This is somewhat hard for me to explain, but what I’m looking for is a technique to correct a fixed-pattern intensity variation in an image. The pattern comes from (say) a projection system and is not inherent to the actual image.
I can produce a 90%-average white frame that exposes these variations, which amount to perhaps as much as 10%, so that the range of values will be from about 85% to 95% in different areas. I then need to make an inverted mask / alpha that undoes those variations. So if applied to the original white frame it would result in a pure image with no variations. Once created, the mask will be applied externally, not within a graphics program.
If I’ve managed to convey that, how would you approach the creating of this transparency / alpha mask, given the original white frame with variations? My tool of choice is PSP 9 (no laughs), but a solution using PS would be fine.
Below your original with the +/- 5% variance, add a second layer that is an even 90% gray (or white, whichever term you prefer!), then:
Add a layer below that is a flat 90%, no variations.
Target your top layer. From the main menu, choose
Image > Apply Image > Subtract. Set
Layer:to your new flat layer,
Offset:to 128 and
Change the blend mode of your top layer to Linear Light.
This will give you a layer that inverts the variations from your base 90%, and will give you a more accurate result than varying the luminance by hand.