[argyllcms] Re: Calibrating problems in darks areas

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Fri, 04 Jan 2008 18:25:01 +1100

Frédéric Mantegazza wrote:

Commencing device calibration
patch 6 of 6
Black = XYZ   0.39   0.44   0.56
Red   = XYZ  51.60  27.75   2.90
Green = XYZ  36.51  76.75  13.52
Blue  = XYZ  20.95  10.65 109.08
White = XYZ 104.16 109.82 118.94

The black level seems very high for a CRT. I'd
expect something like 0.05 to 0.1 for the Black Y value.

How black can you get it if you turn the brightness/contrast
down a long way ?

> Ok, another point I missed: I though these sliders were controling the
> gain, not the offset... I used them to set the white point. But them,
> black level is totally wrong. But if I use them to control black point,
> the white point will be wrong.
>
> How can you set both?

By doing it iteratively in the order the menu presents it. You'll
certainly get into trouble if your controls are reversed. Normally
on a CRT the "brightness" control offsets all the signals,
while the "contrast" control scales all the signals. It's usually
easy enough to figure out which knob does what by playing with
it and observing the display.

The documentation recommends that you at least to black level,
white point, black level and then check, and continue
until it's satisfactory.

See <http://www.argyllcms.com/doc7/dispcal.html#Adjustment>

>>show me yours that exhibit the problem

> I attached the file... I have another photo, but it is not mine, so I can't
> attached it. There is a dark blue sky, with nice gradient.

I can't see anything obvious with you attached file. What is
it meant to show ? Are you sure you haven't simply captured
the original pixel values, rather than the visible effect
after calibration ?

Graeme Gill.


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