[argyllcms] Re: A few questions + an idea for spyd2.c (this time even more useless than before!)

  • From: howdy555@xxxxxxxxx
  • To: Graeme Gill <argyllcms@xxxxxxxxxxxxx>
  • Date: Thu, 12 Mar 2009 07:19:39 +0100

> It's not clear exactly what your workflow is, or where these
> numbers come from. Are they from colprof ? How many points
> are in your test set ?
Yes, there are 2000 measurements made by (if memory serves me well - I will 
post the results
of new measurements today or tomorrow [damn "reset to defaults"...]).

targen -v -d3 -e16 -s64 -g64 -f2000 outfile
// my monitor has huge problems with the color cast in dark grey
// areas, hence -g64. -s64 as I want to emulate the Datacolor's
// approach - even if I had made a mistake here, there are still more
// than 1800 other points left for the Argyll to choose from :)
dispcal -v -qh -yl -gs -a80 -e2 outfile
// -a80 because this seems to be the ambient light. I know the monitor
//  is set to be far too dark than necessary but unfortunately I had to
//  make this compromise to be able to use it for everyday's work

colprof -v -D"My display" -k outfile.cal -qu -cpp -dmt outfile
// I know there is -S missing but I was unable to find the swop.icm
//  file... Maybe I should use the photolab's profile here?

> As has been noted, the self fit errors do not give much
> real indication of how accurate a profile, you need independent
> measurements for this.

That is why I will use Nikolay's testing workflow for this. Maybe it
would be a good idea to add this to the documentation?

> You may well find that the self fit numbers get worse as you increase the 
> number of test points used (since the
> profile will be unable to track the noise), even though the accuracy
> of the profile is increasing.

That was exactly what I was observing. For DE=16 the difference
between -qm and -qu was delta DE=2 (DDE? :))

> Generally average errors < 1.0 indicate that you are either fitting
> a very small number of points, or are close to the repeatability
> limits of the device/instrument.

This would also make a good addition to the documentation which does
not tell anything about the sensible range of DE:
"Make sure you check the delta E report at the end of the profile
creation, to see if the profile is behaving reasonably."
Maybe some example numbers could be given here?

>> The original implementation makes a weighted average of 2 XYZ values read:
>> the "fast" one (with a very small integration time) and the "slow" one

> Actually, the current 1.0.3 release code does something different,
The current 1.0.3 release does not support Spyder3 :]...
The 1.1.0 beta or 1.0.4 beta IMHO do as I wrote - see around line 1543
in 1.1.0's Spyd2.c

> Since the sensor readings and XYZ are linearly related (the
> "level 2" correction doing nothing typically for the Spyder 3),
> I'm not sure this makes any difference in itself.

You might be right here... If there is any difference, it is minor if
the device behaves sensibly. Spyder3 does not :) (but you already know

> It's hard to know what's going on in the instrument. The sensors have
> a typical dark frequency of 0.3 Hz, so should produce at least two transitions
> in 5 seconds when not illuminated, but this doesn't seem to be the
> case. It does seem reasonable to work at the sensor reading level
> if zero sensor readings are to be detected.

Could you comment a little bit about my idea of averaginig non-zero
sensor readings? Are you considering/thinking about it/testing it or
is it lacking something?

> I may have more of a play with it, and determine how long the integration
> time needs to be on my instrument to guarantee non-zero sensor readings.

Take a look at the values of sensv[4] when doing a dark-grey
measurements (there are IMHO no problems with reading a high-luminance
colors). Maybe do as I did - increase the time until sensv[4] is non
zero or max. integration time is achieved.

> You could do something like what you are suggesting, and create
> a profile, but much like using a scanner in place of an instrument,
> you might be disappointed with the result.

When I manage to get the monitor calibrated properly I will try
this then. Maybe it will yield sensible results, maybe not :)

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