[argyllcms] Re: 0.60 CMYK profile misshaped

  • From: Gerhard Fuernkranz <nospam456@xxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Wed, 06 Dec 2006 23:09:21 +0100

Graeme Gill wrote:
> Burckhart Seifert wrote:
>
>> After analyzing the result I have decided to profile the new measure 
>> file again with Argyll CMS 0.53 before delivering the profile. The 
>> reasons are:
>> The preview of 0.60 looks very misshaped in ProfileEditor, in VRML
>> it  looks very different to the preview of the same file calculated
>> with  0.53 and I have some results out of tolerance in some patches
>> of the  MediaWedge.
>
> The resulting profiles from the data you provided don't look
> that different in VRML. They are a little different in behaviour,
> but not grossly so.
>
> Here's the results I get putting 10000 random device values through
> the two profiles A2B tables, between V53 and V60:
>
> Verify results:
>   Total errors (CIEDE2000):     peak = 5.193954, avg = 1.144390
>   Worst 10% errors (CIEDE2000): peak = 5.193954, avg = 2.585372
>   Best  90% errors (CIEDE2000): peak = 2.057085, avg = 0.984281
>
> The v53 result is a bit suspect in any case, because the smoothing
> level was grossly low, and the profile fitting was stopping
> prematurely. It did mean that it better accommodated strange
> behaviour from a device, at the cost of poorer smoothness.
> Given that, the results with V60 do have quite a large self
> fit error (peak 10.5, average 1.93 delta E).

My feeling is that for this particular measurement set the default
smoothness of V0.60 is rather too high for best accuracy (w.r.t. to data
not part of the training set), but on the other hand the device behavior
looks a bit strange anyway; possibly the device is not well linearized.
E.g. for the cyan channel, the max. C* is reached at approx. 70...75%
cyan, and the C* vs. hue plot of the channel also shows a hook with a
_aharp_ bending beyond this limit, while the hue is nearly constant up
this limit. Sometimes it is possibly better, not to use the channels up
to 100%, but to sacrifice a little bit of the max. possible gamut in
favor of achieving more well-behaved overall device characteristics
(i.e. to establish a calibration which eventually maps/reduces 100% cyan
to say 70...75% cyan in this case). The magenta channel seems to be
saturated at approx. 80% too, the yellow and black channels IMO look fine.

Btw, Graeme, I'm wondering whether it might possibly be useful, if
profile would compute per channel calibration curves for the CMYK
channels and use them as prelinearization tables, if single-channel
C,M,Y,K ramps with reasonable resolution are present in the
measurements? (instead of (or alternatively in addition to) the
shaper/matrix/shaper optimization which is done currently)

> Most of the profile fit errors seem to be near cyan == 100%.
> Something funny seems to be going on in that channel at least, as
> it approaches 100%. Is there a calibration system on the RIP ?
> How are the calibration curves targeted ?
>
> The chart is very regularly arranged,

Is this possibly an ECI chart?

Regards,
Gerhard

> which is often not very
> efficient (that's why Argyll generate charts are different).


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