Re: Expression to maintain overall transparancy when blending two layers

When you have two solids that are 50% opaque above one another they will each half the amount of 'light' that is passing through them, resulting in a 50% reduction followed by another 50% reduction... allowing a total of 25% of the 'light' through (25% transparency or 75% opacity).

Unfortunately, the way this works is that as soon as a layer is less than 100% opaque, no number of additional layers at below 100% opacity will total to 100% opacity. If the top layer is 99% opaque and the second layer is 99% opaque that is a total of 99.9999% opacity. 1% or .01 of the 'light' gets through the first layer, then only 1% or .01 of that light makes it through the 2nd layer, so .01 x .01 = .0001).

However, if you add the transparency values of the layers together instead of AE's standard mode of operation, that should work. Try using the 'Alpha Add' blending mode is my short answer =)

Darby Edelen
Lead Designer
Left Coast Digital
Santa Cruz, CA