‘Inverting’ an intensity variation

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.

Answer

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 Scale: to 2.

  • Invert.

  • Change the blend mode of your top layer to Linear Light.

  • Merge down.

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.

Attribution
Source : Link , Question Author : Jim Mack , Answer Author : Alan Gilbertson

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