Please explain this color blending mode formula so I can replicate it in PHP/ImageMagick

Question

I\'ve been trying to use ImageMagick to replicate Photoshops Colour Blend Mode. I found the following formulas in an online guide but I don\'t know what they mean. Do I just

Answer 1

Wikipedia has a good article on blend modes http://en.wikipedia.org/wiki/Blend_modes

They give formulas for Multiply, Screen and Overlay modes.

Multiply
Formula: Result Color = (Top Color) * (Bottom Color) /255

Screen
Formula: Result Color = 255 - [((255 - Top Color)*(255 - Bottom Color))/255]

Overlay
Formula: Result Color = if (Bottom Color < 128) 
    then (2 * Top Color * Bottom Color / 255) 
    else (255 - 2 * (255 - Top Color) * (255 - Bottom Color) / 255)