Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".
I've just been attempting to sharpen images using the L channel of Lab. This should be straightforward but the overall brightness/gamma is substantially altered. An example:
Running -colorspace Lab -channel R -unsharp "0x0.8" +channel on
produces...
I've tried this same method using HSB, HSL, YUV, brightness and lightness channels and everything works normally. Is this an oddity in your implementation of Lab, something to do with the USM, or contention between the two?
LAB may not be totally reversible when transforming to RGB (as LAB has a larger gamut than RGB), and L may not be linear. One or both may be the cause of your results, which I verified on IM 6.6.1-9 Q16 Mac OSX Tiger.
Yup, I understand the difference between Lab and RGB; but the result of flipping modes should be barely perceptible - if at all - to the human eye. Photoshop manages translations Lab/RGB - RGB/Lab well enough so I expected ImageMagick to be similar - swapping to Lab is a fairly standard trick to avoid shifting hues (and noise) when sharpening. Looks like I'm back to two-layer sharpening then!
I understand what you are saying and I would expect a round trip to be reversible.
Post a message in the Bugs forum at viewforum.php?f=3 pointing back to this topic in the User's forum and it will likely get looked at by the IM folks as a potential bug.
As you are offline, I have gone ahead and posted to the bugs forum
