HELP Color error using cmyk

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HELP Color error using cmyk

rich404
Ive had this issue in 2.10.0-2.10.10 and I  really need help:
The cmyk color I apply is not what is detected when I recheck with color picker.

I MUST use cmyk as i'm modeling for an inkjet colored 3d printer.  (inkjets use
c,m,y,k colored inks) I'm trying to make a skin color that works (printer color
is also all off) so I Input 34,37,56,00 into cmyk, fill the selection with it
then use color picker and what I get is WAY off -> 0,4,33,34.  I cant handle ANY
black so this totally screws up the printed color.  Any ideas how to make gimp
use the color I select and not change it??

TIA

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Re: HELP Color error using cmyk

Liam R E Quin
On Tue, 2019-07-02 at 04:57 +0200, walshlg wrote:
> Ive had this issue in 2.10.0-2.10.10 and I  really need help:
> The cmyk color I apply is not what is detected when I recheck with
> color picker.

This is really about the colour picker, and which one you are using,
not about CMYK. It's doing this for me too, although i'm not sure why.

But to work with CMYK you might want to consider using separate+, or
working with Krita, which supports CMYK images directly (and i imagine
can export as a CMYK tiff although i don't know for sure).

> I MUST use cmyk as i'm modeling for an inkjet colored 3d
> printer.  (inkjets use
> c,m,y,k colored inks)

You are editing an RGB image in GIMP, not a CMYK one. it doesn't matter
which colour picker you use, you'll get an RGB colour.

Converting colours from RGB to CMYK for print can be done in multiple
ways (e.g. with undercolour removal to save ink).

I agree the colour selector is misbehaving, though.

Liam


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Re: HELP Color error using cmyk

Liam R E Quin
In reply to this post by rich404
On Tue, 2019-07-02 at 04:57 +0200, walshlg wrote:
> Ive had this issue in 2.10.0-2.10.10 and I  really need help:
> The cmyk color I apply is not what is detected when I recheck with
> color picker.

Actually, after reading th code, what is going on with the colour
selector is that it converts to rgb and back again, and the conversion
is uses is a valid one - you get the same resulting theoretical colour,
if you ignore the way inks mix on paper. In fact changing the amount of
black is often visible, but effectively GIMP (actually BABL) is
subtracting the black from the other colours, which saves ink.

34,37,56,00 is the same colour as
 0, 4,33,34
and you can see this by experimentation.

When GIMP has native CMYK image support, it will need to make
undercolour removal optional, but for now, since the RGB values are the
same and that's all that's stored in the image data, you're stuck with
it.

Liam

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HELP Color error using cmyk

rich404
In reply to this post by rich404
>Ive had this issue in 2.10.0-2.10.10 and I  really need help:
>The cmyk color I apply is not what is detected when I recheck with
>color picker.
>
>I MUST use cmyk as i'm modeling for an inkjet colored 3d printer.
>(inkjets use c,m,y,k colored inks) I'm trying to make a skin color
>that works (printer color is also all off) so I Input 34,37,56,00 into
>cmyk, fill the selection with it then use color picker and what I get
>is WAY off -> 0,4,33,34.  I cant handle ANY black so this totally
>screws up the printed color.  Any ideas how to make gimp  use the
>color I select and not change it??
>
>TIA

I can see the problem.

No solution that I can see. Post a bug report at

https://gitlab.gnome.org/GNOME/gimp/issues/


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HELP Color error using cmyk

rich404
>I can see the problem.
>
>No solution that I can see. Post a bug report at
>
>https://gitlab.gnome.org/GNOME/gimp/issues/

Not only Gimp

A little bit digging and the change in values also happens with online
convertors. I tried a couple, both the same.

CMY(k) 34 37 56 00  

converts to RGB 168 161 112

converts back to CMYK 0 5 33 34

The only suggestion is use Krita and start off in CMYK colorspace. The color
picker there keeps the original values.

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Re: HELP Color error using cmyk

Robert Krawitz-2
On Tue, 02 Jul 2019 14:38:49 +0200, rich404 wrote:
>>I can see the problem.
>>
>>No solution that I can see. Post a bug report at
>>
>>https://gitlab.gnome.org/GNOME/gimp/issues/

It's not a bug -- there's no way to reconstruct the missing
information after it has been transformed from four down to three
dimensions.

> Not only Gimp
>
> A little bit digging and the change in values also happens with online
> convertors. I tried a couple, both the same.
>
> CMY(k) 34 37 56 00  
>
> converts to RGB 168 161 112
>
> converts back to CMYK 0 5 33 34
>
> The only suggestion is use Krita and start off in CMYK colorspace. The color
> picker there keeps the original values.

Yep.  Once you've transformed from CMYK to RGB you've irretrievably
lost that extra information.
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