Re: How does Luminance slider in Camera Raw split colors by their hue? #colortheory #photoshop #lightroom #colors

Kenneth Harris

If you want to puzzle this out, you can use a granger rainbow to see which clean colors are affected.  A long time ago (PS7) I worked out which areas were selected by H/S for each color using calculations.  I don't have my notes in front of me, but IIRC, H/S is colorspace agnostic, it's just ratios, and it corresponds to the peak  angle + rolloff in the old HSV/HSL colorspaces.  Again IIRC, conversion into HSV via the plugin is also colorspace agnostic.  Further, if you take a granger rainbow, and add a Z dimension of saturation, H/S will continue to affect the same ratios.  For instance, run each range in H/S to zero and any images, and it should be greyscale.  Although now I use the logic of how selective color cuts the image apart regularly (via an action), I've totally forgotten what is is, although again, it can be emulated via calculations.  Selective color doesn't cut deeply into unsaturated areas.  If you put selective color to zero on all the blacks, and set to "absolute," the image will turn to white, you can then move and individual range up and see if you you're catching what you want for the purpose of masking, eg, "reds.".  I did not come up with this method, perhaps someone here can attribute, it was circa 2005.  

I have tested how ACR handles it, but I know the hit on peak response is much higher, and the rolloff to low response faster while the range is broader, for a given color.  My sense is that it's still looking at RGB equivalent ratios even though ACR should be in a CIE space.  I'll happily be disabused of my ignorance.

Combining these methods, it's possible to go from a particular/known colorspace into a space that looks ~95% like Lab using only calculations, that is, if you know what kind of reds you want, and what kind of greens you want, ditto yellows/blues, you can get a/b.  The L is trivial.  I could never get the reds perfect, however, too dark.  A granger rainbow was invaluable in testing all that.

Ken Harris

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