Alpha blending comparisons

BlendTest.cs

// alpha blending test
// more info: https://twitter.com/FreyaHolmer/status/1293163103035817985
// add this script to an object in the scene,
// then edit src and dst to see - will it blend?

using UnityEngine;

[ExecuteAlways]
public class BlendTest : MonoBehaviour {

	public Color src, dst; // modify these and see what happens to the ones below
	public Color outSimpleAlphaBlend;
	public Color outHackAlphaBlend;
	public Color outReference;

	public void Update() {
		outSimpleAlphaBlend = SimpleAlphaBlending( src, dst );
		outHackAlphaBlend = HackyAlphaBlending( src, dst );
		outReference = CorrectAlphaBlending( src, dst );
	}

	// rgba: SrcAlpha OneMinusSrcAlpha
	static Color SimpleAlphaBlending( Color src, Color dst ) => src * src.a + dst * ( 1 - src.a );

	// rgb: SrcAlpha OneMinusSrcAlpha
	// a: One OneMinusSrcAlpha
	static Color HackyAlphaBlending( Color src, Color dst ) {
		Color o = SimpleAlphaBlending( src, dst ); // same as simple for rgb
		o.a = src.a + dst.a * ( 1 - src.a );
		return o;
	}

	// reference/correct alpha blending
	// (there is no simple blend mode equivalent)
	static Color CorrectAlphaBlending( Color src, Color dst ) {
		Color o = default;
		o.a = src.a + dst.a * ( 1 - src.a );
		if( o.a <= 0 )
			return new Color( 0, 0, 0, 0 );
		for( int i = 0; i < 3; i++ ) // thanks Unity for no swizzling >:I
			o[i] = ( src[i] * src.a + dst[i] * dst.a * ( 1 - src.a ) ) / o.a;
		return o;
	}

}