Converting RGB -> CMYK and black color

[sheVcza]

Hi,

we try to convert image via imagemagick from RGB to CMYK, but we need to split black into the other channels (CMY), not only into K channel. In the past, this function could do "undercolor", but today not. How can we solve this problem please?

Thanks!

[anthony]

Why not just convert the image to CMY. that is equivelent to a negated RGB image,

though you may need to remove any gamma or sRGB non-linear effects first.

Perhaps you need to explain 'putting black in the other channels'

[sheVcza]

Convert to CMY is not right solution for us. We need convert images for printing.

Perhaps you need to explain 'putting black in the other channels'

If we convert image via ImageMagick, in black color we get this values:
C: 0% | M: 0% | Y: 0% | K: 99%

If we convert image via Photoshop (for example), in black color we get something like this:
C: 75% | M: 68% | Y: 67% | K: 89%
- and this is what we need. It is possible to reach such result in the ImageMagick?

[anthony]

That does not make sense, even with converting a image containing a qamma factor or sRGB colorspace, into linear space.

The IM output is basically black, but the photo shop has a mix of colors. EG grey verses color. totally different.

Also Normal CMYK typically results in one of the color channels being zero.

Do you have an example image of that color? A single pixel would be fine!

[sheVcza]

There is a small example images:

Original RGB: http://www.vyzj-art.cz/im-test/raven_head_rgb.jpg
CMYK converted via Photoshop: http://www.vyzj-art.cz/im-test/raven_head_cmyk_PS.jpg
CMYK converted via ImageMagick: http://www.vyzj-art.cz/im-test/raven_head_cmyk_IM.jpg

Just open it in Photoshop (or other graphic program) and see the difference in black color.

Black color only in K channel is not good for professional printing. That black is'nt enough trustworthy.

[anthony]

When I display the three images in my browser (the first and last images are identical!)
That to mean means IM did the right thing.

However it may also mean that the CMYK was handling was not configured properly in the JPEG images presented.
In fact they may not have the right 'colorspace' profiles defined. As such teh IM jpeg is was probably converted back to RGB before saving, while the Photoshop JPEG is missing the right colorspace profile.

Of course with 4 channels there is a whole range of color division that should (in theory) be equivalent to the original RGB (or in this case sRGB) image. There is also a question of if CMYK should be linear, where your sRGB input image is not.
In this respect IM is NOT doing the right thing, but that could NOT account for the photoshop deciding to use more color ink than black ink.

I just don't know enough to comment further.

Anyone else like to comment?

[sheVcza]

When I display the three images in my browser (the first and last images are identical!)
That to mean means IM did the right thing.

That DOESN'T mean IM did the right thing. You can't see the differences in browser - you must open them in graphic editor, like Photoshop.

So, here is explained what we need (taken from internet):

There's another complicating factor - the black plate. As we've seen, this is really an added extra to the main CMY primaries which, in an ideal world, would combine together to produce black. One of the advantages of this is that equal percentages of cyan, magenta and yellow can be shifted onto the black plate which helps cut down on the possibility of over-inking while giving extra control over the shadows and tone in an image. There are two main options here the first called "undercolour removal" (UCR) where black ink is used to replace equal amounts of CMY only in neutral gray areas and "gray component replacement" (GCR) where they are replaced even in coloured areas. Another advantage of the dedicated black plate is that you can produce richer blacks say translating R0 G0 B0 to produce a "warm black" with added magenta and yellow or, say for a polar image, a "cool black" with extra cyan.

When a very dark area is desirable, a colored or gray CMY "bedding" is applied first, then a full black layer is applied on top, making a rich, deep black; this is called rich black. A black made with just CMY inks is sometimes called a composite black or process black.

So, I hope you can now understand what we exactly need. I think IM used the GCR method, but we need UCR. So, question is simple - can a IM convert image with UCR method or not?