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Newsgroups: gmane.comp.video.image-magick.user Date: Sun, 17 Jun 2007
Hi Anthony,
On http://www.imagemagick.org/Usage/photos/, you have a minor underexposure example.
Before going into -sigmoidal-contrast operator, maybe it is a good idea to first discuss convert's built-in -normalize operator that is similar to GIMP's "auto-levels" or "normalize" feature that stretch pixel values from darkest to brightest.
convert minor.jpg -normalize minor.normalized.jpg
FYI, the following comments were taken out from http://www.linux-mag.com/id/2627/
Normalizing high-contrast line art and graphics can be great. But normalized photos may look unreal, and, as was said earlier, may not print well either. The -contrast-stretch operator can limit the "boundaries" of the normalization, but the -levels and/or -sigmoidal-contrast operator can make "smoother" adjustments.
Also, maybe you'd like to through in how IM can also create the three-color histogram out of it:
convert minor.jpg histogram:- | convert - minor.histogram.jpg
Looking at the histogram, I think it may explain what you wrote:
Sigmoidal contrast brightening tends to de-emphasize the red end of the spectrum. You may end up having to select a parameter that results in the most natural flesh tones, rather than the brightness level you really want.
documented on: 2007.06.17
| Also, maybe you'd like to through in how IM can also create the | three-color histogram out of it:
Actually I did not write it. It was contributed. If you like to edit the page, you are welcome to do so, and send me the results.
Photo processing is NOT a strong point of mine.
| Looking at the histogram, I think it may explain what you wrote:
Also see the more low level operator discussions on the 'Color Modifications' page… http://www.imagemagick.org/Usage/colors/
Anthony Thyssen