gimp colour level algorithm

Question

My first question on this forum, please be soft with me :)

I have a bunch of BMP files (created by the script i wrote ) and i would like to change the color level of them. Currently, i open Gimp, i go to "color level" set the input level values from 0~255 to 200~255 , i click "ok" and i get what i want. But i would like to understand what is the algorithm behind that, so that i can code it. I tried to search , i read the Gimp documentation but i did not find enough information. If someone could help, that would be great !

Thanks a lot for you help :)

Create a bmp with pixels set to all values from 0 to 255 (greyscale or for RGB seperately), process with Gimp, then check the resulting values; this should tell you all you need to know — m69's been on strike for years, Aug 26 '16 at 19:13
Of course i did that - Results are not consistent. i suspect some RGB to HSL translation and something else... but what ? — kalonkadour, Aug 26 '16 at 19:18

score 1 · Accepted Answer · answered Aug 26 '16 at 19:36

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The first googled ling shows documentation https://docs.gimp.org/en/gimp-tool-levels.html

If you don't need gamma correction (middle slider), use simple linear transformation, where V and NV is old value and new (corrected) value for ever color channel (R, G, B), L and H are Low and High levels.

 NV = Min(255, Max(0, (V - L) * 255 / (H - L)))

answered Aug 26 '16 at 19:36

MBo

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Thank you for the link, i did find it too... However, the formula you wrote is not part of this documentation , isn't it ? May i ask where you found it ? Thank you ! – kalonkadour Aug 26 '16 at 21:28
I make formula from thinking that transformation should transform L value 0, H value to max (255), and it is linear . – MBo Aug 27 '16 at 02:53
I just tested your algorithm and it worked. Thanks! Keep in mind that if `H = L` then it will fail due to a divide-by-zero, though. – Sam Nov 12 '18 at 20:20

gimp colour level algorithm

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