CMYK Channel Separation - Odd Result - How Do It?

[fkjt79]

First: I really dig IM! Powerful tool! Having big trouble with this issue:


With this command I would expect all image information separated to CMYK channels:

convert test_black_grey_blue.png -colorspace cmyk -separate test_bgb_%d.jpg

The result is four images but the black channel is different from what I expect.

Example image here: http://qr-code-scanner.com/test_black_grey_blue.png
version: ImageMagick-6.7.6-Q16, windows32

The manual says:
http://www.imagemagick.org/Usage/color_basics/#separate
The last image (the 'Black' or 'K' channel) is especially interesting as it appears to be a negated gray-scale image of the original image. In reality it represents the amount of 'ink' a CMYK printer should deposit on the paper, reducing the amount of color needed by the other color channels.

But the black image information is missing in the separated images.

Is there a way to get the black channel, all the greyscale information that is missing from the CMY images?

Converting to CMY works as expected.

[fmw42]

Cannot get to your link at http://qr-code-scanner.com/test_black_grey_blue.png

convert test_black_grey_blue.png -colorspace cmyk -separated test_bgb_%d.jpg

that should be -separate not -separated

[anthony]

It works for me. I get four images numbered 0 .. 3 the '3' is the black image.

[fkjt79]

Cannot get to your link at http://qr-code-scanner.com/test_black_grey_blue.png

convert test_black_grey_blue.png -colorspace cmyk -separated test_bgb_%d.jpg

that should be -separate not -separated

thanks, corrected.

[fkjt79]

It works for me. I get four images numbered 0 .. 3 the '3' is the black image.

I had forgotten to upload the test image, did now.

are you sure it really is the black image and not something else?

my #3 looks like this: http://qr-code-scanner.com/test_black_g ... cmyk_3.jpg

it says only "blue" but it should say black, too. none of the output images says black...

just downloaded the latest version v6.7.6-5 but it did not help.

[anthony]

The fact that the channel image has color means it is NOT a channel image - It was not separated from the original!!!!

[fkjt79]

The fact that the channel image has color means it is NOT a channel image - It was not separated from the original!!!!

what you say makes 100% sense to me. but I just did it again with the same result.

convert test_black_grey_blue.png -colorspace cmyk -separate test_black_grey_blue_cmyk_%d.jpg

am I missing something? either I or IM is going nuts

[anthony]

Looks like something has gone screwy. I am now seeing cyan colors in my Black channel images
Arrgghhh....

Code: Select all

convert rose: -colorspace cmyk -separate  +append show:

Reporting this are a serious bug...