Photoshop: Convert part of an image to true greyscale?

[G Oliver]

I was preflighting a newsletter, and the customer asked that a potion of a logo be printed as percentages of black, with no cmy. Of course, the provided PS file was cmyk, and the indicated portion was a build containing almost no black. I spent a few minutes trying a couple of approaches to convert that portion of the image to "true" greyscale, before I gave up, saved a greyscale copy of the whole thing, and pieced the two copies back together in InDesign.

So the immediate problem is solved, but I can't stop wondering if there was an approach I overlooked. Everything I found online was about taking a colored image and making it perceptually grey. A curves adjustment didn't work because there was so little black in the original that the edge dithering was destroyed.

Thoughts?

 

[Stephen Marsh]

In the CMYK file, use the channel mixer set to monochrome, using a mask or even a selection will do the trick (does not matter if there is little or no K in the file). You can use a dominant channel as the “lever” or a mix of say 34c33m33y as an “average” of CMY, or weight the channel values differently, keeping the total to 100 so that overall contrast is not adversely affected.

Of course, if the rest of the image is CMY only or only has a light black component, then there maybe registration issues where the CMY and K plates meet.

Edit: Sometimes a heavy GCR may be more appropriate than K only, for the "black" areas only or for the entire image.


Stephen Marsh

 

[Macmann]

Stephen's method gives you ultimate control over the mixing of various channel values to achieve the desired black portion. A down and dirty method in Photoshop would be to select the area you wish to be black only, cut it and paste it into the black channel. Done. No piecing copies together in InDesign. I'm sure there are a half dozen ways to accomplish this. I've used this quick fix in a pinch.

 

[G Oliver]

Thanks for the responses. I knew there had to be something I had overlooked.

I took both approaches out for a spin. Of course, both worked, but I found that Macmann's quick method only works when there's no alpha channel. When I tried to paste over transparent pixels, PS generated a selection rather than painting anything (CC2012).

Thanks again.

 

[BarbWPP]

Hot darn, I have been needing this method for various fixes for a loooong time. I have tried the "cut all channels then paste into the black channel" method, with mixed results (probably operator error), but the channel mixer should take care of that! (I hope).

 

[Macmann]

Responses from G Oliver and Barb made me curious. I did not have any issue with alpha channels but I did see a problem with a layered file. Cut and paste only seemed to work if the file was flattened. Things that make you go hmmm....

 

[Todd Smith]

I always use Hue/Saturation. Make your selection then open Hue/Saturation, check the colorize box, then pull saturation all the way to the left (zero).

 

[Stephen Marsh]

Instead of copy/paste, the apply image command can be used to “stamp” data between channels and or documents without having the overhead of using the clipboard.


Stephen Marsh

 

[G Oliver]

You're right, Macmann. I was incorrectly using 'alpha channel' to refer to the transparency in a single-layer but un-flattened file.

 

[Macmann]

Great tip Stephen! I've been using Photoshop since ver 1.0.7 and even an old timer can learn something new.