Hi,

On Fri, Feb 15, 2013 at 1:45 AM, Lowell PBS4U wrote:

On the Argyllcms list Gerhard Fuernkranz wrote “Btw, regarding gimp, I remember that in the past old versions of the gimp print plug-in did fiddle with the colors during printing, i.e. the DeviceRGB numbers sent to the printer did not match the RGB numbers in gimp's working space. I don't know, however, whether this still applies to recent versions.”

So I thought I'd answer this question, with your help. Does Gimp still “fiddle with the colors during printing”?

Actually I think the problem you are refering to is more that printers do not use RGB at all. The most common color space for printers is CMYK: http://en.wikipedia.org/wiki/CMYK_color_model RGB on the other hand is done for screen display. And so the answer is that unfortunately there is still no CMYK support in GIMP. You can't create a CMYK image in GIMP. This means that when you send the RGB image to your printer, a conversion is done to the other color space, but this conversion is not perfect (basically the result will change with the material).

So that means that GIMP is good for all work made to stay on screen (web, animation, video, etc.), but less for printed work.

Note that for printed work, even if you have a software with CMYK support, your screen is still RGB (so now the conversion is done on display, same problem), and if your printer is not well calibrated, it may still be different from what a professional printer will print. That means that having CMYK support is a start, but that is only one single link in a bigger chain of failure.
That also means that it is a problematic for professionals designers or alike, who will also afford additional material and/or effort to calibrate their screen, scanner, printer, etc. For most basic users, this support is not as useful in reality as most people have badly calibrated material.

But yes, that would still be good when GIMP will support CMYK.

Jehan

PbS4U@hotmail.com

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On Sat, Feb 16, 2013 at 11:14 AM, Jehan Pags wrote:

So I thought I'd answer this question, with your help. Does Gimp still fiddle with the colors during printing?

Actually I think the problem you are refering to is more that printers do not use RGB at all. The most common color space for printers is CMYK: http://en.wikipedia.org/wiki/CMYK_color_model RGB on the other hand is done for screen display. And so the answer is that unfortunately there is still no CMYK support in GIMP. You can't create a CMYK image in GIMP.

Jehan, this is irrelevant to the question :)

The question is whether GIMP does the correct color space conversion and, as a matter of fact, whether it should do it at all, which depends on the kind of printing plug-in in use.

If this is about the Gutenprint plug-in, then any wrongdoings are most likely on its side.

If this is about the printing via stock GTK+ dialog, then it depends on the version of GTK+ and operating system.

Alexandre Prokoudine http://libregraphicsworld.org

Hi,

On Sat, Feb 16, 2013 at 9:22 PM, Alexandre Prokoudine < alexandre.prokoudine@gmail.com> wrote:

On Sat, Feb 16, 2013 at 11:14 AM, Jehan Pagès wrote:

So I thought I'd answer this question, with your help. Does Gimp still “fiddle with the colors during printing”?

Actually I think the problem you are refering to is more that printers do not use RGB at all. The most common color space for printers is CMYK: http://en.wikipedia.org/wiki/CMYK_color_model RGB on the other hand is done for screen display. And so the answer is

that

unfortunately there is still no CMYK support in GIMP. You can't create a CMYK image in GIMP.

Jehan, this is irrelevant to the question :)

Oups ok. My bad then. :-)

Jehan

The question is whether GIMP does the correct color space conversion and, as a matter of fact, whether it should do it at all, which depends on the kind of printing plug-in in use.

If this is about the Gutenprint plug-in, then any wrongdoings are most likely on its side.

If this is about the printing via stock GTK+ dialog, then it depends on the version of GTK+ and operating system.

Alexandre Prokoudine http://libregraphicsworld.org
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On Sat, Feb 16, 2013 at 4:22 PM, Alexandre Prokoudine wrote:

If this is about the Gutenprint plug-in, then any wrongdoings are most likely on its side.

Actually, after another reading of the original mail I think I made a rush judgement. It's quite possible that GIMP is sending incorrect data to the printing plug-in. I'm not sure if Elle looked at this part of the color management implementation in GIMP during her work last year. If that's indeed what's happening, it would be great to fix this in 2.10.

Alexandre Prokoudine
http://libregraphicsworld.org

Hi Alexandre, All,

I didn't touch any of the CMYK stuff. I did write up some additional code that can use the lcms plug-in to convert between RGB and Grayscale, as a kind of practice exercise for converting between RGB and other types of color spaces. But I haven't submitted it as a patch because at present Gimp can't actually color-manage the display of Grayscale images (color-managing the display isn't done by the lcms plug-in).

The Grayscale-RGB code was easy to write because there is already code in the lcms plugin to detect whether an image is RGB or Grayscale. Unfortunately the CMYK code is not part of the lcms plugin. If anyone wants to work with me on this issue, I can write up the conversion code.

It might be easier if all the conversions between different color spaces is handled in one place, preferably the lcms plugin itself. Then it would be very easy (a lot of code, but straightforward to write) to add conversions between LAB, XYZ, CMYK, RGB, and Grayscale.

Elle Stone

On 2/16/13, Alexandre Prokoudine wrote:

On Sat, Feb 16, 2013 at 4:22 PM, Alexandre Prokoudine wrote:

If this is about the Gutenprint plug-in, then any wrongdoings are most likely on its side.

Actually, after another reading of the original mail I think I made a rush judgement. It's quite possible that GIMP is sending incorrect data to the printing plug-in. I'm not sure if Elle looked at this part of the color management implementation in GIMP during her work last year. If that's indeed what's happening, it would be great to fix this in 2.10.

Alexandre Prokoudine
http://libregraphicsworld.org
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