CODEWITHSUNDEEP
image-processingimagemagick
cg433n
cg433n

Reputation: 729

ImageMagick: Promote grays to CMYK black?

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.

Upvotes: 3

Views: 992

Answers (2)

Mark Setchell
Mark Setchell

Reputation: 207873

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".

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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.

Upvotes: 1

Kurt Pfeifle
Kurt Pfeifle

Reputation: 90305

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.sstatic.net/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.

Upvotes: 0

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