[argyllcms] Re: Collink and device link profiles

  • From: Michael Schulz <ms.typografik@xxxxxx>
  • To: <argyllcms@xxxxxxxxxxxxx>
  • Date: Fri, 08 Jul 2011 12:14:34 +0200

Hello,

it was not in my intention to blame Argyll producing colour casts, the
description "colour cast" might be the wrong word - it's the impression of a
warmer looking result. If you might take a look at the channels of the test
images (with photoshop) you can see and measure that there is another mixing
ratio of cmyk between input (ISOcoated_v2_testimage.tif) and output
(DLconv_ISOcoat_v2-ISOuncoat_kt_fk.tif): more amount of yellow in the output
than in the input. I'm trying to understand what happens respectively what
I'm doing wrong or what I could do better...

The command line of the example I used was:
collink -v -qh -G -kt -fk -cpp -dpp ISOcoated_v2_eci.icc ISOuncoated.icc_eci
DL_ISOcoat_v2-ISOuncoat_kt_fk.icc

Best regards
Michael Schulz


Am 08.07.2011 10:47 Uhr schrieb "Graeme Gill" unter <graeme@xxxxxxxxxxxxx>:

> Michael Schulz wrote:
>> thank you for the reply and for the indication how to check the result with
>> cctiff. In the meantime I did more trials with collink's device link option.
>> What I found out is, there is a slight yellow colour cast in the converted
>> images (rgb2cmyk and cmyk2cmyk) - I misunderstood this as an over
>> saturation. It becomes visible in skin tones and noticeably if one compares
>> the yellow colour channels of images that have been converted across the PCS
>> with device link conversions.
> 
> Hi,
> it's not possible to say why you see this effect without
> being able to exactly duplicate your situation, including devices and
> viewing environment. I'm not aware of any tendency of Argyll device
> links to have any color cast - they merely do what the profiles tell
> them to do. Different intents and profiles will interact in different ways.
> 
>> I don't understand where it comes from but I assume that it might be the
>> black ink generation and not the gamut mapping. I tried -ke, -kt, -kx, -kr
>> and -kh; beside of the yellow colour cast -kt gives the best results. Does
>> it makes any sense trying a black ink generation with the -kp... flag?
> 
> It's unlikely to be black ink settings because it's "closed loop" - unless
> you are outside the devices gamut, you always get an accurate color value as
> best
> the device profile can describe the device behaviour. If it does appear to
> have an effect in a real world viewing situation then you may be experiencing
> metameric failure due to your illuminant not matching what you've told
> Argyll. (Note that by default you get D50 spectrum, the ICC standard, and
> there are almost no real D50 spectrum viewing booths).
> 
> You aren't choosing a particular black generation if you are using -kt -
> you are letting the incoming CMYK choose it. If you want to choose a
> particular
> black generation then yes, you need to use -kp and choose appropriate
> parameters, as outline in the tutorial:
> <http://www.argyllcms.com/doc/Scenarios.html#PP6>
> 
>> What I don't understand in your answer is the last paragraph - why do one
>> gets multiple gamut mapping transformations? I thought with device link
>> profiles there is only one B2A table used?
> 
> But if you were to use a perceptual A2B table to "preview" the
> result of a device link transform, then you may be applying a reverse
> gamut mapping in the process (depending on what software made the
> device profile). This can result in an over saturated looking result.
> 
> Graeme Gill.
> 



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