[argyllcms] Re: Calibration issues using argyll

  • From: Peter Karp <pmailing@xxxxxxxxxxxxxxxx>
  • To: Steffen <argyllcms@xxxxxxxxxxxxx>
  • Date: Wed, 10 Sep 2008 22:29:03 +0200

Hi Steffen,

> But what I don't understand then is this: why does iColor seem to do a
> better job calibrating the neutral axis than argyll? Shouldn't this 
> effect be the same no matter which software is used to do the 
> calibration?

iColor Display was tuned to give a visual good gray balance.
Especially for the hardware calibrated Quato displays, which is the
"main task" for iCD, it does a good job.

You have not specified which blackpoint setting you have choosen for
the iColor Display calibration. There is one setting to calibrate to
the native blackpoint (meaning as dark as possible) or you can choose
to calibrate to a specific blackpoint luminance with "neutral black".
This setting will use the specified blackpoint and will try to match
the blackpoints color temperature as close as possible to the
whitepoint. This is similar to choose the "blending" in Argyll. But as
Graeme said this is a trade off between a low black point and a more
or less neutral color temperature in the shadows. In my experience a
totally corrected black point is not needed and will result in a too
high black point. "Too high" means here you will loose contrast. You
won't see 1000K higher color temperature in the 0/0/0 black, when a
little above (for non-zero RGB values) you will get close to the white
point color temperature. You could upload or e-mail the generated
profiles and other data files. That might be helpful to find the
reason for your observations.

The calibration matrix used should not make a difference. Your Eizo
uses a "sRGB"-panel type and iColor Display uses the LCD readings from
the SDK. In this case no special calibration matrix is used, but the
default X-Rite factory one for LCDs. I assume that Argyll also uses
the LCD values and neither the raw measuremet data nor the CRT values.
So the instrument readings should be similar.

> The iColor Software supports a test of the calibration results
> according to UGRA standards. Yet again I cannot say if it has any
> relevance.

It has some relevance but not in this regard you're trying to
understand. It's a closed loop test and therefore the device will not
tell you "absolute" truth you are looking for here. The test is useful
to some extent, but because you normally won't know if your
colorimeter the use is limted.

>  Also, I am only able to check profiles created
> by the software itself, not third-party profiles created by Argyll. 
> Maybe there is a tag I can change to trick it into doing it.

There's no trick. If you want/need to test non-iColor Display
calibrations (for example Argyll) you would have to use the standard
UDACT not included in iColor Display. The integrated UDACT test is
"dongled" to the iColor Display profiles.

>> If you tell me which platform you are running on, I can make available
>> a version of dispcal with an extra option that would let you alter
>> the blend rate used to the native black point target, to see
>> if this improves the appearance of the calibration.

I tried the download, but it didn't work (any longer?).

On a related note: One idea I had and always wondered why nobody
seemed to realize (at least AFAIK) is the option to specify for a so
called software calibration (via vcgt) if the calibration curves
should be smoothed, thus giving smoother gradients sacrificing a
little bit of the profile accuracy.

> After I created a profile, I normaly check out the results by
> browsing to two different locations on the web. The first one [1]
> renders a test image in which numbers are displayed with different
> neutral RGB values. Profiles generated with iColor produce results
> where all numbers are visible and none of them seems to have any
> color cast to the naked eye.

You have to be extremly careful what you check with such a test
picture. Does it use "raw" values or are ICC profiles used for the
test picture and the monitor? Second you need to know that if you look
if certain steps will be visible in a ramp is highly dependent on the
ambient _and_ the gradation you used for the calibration. There's not
a single "best" gradation.

> Profiles created with Argyll also reveal every number, but there is
> a slight greenish or reddish hue on numbers on the dark area from 4
> downwards.

That's the interesting point here. Without having compared both
solutions I guess the reason is the "resolution" (means number of
measurements) in the shadow range which is used as the basis for the
calibration (vcgt) curves.

> The second image I open is to see how many visible bands in a
> supposedly smooth gradient appear. Here, both programs produce
> profiles that show banding

Which you will always have when you have a "software" calibration
opposed to a hardware calibrated display. That's because of the
limited 8Bit/channels resolution for the vcgt "calibration". You have
an 8Bit/channels input signal, do some corrections in 8Bit/channels
for the output. Then you will loose some levels due the inevitable
roundoff errors. Smoothing the calibration curves should improve this
to some extent. For example the default vcgt-curves for Apple displays
(default ICC profile for Powerbook for example) are smoothed and
produce a decent smooth gradient IMO. I don't know if Argyll supports
such a smoothing. But you could do this "by hand": read out the vcgt
curves (xcalib or X-Rites CalibrationTester might be helpful but I
think in the Argyll files you also can see the calibration curves),
import the values to Excel or somewhere else, smooth the function (or
better said values representing the transfer function) and create a
new profile including the smoothed calibration.

Best regards Peter

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