Digital BW, The Print

> From: Steve Kale [mailto:stevekale@...]
>
> I typically use the split channels method to render B&W from
> colour but have recently
> begun playing with the channel mixer.  I guess I need some help
> as to what the sliders do
> - I am simply fiddling in the dark at the moment.  Should the sum
> of the Red, Green and
> Blue slider percentages ideally add to 100?  What is the purpose
> of the Constant slider?  If
> a channel is raised beyond 100% what does this mean? (I found
> that some quite dramatic
> effects can be produced by say raising the red to 200% and having
> -50% for the other two
> channels but it seems that skies can get torn up by doing this.)

In the general case, the Channel Mixer just lets you do any linear mixing of
channels, either to a RGB target (in which case there are three sets of
sliders) or to monochrome. It is really only widely useful in the latter
case, so I can't help wondering why Monochrome isn't the default. Also, the
constant slider is really just there for completeness--it's rarely useful,
either.

And you're right that in general you want the other three values to add up
to 100. For an underexposed image, you might correct for it by making the
sum larger, but in general I think it's best to do that afterward with
Curves or Levels.

And you're right that you can get some really dark IR-like skies by
subtracting green or blue from red.

The main thing to keep in mind is that the Channel Mixer isn't likely to
produce a finished image. It just gives you a fairly simple global way to
mix them, but you still need to apply a curve to get the different zones in
the right place.

I sometimes use split channels, too, in images where I need a very different
mix in different part of the picture. I split them, layer them, and then
paint the layer masks to control the mix. However, for even more control,
duplicate an image, use the Channel Mixer twice to optimize different parts
of the picture, then layer them and paint the layer mask. Or sometimes, I
just use the channel mixer, then use the history brush to paint back some of
the original image, if the mix produces generally pleasing results but a few
clipped highlights here and there.

--

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@...
>
> Thanks
>
> Steve
>
>
>
>
>
>
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