Gradient Map per channel

[Doug Nelson]

I've no idea what use this might be, but I got to wondering (as I tend to do) how you could go about applying a Gradient Map adjustment layer to each individual color channel. I got about 75% of the way there, then got sleepy. So I thought I'd throw it out for discussion.

Criterion:
Must be GM adj. layer, must show results interactively in document window.

[Doug Nelson]

Ok, either I'm not as sleepy as I thought, or much too sleepy. But I answered my own question using a variation on one of Richard Lynch's Hidden Power techniques.

Each fill layer is set to screen, each GM layer is set to luminosity.

And I still have no idea what use this is, although it might be an intuitive method for severe color correction in one tonal area.

[Stroker]

GMap individual channels -
I would use Advanced Blending options and turn off channels.
For Red only, turn off G and B.
Luminosity seems to be the 'identity' blending mode.
Not entirely sure why Lum is it, just yet.

The rig that you did with the filled layers and gmaps is a variation on the colour cast technique.
Basically in the same family as what you posted in the Fleshtone Experts thread.
When I get the chance, I'll get over there and babble about that for a minute.

[byRo]

Stroker, almost there but...
True the blend-if will limit the application of the gradient map adjustment to only the (let's say) Red channel BUT the input on which it will act will still be the overall luminosity (+-60% Green, 30% Red, 10% Blue) and not just the Red channel value.
Can't try it now, but I'd think you'd have to put in a channel mixer, mono using 100% Red, to get the complete 0 to 255 luminosity range, top that with the gradient map and pack these two into a layer set with blend-if set to just Red.

(Doug, using 3 different masks seems a bit like cheating!)

Rô

[Doug Nelson]

Originally Posted by byRo

(Doug, using 3 different masks seems a bit like cheating!)

I'm sneaky that way

I was mostly looking for a quick/dirty way to test out if there'd be any use at all for this kind of thing. A more elegant design would be appreciated.

[Stroker]

Ro, I just tried it again and it works for just one channel for me.
Just like using the drop-down in Levels or Curves.

I think the Lum colour<>grey paradigm is why Lum is identity.

The paradigm seems to be one-way, though.
Say I set it up for R only.
If I put pure blue where R is high in the original, the high reds will be mapped to darker shades of grey.

Doesn't seem like it would work that way, like you said.
Weird.
Must be missing something.
Could be the saturation values in the stopper values in conjuntion with blending mode...?

edit:
Hang on. Doing too much at once.
I found a way to figure thing out...

[Stroker]

Got it. The trick is in 'negating' the Lum paradigm. You have to counter-weight the values.
This is actually very similiar to normal shader that I built for 3DS Max.
In my normal shader, had to 'multiply backwards'.
Same kind of idea here. Sort of.

To do gradient map just the Red channel.
New layer.
Fill with R=0, G=255, B=255.
Set blending mode to Difference.
Advanced Option > Turn off G and B.
Clip a G-Map and get busy with the Cheese Whiz.

Follow the pattern for G and B.

I'm a genius.

edit:
Technique can be reduced to one adjustment layer per channel.
This variation will also work with adjustment layers other than GMap.
For example, you can Threshold just the R channel.
Umm... sort of. Depends on the adjustment layer being used.
Not too hard to figure out.

edit some more and more and more
Ah! Driving me nuts.

Figured it out.
Going to soak my heak for a few hours.