[argyllcms] Re: What does dispcal actually do?

  • From: Ben Goren <ben@xxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Wed, 28 Nov 2012 12:30:22 -0700

On 2012-11-28, at 12:13 PM, Nikolay Pokhilchenko wrote:

> There is two aims of calibration: correct brigtness response and neutrality 
> along the neutral scale R=G=B.

Another way to put it is that, before calibration, RGB values simply equate to 
whatever voltage responses the hardware happens to have. After calibration (by 
whatever means -- vgct matrices or voltage knobs or whatever), a gray ramp 
displayed on the monitor will be a neutral gray with the chosen chromaticity 
and gamma. Single-channel ramps will have a comparable luminance (but, 
obviously, not chroma) response.

Profiling provides a mechanism whereby known absolute colors (commonly 
expressed in Lab or XYZ space) can be translated to corresponding RGB values 
for that display using that calibration. The profile associated with an image 
(sRGB, Adobe RGB, etc.) maps the RGB values in the image to an absolute color 
space, so the combination of the two profiles provides a way for absolute 
colors to be recorded and displayed.

As with anything analog, the physical response of the display drifts over time. 
To compensate for this, a new calibration can be created with the same 
parameters as before, after which the original profile generated earlier will 
again properly characterize the display's behavior. Sometimes the drifting is 
enough that calibration improves the response but it's still not a good enough 
fit to the original response; at that point, a fresh profile needs to be 
created as well.



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