SNH on different colour spaces

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Joined: Fri Jan 26, 2007 1:24 pm

Postby derekfountain » Mon Jan 19, 2009 12:16 pm

I'm not sure if this is a bug or not, but it strikes me as weird so perhaps someone will explain... Using CM 3.0.12 on CS2:

Open a new image of any convenient size. Select the gradient tool and switch to one of the rainbow gradients. Now drag the gradient from left to right on the image, giving a nice rainbow. Now open CM.

Start in LAB mode. Put a shadow point in the middle of the deepest blue area, and a highlight point in the middle of the brightest yellow area. Now put a neutral point anywhere. Grab the neutral point with the mouse and drag it around a bit. The colour curves dance and you get a (rather grotty) kaleidoscope effect. Now switch to RGB and drag the neutral some more: same effect. All OK so far.

Now switch to HSB mode and drag the neutral about. This has no effect, which I think I understand. In fact, in HSB mode, moving the shadow and highlight points doesn't do anything either. I think that's right too (noting the hue clock always says the brightness is 0%) but I can't explain why.

Finally, switch to wgCMYK mode. Again, moving the neutral point around has no effect. Is that right? Moving the shadow point around has no effect, and moving the bright point has a weird effect I can't really describe.

Would anyone care to explain what's going on? wgCMYK mode interests me most...

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Joined: Thu Mar 26, 2015 1:53 am

Postby -default » Mon Jan 19, 2009 8:33 pm

Re HSB mode - there is no concept of neutral in HSB, only an overall reduction of saturation.  The rainbow gradient always has one of the channels pinned at 255, and  the brightness of HSB is determined by the max value of the rgb channels.  0% means zero ink, or max brightness.  You should be able to control this by changing the curve units.

Re wgCMYK.  I don't have the opportunity right now to experiment - being in an airport on the way to Costa Rica - but try using a more natural image than the rainbow one and see if you get more reasonable behavior.  There could very well be a bug.  I'll play with this myself later.

Thanks for an interesting question.

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