Replacing a Color Channel

[Jakaleena]

I usually look at the channels first thing.

If I see that most (or a lot) of the damage I'm going to try and repair resides mainly in one channel instead of being spread throuought all three of them, then I replace the channel.

A good example is in retouching challenge #3 - lost weekend. If you download that one and look at each channel, the red channel contains a huge amount of damage that isn't apparent in the green and blue ones.

In less than 2 minutes, by replacing the channel and making a curves adjustment, I came up with this difference - a much better starting point for the job...

[Stephen M]

Some related PDF links of interest:

http://www.ledet.com/margulis/Poetry.pdf

http://www.ledet.com/margulis/Numbers.pdf

Aimed at colour correction - but the mechanics of channel blends/swaps using channel mixer/calculations/apply image are similar.

There is a whole book devoted to these types of Channel Operations (CHOPs) - which I first found in Kai's Power Tips all those years ago...do a search for Photoshop Channel CHOPS if you would like to find out more.

Another eye opener is LAB based retouching for severe restoration - splitting colour from tone can be CRITICAL and LAB does this while RGB/CMYK do not.

http://www.ledet.com/margulis/LABCorrection.pdf

Regards,

Stephen Marsh.

[rosieb]

I can't figure out what I'm doing wrong, but on the picture I'm working on, the blue channel is the problem, too. I thought I could just follow all your directions (which are very clearly written and easy to follow), but there doesn't seem to be any change once I've finished. It still looks as bad as it did before.
What do you think I'm doing wrong?
I really have learned a lot from reading the tuts here, by the way.
Thanks!
Rosie B.

[Jakaleena]

It's really difficult to know why it's not working for you without actually seeing what you're doing. Most of the time, when replacing the channels doesn't work, there's a small part of the procedure that's getting missed.

If you'll post a sample of the image you're working on, I can try switching the channels here and see if I have any sort of problem with it.

[EdwardA]

Hi,

I'm glad I found this place. I have had Katrin's book for several months and this is one of a couple of examples I keep messing up on.

Using PS7 when I move the lighness layer to the blue channel I end up with an Alpha 1 and not a replaced blue channel. I noticed in the example here that (select all ) is added. I used that and when I move the layer I get a red cast onthe photo with an additional lightness layer.

The way I read both examples I should end up with only 3 channels red,green and blue.

Any ideas what I'm doing wrong?

More mess ups to follow.

thanks
Edward A

[CJ Swartz]

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.

[CJ Swartz]

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.

[Andrew B.]

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.

[Doug Nelson]

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]