[argyllcms] Re: Strange contrast clipping during calibration

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 23 Aug 2012 10:24:39 +1000

János, Tóth F. wrote:
> Changing the luminance targets to "native" didn't change anyting.

No surprise there - the white point needs to be native for the algorithm
to be used that targets a dynamic 100% in all channels. A non-native white
point targets a calculated white point value.

The puzzle is why the target that is computed from the additive model
doesn't actually lie close to the gamut surface (ie. doesn't have one channel 
at 100%).
In the previous log you sent me, you'll notice that the clipped value was 
expected to
have an RGB value of 0.886559 0.823500 1.000000 for a white point target of XYZ
83.058731 87.390821 95.159898, but the eventual RGB to meet the white
target was 0.9106000 0.8348100 0.9275700. So something is out of
kilter with the initial model and the actual device response.

It's possible that this is due to the inaccuracies of the rough models device
curves. I will change it to just use the matrix to compute the gamut
boundary clipping, as this may solve the problem. You can try this out
using <http://www.argyllcms.com/disptools.win32.zip>

Graeme Gill.

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