Severe contrast enhancements: on 45% rule — and cats

Started Jun 28, 2013 | Discussions thread
ilza
Forum MemberPosts: 74
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Re: Changing contrast without distorting colors: 45% rule and another cat
In reply to knickerhawk, Jul 12, 2013

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.)

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?

Here's a quick and dirty luminosity blend of your image. Hue hasn't been changed by it. (The cat is sitting at the bottom of the rock pile a little more than a quarter of the way in from the right.)

Hue was not changed… This still has very little to do with the perceived change of colors. And compare the white wall on your image and on mine. Your manipulation is very mild. IMO, to be able to clearly recognize color shifts related to manipulating L, it is better to start with severe manipulations, where differences are more pronounced.

Ilya

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