ebiten: add a mode for automatic gamma correction

[hajimehoshi]

Operating System

• Windows
• macOS
• Linux
• FreeBSD
• OpenBSD
• Android
• iOS
• Nintendo Switch
• PlayStation 5
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• Web Browsers

What feature would you like to be added?

Currently Ebitengine doesn't distinguish linear sRGB and gamma-corrected sRGB. An image file basically has gamma corrected values, while a shader should treat linear values. So the rendering result is not actually correct.

We can use automatic conversion in graphcis libraries:

• OpenGL's GL_SRGB8_ALPHA8
• Metal's MTLPixelFormatRGBA8Unorm_sRGB
• DirectX's DXGI_FORMAT_R8G8B8A8_UNORM_SRGB

With this, we can send pixel data to GPU as gamma-corrected sRGB values, and when a color is picked in a shader, GPU automatically converts them to linear sRGB values.

As this is a breaking change, this should be done in v3. We might be able to have an option for v2 as an experimental flag.

The current concern is that I am not sure if premultiplied-alpha gamma-corrected values make sense. This might confuse the graphics library's conversion.

Why is this needed?

No response

[hajimehoshi]

In Ebitengine, we use the same image object for a regular image and an offscreen image. It's OK to use GL_SRGB8_ALPHA8 when we create an image from an image file, but it's not OK to use it for an offscreen image, as an offscreen image data is already linear sRGB data.

Thus, I don't think it is feasible to introduce such conversion in Ebitengine without changing the API drastically.

[hajimehoshi]

Maybe a more feasible proposal is like following:

• Add an option to de-gamma values at WritePixels and NewImageFromImage
• Add an option to gamma values at final rendering
  • You can do it by yourself by DrawFinalScreen.

so that the internal texture format is always linear sRGB.