[argyllcms] Re: Wrong gamma reading with Spyder 3.

  • From: Florian Höch <lists+argyllcms@xxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 09 Nov 2010 19:35:18 +0100

Hi,

Am 09.11.2010 19:01, schrieb Graeme Gill:
Darzur wrote:

It shows, that despite of agreement shown in above report (0.2% error
fo 50% mark) LUTs were corrected by 5% down at 50% mark.

But you don't indicate where this curve comes from. What are the axes ?

It's just a plot of the values in the .cal file (but scaled to 0...255 instead of the Argyll range of 0.0...1.0).

So image was
darkened by 5% while it should reamain almost unchanged (due to 0.2%
error). This report (with 0.2% error) was done on uncalibrated monitor
just before calibration and curves shown above are a result of this
calibration. It's strange for me that correction to LUT's is so big
(5% darker at 50% mark) while it should be almost nonexistent (0.2%).

Hmm. I'm not really sure if there is a problem, or if any problem
is really fixable. Because the display has not been adjusted
at that stage, it's possible that the targets are approximate, and that
therefore the 50% target during calibration is slightly different
to the one computed during the actual calibration. Since there is
lot of interaction amongst the target values (ie. white level
may change due to the white point to keep it in gamut, and
then this will change the target curve, and even more complex
interactions with regard to the black points influence on
the target curve), I'm not sure that there is actually anything
wrong with this. I would need to look into things in a lot
more detail to determine how it actually works. If the result
is too dark, reduce your target gamma.

I think the whole 'issue' is that a too low quality (dispcal -qv) was used in the creation of the .cal file.

Regards
--
Florian Höch


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