At first glance, decreasing the luminosity of a near-white colour is a simple manner. You just make it a near-gray one. However, there are instances in which this leads to undesirable results. If the color has been previously artifically shifted towards near-white just to make it brighter (such as ACE does for out-of-gamut luminosities), the darker version of that colour should be re-saturated. Some skin tones end up with colourless dark spots. White highlights on coloured objects are not expceted to become grey, but to revert to the colour of the underlying object.
[...] Questions abound. How do you decide which colours should be re-saturated? How do you decide how much to adjust the saturation, without over-saturating?
Here we have a picture of sky and water, above and below the Caroline Islands (Photo credit: Dr. James P. McVey, NOAA Sea Grant Program). The image isn't too interesting as a subject for contrast enhancement, but it does serve to highlight the differences between our several colour-processing algorithms.
[ Strength & Brightness adjust: 1.0, Iterations: 40, Detail Size: 48, Smoothing: 12 ] [ Grayscale version ]
The original image is followed by ACE's attempt to adjust the luminosity while retaining the chromacity. However, where ACE makes the sky darker, it does not do so by deepening the blue to match the ocean beneath it, but by turning it a dismal bluish-gray. In truth, the sky has the same measure of saturation as it did before it was darkened, but we still cannot help but feel that ACE has done the "wrong thing" here.
Here are two alternative methods:
The first is the method that ACE used in earlier releases. The sky retains a more pleasing blue, however our fluffy white cloud has turned cyan. Clearly the saturation has gotten out of hand. The second is the same process, but with Martin Weber's corrective fudge-fator for light colors added at the end. It is sucessful in keeping some of the colour out of the clouds, but is it enough?
Note: Lest you think this problem only plauges blue skies, know that a similar thing happens with skin tones, as will be seen in the image of the nudibranch below (Photo credit: NOAA). Nudibranch are must less troublesome about having their pictures posted on the web than people are.
After ACE: [Strength: 0.8, Brightness Adjust: 0.3, Iterations: 40, Detail Size: 22 Smoothing: 6]
Version 0.6's method:
With light-color correction factor:
see also Grayscale output
Questions, comments, reservations, and pointers to appropriate resources are strongly encouraged and appreciated. Please mail them to the address at the bottom of this page.