BlendSourceOver does not consider the alpha channel of the source color

[25349023]

Operating System

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What feature would you like to be added?

Currently, the ebiten.BlendSourceOver (Note: same issue applied to BlendDestinationOver) is defined as following:

// BlendSourceOver is a preset Blend for the regular alpha blending.
//
//     c_out = c_src + c_dst × (1 - α_src)
//     α_out = α_src + α_dst × (1 - α_src)
BlendSourceOver = Blend{
	BlendFactorSourceRGB:        BlendFactorOne,
	BlendFactorSourceAlpha:      BlendFactorOne,
	BlendFactorDestinationRGB:   BlendFactorOneMinusSourceAlpha,
	BlendFactorDestinationAlpha: BlendFactorOneMinusSourceAlpha,
	BlendOperationRGB:           BlendOperationAdd,
	BlendOperationAlpha:         BlendOperationAdd,
}

However, it doesn't consider the transparency of the source color. For example, says source color = (0, 1, 1, 0) and dest color = (1, 0, 0, 1), with BlendSourceOver we'll have result color = (1, 1, 1, 1), which does not make sense to me. In fact, as long as the source color is semi-opaque, then the result color would be kind of weird.

Hence, I think that BlendSourceOver could be instead defined as following (or something else that could take the source's alpha channel into account):

// BlendSourceOver is a preset Blend for the regular alpha blending.
//
//     c_out = c_src × α_src + c_dst × (1 - α_src)
//     α_out = α_src + α_dst × (1 - α_src)
BlendSourceOver = Blend{
	BlendFactorSourceRGB:        BlendFactorSourceAlpha,
	BlendFactorSourceAlpha:      BlendFactorOne,
	BlendFactorDestinationRGB:   BlendFactorOneMinusSourceAlpha,
	BlendFactorDestinationAlpha: BlendFactorOneMinusSourceAlpha,
	BlendOperationRGB:           BlendOperationAdd,
	BlendOperationAlpha:         BlendOperationAdd,
}

If it is not possible to change the original preset variable, adding another new blend mode would be great, too.

The description of the over operator of color composition on the Wikipedia might help: https://en.wikipedia.org/wiki/Alpha_compositing#Description

Why is this needed?

No response

[hajimehoshi]

For example, says source color = (0, 1, 1, 0)

This is not premultiplied color value. Note that Ebitengine requires premultiplied color values.

See also https://github.com/tinne26/kage-desk/blob/main/docs/tutorials/premult.md (CC @tinne26)