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Replacing a Color Channel

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Old 06-11-2003, 08:24 PM
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CJ Swartz CJ Swartz is offline
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The way I read both examples I should end up with only 3 channels red,green and blue.
Yes, that's the way it works. The method I use is a bit different from Jak's, and Katrin Eismann's, and I don't remember where I read it. I click on the Blue channel in the original, and the Lightness channel in the dupe to select those channels -- just as the tutorials state. Then I do a Select All (Ctrl-A) of the Lightness channel and a Copy (Ctrl-C) of that channel. Then I go to the Blue channel of the original image and do a Paste command (Ctrl-V). The Lightness channel is pasted in the place of the Blue channel.
NOTE: I typed it wrong, Chuck caught my error -- thanks to him for getting me to correct it.

I know the steps in the tutorials (Jak's and Katrin's) must work, but I ended up with the new Alpha channel instead of a replaced Blue channel -- just like you. So I do it this way.

This may often result in a color change, but it is usually one that you can correct with Levels or Curves.

Last edited by CJ Swartz; 06-11-2003 at 08:56 PM.
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Old 06-11-2003, 08:57 PM
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CJ Swartz CJ Swartz is offline
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Is that a select all and copy in the lightness channel and a paste into the blue?? Or am I confused now?
No, Chuck, YOU'RE not confused, but my fingers were when they originally typed the wrong word. It is corrected now...I think.
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Old 06-11-2003, 11:52 PM
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Andrew B. Andrew B. is offline
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If I may, here is an alternate method that I think provides greater flexibility.

1. Duplicate image.
2. Change duplicate to LAB
3. Click on the Lightness channel, Ctrl A, Ctrl C.
4. Back in the original image, paste the Lightness channel as a layer (not a channel) above the image.
5. Double click on this new layer to bring up the Layer Style menu.
6. Under Advanced Blending, uncheck R and G.

What you have now is the Lightness channel doing a nondestructiove replacement of the blue channel. And with this in place, you can dial in the blue channel with great flexibility.

For example, you can adjust the opacity on this new layer and it will give you a mix of the lightness and blue channel, and you will be able to see the result live, as you dial it in. Still not right? Copy and paste the green channel onto a new layer and group with the lightness channel. Now slide the opacity slider of the green layer. This will let you do live previews of blending the green with the lightness to replace the blue, or even just the green.

You can also add one or more adjustment layers, with or without layer masks. And it all works as long as you have everything grouped with the layer that is pointing at the blue channel.

This method can also be used for simply working on a single channel in a nondestructive way. For example, lets say you want to hand touch up the blue channel. Copy the blue channel and paste it as a layer above the picture. Uncheck R and G. Now do whatever touchup you want, use masks, adjustment layers, etc. on this.
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Old 07-24-2003, 04:10 AM
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Doug Nelson Doug Nelson is offline
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Post Replacing a Color Channel

I have learned here at RP to always check my channels as the first step in any retouch job. It's a great tip, so I thought I'd pass along the "how to" of replacing a bad color channel. [details]
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