Hi Marco,
yes I thought in fact that PS lum would not cause any hue shifts.
Hi, Hening.
It causes
less of a hue shift, but still some, compared to what happens in
Normal blend mode.
I retrieved a post where digitaldog refers to your recipe for a luminosity mask, and I'll try it out ASAP.
What Andrew
[digitaldog] is referring to is the use of an additional layer filled with
white, or black, or a perfectly neutral RGB mix (R=G=B), set to
"Color" blending mode, above a full-color image. From that, you can proceed to create a
luminosity mask:
[blockquote]Select All
[Command-A], then Edit > Copy Merged
[Shift-Command-C], then paste above the full-color image the Grayscale layer which now resides in your clipboard
[Command-V]; also, toss the previous black/white/neutral layer into the trash (it was only meant to be used as an intermediate tool).
Next, set the newly-created luminosity layer itself to Luminosity blend mode, then clip it to a
Curves or
Levels adjustment layer right above it. You may now proceed to make your tonal/color changes that way.[/blockquote]
That is a trick which allows the user to restrict the color modifications in the underlying image to changes in luminosity and saturation alone, while keeping the image's
hue constant — though, strictly speaking, that is not actually completely true either. One can only go so far in lightening or darkening a color without changing its hue before it "crashes" against the "ceiling" or the "floor" of the RGB space's own boundaries. When that happens, the hue is
forced to change, by necessity.
If you sample the colors before and after the changes using
HSB in Photoshop, the Hue angle
appears to remain constant. But HSB is device-dependent, meaning that its results vary depending on which RGB profile one is using,
and thus it is not reliable as a measure of absolute color. On the other hand, if you use LCH (
Lightness,
Chroma,
Hue — which is device
independent, and therefore far more reliable), you will see that
a hue shift has indeed occurred. (Unfortunately, Photoshop does not offer an LCH sampler, in spite of the fact that the math behind LCH would probably be fairly simple to implement. As things currently stand, you need professional-level tools — like MeasureTool, or ColorLab — to find out the LCH values in a file.)
One advantage of using adjustment layers clipped to a luminosity mask is that even the peak values undergo a change in appearance
without having to touch the endpoints in the adjustment layer, whereas they normally remain constant unless you change the endpoints of your Curves or Levels controls.
The directions I provided above should be sufficient for you to try out this procedure, if you are interested. One warning, though: truth be told, keeping the hue constant (in terms of HSB values, anyway) creates results that, more often than not, end up looking unnatural and, frankly, kind of ugly. That is because of the way that we, as humans, perceive color — non-linearly and highly dependent on elements in the surrounding environment.
My bottom line is that, if for some reason you absolutely need to keep hue changes
to a minimum (while still not perfectly constant), there are ways to do that. But tonal and color changes without some correspondingly substantial changes in hue (besides lightness and chroma/saturation) often appear unnatural. There are valid reasons for the way adjustment layers work in Photoshop in their "default" behavior, and those are often practical and sensible ones — to keep the images looking as natural as possible, according to the way our human vision system operates.
