Thanks Kalle for the fast response.
I also need to animate the transparency in a rather controlled way.
Example: I need to have the greys RGB 160 to 165 to be visible (to make the bottom layer transparent) then it crawls to 165-170 then 180-185, increasing or decreasing in value -but with a constant clamp maybe 5 to 10 range.
I need the visibility (narrow section)to slide/animate along the grey scale image provided.
Similar to gradient wipe but only a narrow middle section to be visible.
Thanks.
Dave