Color management: colors out of gamut

[J0j0]

Hello!

I'm new to Scribus and DTP and running into trouble with colours that are out of gamut in imported images. I've searched the forum for "color management" and checked the corresponding Scribus wiki page, but I couldn't find a solution to my problem.

What I'm trying to achieve is the following: I'm trying to create print version of a comic I bought digitally (private use). The comic is originally in PDF format and the images have embedded colour profiles: the output to

Code Select Expand

pdfimages -list <file> reads

Code Select Expand

page   num  type   width height color comp bpc  enc interp  object ID x-ppi y-ppi size ratio
--------------------------------------------------------------------------------------------
   1     0 image    1568  2489  icc     3   8  jpeg   no       170  0   300   300  306K 2.7%
   3     1 image    1550  2437  icc     3   8  jpeg   no        13  0   300   300  592K 5.4%
   4     2 image    1550  2435  icc     3   8  image  no        17  0   300   300 2479K  22%
...
('icc' for all images)

From this file, I'm trying to create one with the ISO Coated V2 300% colour profile required by the printer. These are my settings in the "color management" tab:

As a result, all darker colours are shown as out of gamut.

I'm not trying to achieve a print that shows the colours as shown on my screen, I'm looking for a way to suitably convert the embedded colour profiles of the images to ISO Coated V2 300% (I hope I don't have a misconception here and this is possible with the information available to me). I'd much appreciate any pointers as to how to do this. (I'm on Ubuntu 22.04.1 LTS and using Scribus Version 1.5.8 )

Best
- Jo

P.S.: This is only the next step, however, the wiki page "color management" and a web search left me somewhat confused as to what PDF version to export to eventually. Any recommendations?

[Nermander]

I am not very good at color management, but I have read a lot to try to understand it.

Out of gamut means that the color cannot be reproduced by a printer using the selected output profile.

Since you mention the darker colors, my guess is that printing these with CMYK generates more ink than 300% per pixel. If that is the case, brightening the images could maybe help.

Also maybe changing rendering intent may help. If it is a comic book, maybe you do not need to use perceptual (which is mostly useful for photos, if I understand right).

[J0j0]

Thank you for the quick response!

I tried brightening an image within Scribus with Image Effects, however, as a result, the "out of gamut" marker colour was brightened as well ... I totally did not expect this.
 


I can of course edit each image in Gimp, though I'd prefer not to (does Gimp preserve the embedded colour profile? I vaguely remember having issues with creating images with non-sRGB colour profiles in Gimp before)

I also played around with rendering intent, but none of the given options resolve the out-of-gamut issue.

[utnik]

hello jo

depending on your images, this may not be an issue at all.
the 'out of gamut' warning just tells you, that this part of the image will be adjusted to match the export profile.

for images you normally select 'perceptual' as the rendering intent. that means, the whole color range will be adjusted to the range of the output profile in a way, our eye will (more or less) just detect the reduced contrast.
for graphics it is in most cases 'relative colorimetric' – even here the full color range will be adjusted to preserve local contrasts as good as possible.
but there are at least two situations where you should deal with this 'out of gamut' warning seriously.

• if it is not just about a darker tone, but about a color out of the extreme green or blue corners of the rgb space, which will really look different (in the red corner the difference is smaller...)
• in cases where graphical elements need to match with parts of an image (here you have to deal with your colors and the color management in an advanced manner...)

otherwise let do the color management its job (it will take care of the ink coverage and the color conversion to the output profile...) then take a look at the exported .pdf file and decide if there is some need to adjust anything.
if your work is color sensible, you should consider a good print proof anyway. (contact your print shop.)

utnik

[J0j0]

Hello utnik,

Thank you so much for your reply, this was very helpful - I somehow did not realise that the colour management is intended to take care of out of gamut colours as well.

Could you please help me understand what would be considered `extreme green or blue corners of the rgb space'? I understand this as meaning that the G- and/or B-value of the RGB exceed a certain threshold (e.g. anything with G- and/or B-values above 240 on a [0,255] scale).

I checked all images in my file: colours out of gamut are indeed mainly dark colours, however, there are a few colours that are neither dark nor red tones (in HTML notation: f6ec3c, 27bcb2, 5eccf2, 45b75c and 83d4f3). If these fall into the `extreme green or blue corners of the rgb space' category, how could I deal with this - would I necessarily require colour proofing or are there alternative approaches?

It is actually hard for me to tell how important color management is for this project - I did not create the images and my screen might not display them exactly as the author intended them, so I do not insist on exact shades. However, I would like to avoid significant loss of contrast or colours that do not make sense in context, such as a green sky. Since I've run into similar colour issues with a print project before, I'm now trying to do better...

Best
- jo

[a.l.e]

hi jo

you might want to have a look at

https://en.wikipedia.org/wiki/Color_space

and more specifically to



https://en.wikipedia.org/wiki/Color_space#/media/File:CIE1931xy_gamut_comparison.svg

or



https://en.wikipedia.org/wiki/Color_space#/media/File:Colorspace.png

(the second one is more what you want, but it's not so nice...)

some colors are not common to all the RGB spaces (the screen colors) and many colors of the RGB spaces cannot be represented in CMYK (the printer colors).

ciao
a.l.e

[utnik]

hi jo

...help me understand what would be considered `extreme green or blue corners of the rgb space'?

this should be done with ale's post.

...there are a few colours that are neither dark nor red tones (in HTML notation: f6ec3c, 27bcb2, 5eccf2, 45b75c and 83d4f3). If these fall into the `extreme green or blue corners of the rgb space' category, how could I deal with this - would I necessarily require colour proofing or are there alternative approaches?

well, they are not really 'extreme', but when you look at the second color scheme in ale's post, you'll see that they are around the edges of the cmyk space.
if you fear a color issue and don't trust your screen (mine is very roughly calibrated, but it works for my needs...) you should really ask for a proof.

...I would like to avoid significant loss of contrast or colours that do not make sense in context, such as a green sky.

the loss of contrast will be there. the cmyk model of the printer can't handle the same contrast as your screen. but complete color changes shouldn't happen with the colors you listed here.

utnik

[Nermander]

The "image effect" do not alter the underlying image, which is what you would need to brighten the images. As the name suggests, the image effects are applied to the image, they do not alter the image.

[J0j0]

Thank you for your replies!

@a.l.e & @utnik: It took me some time to fully understand your response. I'm familiar with the graphs comparing colour spaces, though so far, I mostly thought of them as illustrative of the range of different spaces. I didn't know how to use these graphs to quantify 'how much' a colour is out of gamut. The key was that I wasn't familiar with xyY colour representation This helped a lot for me to get an idea of how significant the difference to the printed result might be, thanks! Generally, it seems that none of the colours are 'extreme' sRGBs (probably because the author prints some of their work themselves), so I was happy to leave adjustments to Scribus' colour management function. The files are with the printers now and I'm excited to see the result next week!

@Nermander: Thank you for your reply,  I'm aware of the function of "image effects", I checked the wiki when I applied them to make sure I didn't misuse them. However, they do affect the colours in the eventual output, so I would expect that colour management is applied to the colours that result from "image effects", not to the colours in the original image. Is this not the case? Along these lines, I also would have expected the out-of-gamut marker colour to be unaffected by "image effects" (I'd consider the behaviour I see as a bug).

Best,
jo

[utnik]

hi jo

...I'd consider the behaviour I see as a bug

me too.
maybe you could file a report here.

utnik

[J0j0]

Hi utnik,

I checked the mantis bug database for this behaviour - it's actually a known bug since 2006... (ID 0004270). I added a note.

Best,
jo