blendfunction_alphablend 叠色成绿色_acsrcalpha(3)

If the destination bitmap has an alpha channel, then the blend is as follows.

Dest.alpha = Src.Alpha + (1 - SrcAlpha) * Dst.Alpha 

If the source has both theSourceConstantAlpha(that is, it is not 0xFF) and per-pixel alpha, the source is pre-multiplied by theSourceConstantAlphaand then the blend is based on the per-pixel alpha. The following equations show this. Note thatSourceConstantAlphais divided by 255 because it has a value that ranges from 0 to 255.

Src.Red   = Src.Red   * SourceConstantAlpha / 255.0; 
Src.Green = Src.Green * SourceConstantAlpha / 255.0; 
Src.Blue  = Src.Blue  * SourceConstantAlpha / 255.0; 
Src.Alpha = Src.Alpha * SourceConstantAlpha / 255.0; 
Dst.Red   = Src.Red   + (1 - Src.Alpha) * Dst.Red 
Dst.Green = Src.Green + (1 - Src.Alpha) * Dst.Green 
Dst.Blue  = Src.Blue  + (1 - Src.Alpha) * Dst.Blue 
Dst.Alpha = Src.Alpha + (1 - Src.Alpha) * Dst.Alpha 

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