: '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

@Tiffany317

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.

Vote Up Vote Down

@Sherry646

I think I have a workable process for this. The steps are:

1) Capture the initial average 90% white frame with +/- 5% areas

2) Reduce the overall luminance to 50% average -- pixels that were exactly 90% proportionally become 50%, those at 95% become 52.7% etc.

3) Invert the image: make it negative. Anything at 50% stays the same, anything at 53% becomes 47%, and so on.

4) Increase overall luminance so that 50% pixels are again at 90%, or whatever higher level allows no clipping (nothing tries to exceed 100%).

5) The resulting image can be used as a transparency layer, reducing the intensity of those areas that originally were too bright, and likewise for ones originally too dark. The overall amount of correction can be varied using gain.3

If anyone sees an issue with this, or doesn't understand my thought process, please comment. I feel pretty good about it, but it's still just an idea.

Vote Up Vote Down

@Yeniel278

This can be done yes, I would call this operation equalizing or normalizing the image. That white image is the alpha, if you can call it that.

As per comments and feedback:

It is possible to drop all pixels down to 85%. There are 2 possible physical phenomena: subtractive or multiplicative masks, so I'm outlining both


Subtractive for emitters such as digital imaging units. First subtract 85% percent form your reference image then invert the image and subtract the source with that. You now have a mask of how much to subtract the color with.

Mathematically speaking it is:

1-(error-85%filledLayer)




Image 1: Top original difference capture (synthesized and HIGHLY exagarated), resulting mask trough subtraction, subtracted mid from top


Alternatively since your physical mask is most likely multiplicative (due to physics of semitransparent maps). You can use divide instead divide a 85% layer with your original error image. That is then your mask.

Mathematically speaking it is:

85%filledLayer/error




Image 2: Top original difference capture (synthesized and HIGHLY exagarated), resulting mask through division, multiplied top and mid

All functions used exist in both Photoshop (as that's what I used) and PSP, as well as Matlab and Mathematica.

Make sure that:


Your color correction mode is converted to a linear working space. Otherwise calculations won't work! This is highly unnatural for graphics designers but makes the math MUCH easier.
That your manufacturing process has a well defined color profile that can adjust to your linear data.


NOTE: it's way better if you do this processing in Mathematica or Matlab since it's far easier to formulate this as mathematics.

PPS: I'm not a graphics artist but a mechanical engineer.

Vote Up Vote Down