[argyllcms] Re: question about monitor calibration

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sat, 19 Jan 2013 13:05:10 +1100

Alberto Ferrante wrote:

Hi,
        there are a number of assumptions involved in all this,
and the distinctly different purposes of calibration vs. profiling need to
be understood.

> viewing condition adjustment (-a) is used in dispcal. If I specify -a
> 11, that is the number measured through my colorimeter, the calibration
> profile makes the gamma visibly higher (i.e., the images look visibly
> darker).

An ambient of 11 Lux is quite dark. That's why the gamma is increased.
The ambient light correction is based on assumptions about the
source viewing conditions ambient light level and brightness,
and the correction will increase gamma if the source is brighter
than the display, and decrease the gamma for the reverse.
The source assumptions are base on the gamma selection. A power
curve, Lab curve or sRGB assume sRGB viewing conditions as source (250 Lux).
Rec709 and SMPTE240M assume TV studio conditions (1000 Lux).

> If I use a color-managed application and I use the color
> profile (obtained through dispread) the images gets back to the original
> gamma.

Yes, that's how it's supposed to work. Calibration has two benefits:
It can improve the look for non color managed applications, and
for color managed applications/profiling, it can improve the
apparent device behaviour to improve the quality of the profile.

Because profiling records the response of the calibrated device,
then (of course!) when you link this with a source colorspace, the
CMM strives to reproduce the source color space accurately,
removing as much of the destination devices "personality" as
it can, which includes any effects of calibration. So any change
to the device calibration _should_ have no effect if color management
is working correctly. To change the appearance of an image you need to
alter the colorspace it is defined to be in (ie. the source profile
that it is tagged with), or introduce a deliberate color adjustment
in the CMM process (say, by using an abstract profile in between the
two device profiles, or introducing viewing condition appearance adjustments).

> Now I have two questions:
> 1) why is the image getting darker by using -a? Shouldn't it be lighter
> to cope with a non dark room?

See above. You have specified that you are viewing in a very dark room.

> 2) When the color profile the luminosity correction is disregarded: this
> does not make sense to me, because it is like assuming that, when a
> color-managed application is used, the lights in the room get adjusted.

Calibration doesn't directly affect profiles/color managed response (see above).
To make viewing condition adjustments in color managed workflows
you need to account for it in the CMM linking/gamut mapping.
See ArgyllCMS collink -c and -d parameters: 
<http://www.argyllcms.com/doc/collink.html#c>

Hope this helps clarify things.

Graeme Gill.

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