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Nondestructive Channel Operations

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Old 01-14-2003, 11:19 PM
Andrew B.'s Avatar
Andrew B. Andrew B. is offline
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Nondestructive Channel Operations

Methods for channel blending usually involve destructive methods such as Apply Image or Channel Mixer. But there is another method that is not only nondestructive, it also allows you to use the power of layers when dealing with channels. Here's how it works:

Let's say you have a dirty blue channel, and you think blending in part of the green channel might fix it.

1. Go into Channel palette. Select and copy the green channel. Go to the layer palette. Paste. Now you have the green channel's "grayscale" sitting on top of the Background layer of your image. Activate the Blending Options menu for the "grayscale" layer. Under Advanced Blending|Channels, uncheck the boxes for "R" and "G." Click okay.

2. What you see on your monitor is the image with the grayscale of the green channel substituting for the grayscale of the blue channel. And as you lower the opacity you blend in more of the blue channel and less of the green channel, until you get it to where you want it.

3. Now, maybe you want to see how the L channel works here instead. You take it from a second copy of your picture, put it in there, and set its advanced channel blending to "B" only. Now you click back and forth between the L layer and the G layer to see which one works better. And because it's too light, you add an adjustment layer, set its advanced blending to "B" only, and reduce lightness. All in a nondestructive way.

And it doesn't end there. Add the fact that you can use layer masks and blending modes. And you can use this to hand-retouch a channel without wrecking it. Just make a copy into a layer, have it feed into the original channel, and then brush away. You see the changes live, and never touch the actual channel.

The amount of flexibility this brings to channel operations is mind-boggling. But it has not been tested much. So have at it, and report back how it works for you.

BTW, I did not invent this technique. It was suggested to me during a conversation with a user named Witkacy over in the Photoshop featurerequest group. I immediately began testing this and I'm posting everywhere I participate so others can try this out.
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Old 01-15-2003, 12:38 AM
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d_kendal d_kendal is offline
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Hi Andrew, excellent and very interesting tip, big thanks. I'll definitely have to do some experimenting with this one, sounds very promising.

- David
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Old 01-15-2003, 12:38 AM
hanuman hanuman is offline
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I've just read your post and getting it sorted out in my head, haven't tried it yet and will have to be offline before doing it. I got a bit lost between 2 and 3. Steps 1 and 2 in RGB mode and 3 in LAB mode, that's right isn't it?

I've been reading quite a bit about working in LAB mode recently and Dan Margulis seems to be one of the LAB mode's great advocates. My introduction to LAB was in his book Makeready a few years ago. Something in one of his articles also seems vaguely familiar to this, or maybe it was an article on CHOPS by somebody else, my mind is in Channel Blended Mode!

I've also been following a thread on the Color Theory List on sharpening options by RTP member Stephen Marsh and have to try it as well.

- Mind-boggled Hanuman
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Old 01-15-2003, 02:00 AM
Andrew B.'s Avatar
Andrew B. Andrew B. is offline
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Hi Hanuman.

What I posted was just a "for instance" of the types of situations where this method might be superior to other methods, primarily because it is easier to preview and make changes. But the entire operation is done in RGB, with the exception of obtaining the the L channel for use in RGB mode. And maybe the easiest way to run the steps is to get this first. So try this:

1. Open your picture. On the menu, click Image|Duplicate. You now have a second copy of the image in your work space. Go back to the menu and click Image|Mode|LAB. Go to the Channels palette and click the Lightness channel. From the menu click Select|All. Then Edit|Copy. You are done with this second copy of the image, so just close it without saving.

2. Click on the Layers Palette. Edit paste. Rename this new layer "Lightness Channel" Go the channels palette. Click the green layer. Select all. Edit, Copy. Click the top "channel" so all three are activated again. Go back to the Layers palette. Edit, Paste. Rename this layer to "Green Channel." Click the eyeball icon on the Lightness Channel layer to turn it off. You are now ready for your first step on the Green layer.

BTW, even though it is pretty much a "no no" to use a Brightness/Contrast layer to adjust a photo, this is the kind of layer one might use to adjust a channel. So when you get to this step, try this kind of adjustment layer. Also, you might have to use two of these to bring the Lightness Channel down far enough.
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Old 01-15-2003, 05:46 AM
dcarr dcarr is offline
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That is a good technique. Check out the tutorials section for more channel information as well as some other great tutorials.
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Old 01-15-2003, 09:22 AM
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Ed_L Ed_L is offline
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Thanks for posting that Andrew. Just another of Photoshop's many great ways (that I didn't think of) of doing things.

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Old 01-15-2003, 10:44 AM
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DJ Dubovsky DJ Dubovsky is offline
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Big thanks for the cool tip. I always love new ways of doing things especially ones that are not distructive.
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