ImageMagick: Promote grays to CMYK black?

Question

Is there a way to move all gray colors of a CMYK image (e.g. a CMYK .tiff) into the black (K) plate with ImageMagick?

(In Adobe Acrobat Pro, this functionality is labeled: "Promote grays to CMYK black")

Here's an image to experiment with: Example Image

You can view an example of this process on Wikipedia.

If you add an image to test on, I will try and work out an answer. — Mark Setchell, Dec 11 '14 at 11:07
Thank you for updating your post. I have had a read and I presume you are referring to Gray Component Replacement (GCR) as detailed here... http://cmc.printing.org/how-color-separation-with-gcr-gives-you-more-consistent-color/ Before I do too much work, is that what you mean? Also, if GCR is what you seek, do you mean 100% GCR or say 50% as specified in the example? — Mark Setchell, Dec 11 '14 at 15:48
Also, the image above is a JPEG in sRGB colorspace, not a CMYK TIFF. — Mark Setchell, Dec 11 '14 at 15:49

score 1 · Answer 1 · answered Dec 15 '14 at 14:59

Also not a full answer as such, but hopefully useful towards producing one - by Kurt, myself or others. I looked at the Photoshop method of GCR and am adding the characteristic curves that Adobe seem to use for GCR. There are 5 levels, ranging from "None", through "Light", "Medium", "Heavy" and "Full".

enter image description here

I presume the "Light" curve is showing that no black ink is added into the mix till it would be over 50%, and the "Medium" shows the black would have to be only 25% before any gets added, and the "Heavy" shows that only 12-15% of black is needed before black ink gets added into the mixture.

I also add the following reference to assist any other answerers... see PDF here.

Kurt Pfeifle · Answer 2 · 2014-12-11T23:27:03.757

Taking into account that the provided example image is NOT a TIFF (as announced), and does NOT use a CMYK color space (as announced), but is a JPEG using sRGB, here is how you would convert it into a TIFF file using CMYK, where the black channel is used:

 convert                               \
    https://i.stack.imgur.com/HFnCz.jpg \
   -colorspace cmy                     \
   -colorspace cmyk                    \
    cmyk.tiff

To separate out the different colors again and show them as grayscale images each, use these commands:

 convert HFnCz.tiff -colorspace cmyk -channel c         -separate channel_c.png
 convert HFnCz.tiff -colorspace cmyk -channel m         -separate channel_m.png
 convert HFnCz.tiff -colorspace cmyk -channel y         -separate channel_y.png
 convert HFnCz.tiff -colorspace cmyk -channel k -negate -separate channel_k.png

I did output to PNG in order to keep the file size a bit smaller...

Here are the 4 color separations. Top left is C, top right is M, bottom left is Y, bottom right is K:

Top left is Cyan, top right is Magenta, bottom left is Yellow, bottom right is blacK

Update

I made a mistake in my original answer. The -negate command parameter should only be there for the blacK channel.

Hello Kurt. Do you believe this to fully answer the OP's request to "move all grays to the black plate"? Or are you suggesting this is a starting point for further processing? — Mark Setchell, Dec 11 '14 at 22:51
This may only be a starting point for further *discussion*. Because to '***move***' all grays is not possible, when there are no grays in the original (which is RGB) in the first place! What you can do is '***convert***' for each pixel, equal amounts of R, G and B values to respective values of K (either fully, or a certain amount...). — Kurt Pfeifle, Dec 11 '14 at 23:23

ImageMagick: Promote grays to CMYK black?

2 Answers2

Update

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