with regard to the channel clipping, i did some playing around earlier and found these things:
as long as one of the colors contains one or two RGB value of 0, then increasing saturation does nothing.
if one or two RGB values is 255, then increasing saturation does nothing.
increasing saturation can lead to individual R G or B clipping or maxing out at 255. if that happens, then increasing saturation any more, does nothing.
That’s how it is.
Finally, the Hue/Sat.-tool lets the data bump against the border of your RGB working space which is indicated by R or G or B= 0 or 255. When this happens with too many pixels, posterization can occur. Possible solutions:
/a) To convert to a larger working space like ProPhoto RGB which offers more headroom. That way, there’s of course a certain risk to overdo it; to create fancy saturated colors (or even imaginary colors) which lead to output-referred problems (monitor / print). So it can be wise to combine this with a Selection e.g. as suggested in point d).
/b) Saturation Curves, e.g. such as offered by the Curvemeister plug-in.
/c) To convert to Lab mode and to work with S-shaped curves on both color channels, a & b. Probably great, but not really my field of competence.
/d) Saturation Masks
. Just recently, I’ve learned & implemented the following technique:
/> Create a Selective Color layer. For all R, Y, G, C, B, M color ranges set Black to -100%, respectively. For the White, Gray, Black ranges set Black to +100%, respectively (everything Absolute).
/> Strg + click on the RGB composite channel (to select everything visible)
/> Strg + Shift + I to Invert the selection
/> Delete the Selective Color layer (while the selection stays unaltered)
/> Create a Hue/Sat.-layer, preferably in Saturation blend mode (+ mask from the selection). Confirm by OK.
/> Alt + click on the mask
/> set a Threshold with the respective tool (to protect objects/areas which are already quite saturated).
/> apply a Gaussian blur of some pixel width
/> tune saturation depending on needs
Now I’m sure that I’ve forgot or confused anything. If so, please let me know.