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This probably happens to all of us at one time or another, but it's still embarrassing.

Recently I suggested pasting a CMYK file's black channel into the same file in RGB mode. I've just discovered a note in my files stating that this novel technique was originally suggested by Lee Varis. I apologize to Lee for failing to give him credit for it. It works so well that I can't help but regret that it wasn't mine, but the truth of the matter is that I probably wouldn't have thought of it myself. Have to be more careful in the future.

Howard Smith

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Hi Howard,

Grateful if you could elaborate a bit: what's the purpose of doing this? What are the steps involved?

Mark Segal

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Here you go, Mark! My original post, which I can no longer find, probably didn't explain it very well. The details of the method are copied from my current notes. A little background information, though. For several years I've converted from RGB to CMYK and back again just to be able to do some editiing that was best done in a file that had a black channel. It was a nuisance, but until Lee's post I didn't know any other way besides using Calculations with the RGB file. I'm a bit impatient, and that was relatively time consuming, so it never appealed to me. Lee's method is both quick and effective.

You can get the Black channel from the CMYK file to the RGB file in a couple of ways. You can use Apply Image or you can copy the Black channel and paste it into the RGB file. Either way you probably want to use a blank layer into which you can paste it. The result is not going to be pretty, but then you haven't done anything with it yet. Change the blending mode (in a quick test tonight I used Multiply mode) from Normal mode because Normal mode gives a misearable appearance to the image. After changing the blending mode to whatever works best for your image, you also have to adjust the opacity of the Black channel copy to get the final result that works best for you. You no doubt know-but others may not-that a channel is just a grayscale image that does not respect color spaces. You can copy it from one to the other with impunity and without doing any converting.

Everyone looks at things differently. If my explanation leaves you with questions, don't hesitate to contact me off line for clarification. If you are critical of the method, or if you can find a way to improve on it, you might want to post your criticisms and comments for all of us. As everyone here is probably well aware, criticism means nothing to me except when it corrects my thinking or otherwise adds to my knowledge.

Copied from my current notes:

From a suggestion by Lee Varis on the ColorTheory forum: (date not recorded)

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Howard,

Thanks - your explanation of what to do is fine. I also discovered that Dan has an extensive illustrated treatment of this subject in Professional Photoshop 5th, Chapter 10 from page 234.

Mark Segal

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I sometimes convert the RGB file I work on to a short-black CMYK profile, copy the black channel, go back in history to RGB, paste the black channel in Quickmask mode, exit Quickmask and make a curve adjustment layer for the black point. (This way some of my RGB files have an adjustable "K-like-channel" on a layer : )

Laurentiu Todie

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Although the black channel by itself can be coaxed into providing some benefit using layer apply modes the simplest thing you can do is to drag the CMYK document back into the RGB original and then change the layer mode to luminosity. The CMYK document presumably has had black channel editing to enhance shadow detail and sharpening but converting to CMYK generally reduces the color gamut. Simply applying the new CMYK luminosity over the original RGB color can give you the best of both worlds. When you drag the whole CMYK document into the RGB document just accept the profile conversion back to RGB – change the layer apply mode to Luminosity and you're done.

regards,

Lee Varis

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You can also create a black channel by applying the red channel and then the blue channel onto the green channel using screen mode converting the result to greyscale and pasting it into the RGB original. This will give you a skeleton black which can be used in overlay mode or luminosity mode to solve density or contrast problems.

Since there seems to be a concern these days about not giving credit to the person who originally suggested this idea, I will give the credit to Dan.

Regards,

John Denniston

www.johndenniston.ca

www.dirtbikephoto.com

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The idea is that if you think you know what kind of K will be used in the final conversion to CMYK, but you're not the one doing the conversion (for political reasons or… "workflow"), the negative of that K can be used as a mask in RGB to make black point with curves.

[I'll give myself credit : ) ]

Laurentiu Todie

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First of all, thank you, Mike Russell for your sensitivity in sharing your images of the tribute to Bruce Fraser. You make me sorry I never had an opportunity to meet the man.

And my sincere thanks to Mark Segal, Andrew Webb, Lee Varis, and John Denniston for sharing their techniques and ideas with all of us. Putting all of this together will enable each of us to develop a solution that works best for our workflow and our images.

The comments made by Lee Varis helped me see something that should have been obvious but slipped right past me. When one uses Apply Image to paste the copied Black channel into a blank layer above the RGB composite, there is a noticeable difference in the luminosity of the copied channel. With the one image that I used for a quick evaluation, the Black channel becomes darker when used in the RGB file. This can be avoided by using Ctrl-A to select the entire channel, then Ctrl-C to copy it before using Ctrl-V to paste the copy into a blank layer above the RGB composite. The result is now identical in appearance to the original Black channel. Mac enthusiasts no doubt recognize that they must substitute Cmd for Ctrl.

The problem we encounter here, no matter how we approach it, is that we probably aren't going to get quite what we're looking for. Lee uses Luminosity mode, I use Multiply mode, and who knows what other modes might work for other images? Since we aren't working on the same image, that's probably why we can't use the same mode. Doesn't really matter because there's a simple solution that enables us to have our way with the Black channel. Place a Curves Adjustment Layer directly above the Black channel copy. Change the Adjustment Layer's mode to Luminosity. Press Ctrl-G to create a Clipping Group with the Curves layer, confining your edits to the channel copy. Now you can adjust it endlessly until you get the contrast you were seeking. This seems to be about as close as one can get to editing the Black channel without working in CMYK mode. You'll still have to experiment with which mode works best for the copy itself.

Those who may object to our taking this relatively minor subject and beating it to death should consider an observation made by an old gold miner patiently panning for gold. "It's just specks that I'm gathering, but at the end of the day they're worth a lot of money."

Howard Smith

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Another interesting approach to try is to paste the black channel into a new layer in the RGB document, run the high-pass filter at suitable parameters (i.e., see what works; I often use small numbers), and set the blending mode of the layer to soft light or overlay (or one of the other overlay-type modes), and set opacity to taste. This actually causes edge-level lightening based on the content of the black channel, creating an effect of sharpening and clarifying the black channel content without the darkening described in other methods.

Actually the ramifications of using any pre-built or constructed channel as a source for high-pass sharpening can make a good case for thinking of high-pass sharpening more often than it tends to be in this forum.

Rick Gordon

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Rick Gordon writes,

I did some testing of this method in preparation for PP5E. Although it gives good results I did not find any image where some simpler way worked as least as well.

One area in which it does seem to offer something hard to get with other methods is in *unsharpening* an image. Sometimes there is a lot of action going on in the background that distracts the viewer's attention from the object that we want to emphasize. This happens a lot when the background has a lot of natural greenery, for example.

In these cases people sometimes select the background area and blur it. This usually results in the appearance that the foreground object has been cut out and placed back into the image.

I've previously suggested the better method of selecting and blurring the black channel only, which takes focus away more naturally. However, that method is a nuisance if you are working in RGB. Also, I seem to get more convincing results with the opposite of Rick's method--by pasting a copy of a hypothetical black on top of the RGB file, running High Pass at, say 10 pixel Radius, and then *inverting* the result before changing the layer to Overlay mode and reducing its opacity.

Doing this robs the affected areas of shape in a natural way--it just seems to unsharpen them, without making it obvious what has happened, as a blur often would.

Dan Margulis