public class GPUImageNormalBlendFilter
extends GPUImageTwoInputFilter
This equation is a simplification of the general blending equation. It assumes the destination color is opaque, and therefore drops the destination color's alpha term.
D = C1 * C1a + C2 * C2a * (1 - C1a)
where D is the resultant color, C1 is the color of the first element, C1a is the alpha of the first element, C2 is the second element color, C2a is the alpha of the second element. The destination alpha is calculated with:
Da = C1a + C2a * (1 - C1a)
The resultant color is premultiplied with the alpha. To restore the color to the unmultiplied values, just divide by Da, the resultant alpha.
http://stackoverflow.com/questions/1724946/blend-mode-on-a-transparent-and-semi-transparent-background
For some reason Photoshop behaves
D = C1 + C2 * C2a * (1 - C1a)