If you had a contone image and wanted to subtract a specific RGB value linearly throughout the entire image, how would you go about it?

So R100,G100,B100 minus R25,G10,B5 would result in R75,G90,B95, and every other pixel would also have precisely R25,G10,B5 subtracted from it.

Doug, the 1st way that comes to mind would be to create two layers or documents. Select the 100-100-100 one and Image>Apply Image and select the Subtract Blend mode. There may be other ways but this is the 1st that comes to mind.
Regards, Murray

Easy

%ffp
R: r-25
G: g-10
B: b-5
A: a


Filtermeister is great

Ken.

Linear Light

R minus 25 is 128 - 25/2 approx R=113
G minus 10 is 128 - 10/2 is B=123
B minus 5 is 128 - 5/2 approx G=125

I use this all the time for colour balance (cast) and other tonal shifts. The Lab variation is absolutely spectacular.

Stroker,

Could you explain, for a dodo who doesn't speak...Strokerese, how one might actually go about using this information?

dc

Starting with a photograph in RGB:
- new layer and fill with 50% grey
-- set blending mode to Linear Light
-- bring Opacity down to 50%
- Adjustment Layer > Hue/Sat
-- clip this to the 50% grey layer
-- in the H/S dialog, turn on Colourize
-- adjust Hue and Sat sliders to taste

The Hue slider will set the hue, angle, or direction that you want to shift hues to. The Sat slider will set the amount that you shift. That is, lower Sat means less shift and higher Sat means more shift.

One of the problems of doing this in RGB with LL is that you could have a very adverse affect on Luminosity. So, you might have to use your favorite method to tweak Lum after you balance.

One way to avoid messing with Lum in RGB when doing this technique is taking things into Lab mode. Same as the above, except you use Advanced Blending to turn off L channel for the 50% grey layer.

More junk about this:
Hacking H/S for CB in Lab (http://www.photoshoptechniques.com/forum/showthread.php?t=16806)
Still a bit strong on the Strokerese, but should be more than enough to get you playing.

I could talk your ear off when it comes to colour balance and manipulating tones (adding, subtracting, whatever). But I'll spare ya.

edit:

Some minor discreptencies about what I've said here and what I said almost a year ago at PST. Functionally the same and shouldn't be too hard to sort out.

Stroker,
Perfect. Thanks a jillion. Great stuff. I'm off to give it a workout.
dc