 All the flavors produce the same result when the top layer is pure black; same for when the top layer is pure neutral gray.The Photoshop and illusions.hu flavors also produce the same result when the top layer is pure white (the differences between these two are in how one interpolates between these 3 results).These three results coincide with gamma correction of the bottom layer with γ=2 (for top black), unchanged bottom layer (or, what is the same, γ=1) (for top neutral gray), and γ=0.5 (for top white).