Severe contrast enhancements: on 45% rule — and cats

Started Jun 28, 2013 | Discussions thread
knickerhawk Veteran Member • Posts: 5,393
Re: Changing contrast without distorting colors: 45% rule and another cat

ilza wrote:

knickerhawk wrote:

I don't have Gimp installed and haven't used it, so I'm not sure what y=5 corresponds to in any scaling available in Photoshop.

Exactly the same. Gamma is the one of the primary ways of image manipulation: it describes the contrast enhancement literally. If you have two pixels with levels differing by 1%, after applying γ=5, their levels will differ by 5%. (And γ=1 is DO-NOTHING.)

Googling, I can see that in Photoshop the UI for gamma is exactly the same as in GIMP: 1/γ is the middle of 3 number in the Levels dialogue. So just set this number to 0.2. (The formula for γ=5 transformation is newL=L⁵; here L is normalized to change between 0 and 1.)

OK.  I did the gamma adjustment as suggested and it does seem to cause the contrast/brightness rendering to closely match what was going on in your samples.

I'm also having difficulty understanding the problem you have with a straightforward curve adjustment set to blend in luminosity mode or a curve adjustment to the L channel in LAB. Depending on the contrast already present in the image, both work well without f'ing up color.

It quite well may be that if you do very minor adjustments, and do not have “better tools” to compare your results to, the results will be satisfactory. Definitely, the colors will not be “f’ed up”. They just will be not the best one’s photoshop-fu can buy.

However, for printing, one may need to compress contrast ~1.5 times (γ=⅔). This already creates noticeable unnaturalness of colors (at least as my eyes can see it) if one uses the “touch only the brightness channel” method. With gamma still pretty close to 1, as above, the 45%-rule reduces these perceived color shifts to practically nothing.

With gamma very far from 1 (or very nonlinear tonal curves), using the naive ways such as N%-rule does not fix the color shifts problems completely (the colors in all the images I provided are hideous). Still, 45% image is, IMO, much closer to the initial colors than 100% one. Do not you agree?

I better understand now what you're trying to do and am able to pretty closely match your output with a luminosity-blended version of the gamma adjustment and then an additional saturation adjustment layer.  I will need to play more (and do some test prints) before I can really comment on which method appears best to me.  I rarely bother with gamma adjustments (too blunt) and I haven't noticed the printing adjustment related problems you describe.  Perhaps I haven't been looking close enough (being a humanities guy, and what not...)    But I thank you fro giving me something to play with (actually, damn you, for giving me another distraction to keep me from more pressing chores...)

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