSettings.antialiasingLevel);
            }
        }

        // Clear render texture
        renderTexture.clear(sf::Color(0, 0, 0, 255));
        // Draw gradient sprite
        renderTexture.draw(sprite);
        // Finished drawing to render texture
        renderTexture.display();

        // Clear window
        window.clear(sf::Color(0, 0, 0, 255));
        if (bypass_texture)
        {
            window.draw(sprite);
        }
        else
        {
            // Draw render texture drawable
            window.draw(renderTextureDrawable);
        }

        // Finished drawing to the window
        window.display();
    }

    return 0;
}

[Sakarah]

Independently on how we plan to solve the issue, I updated the code to test the PR in a more reproducible way.

[Sakarah]

@binary1248 Any remarks on the current state of this PR ?

[binary1248]

Looks good to me now. Test works on my system.