[argyllcms] Re: Profile input white not mapping to output white

  • From: Gerhard Fuernkranz <nospam456@xxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Mon, 26 Nov 2012 17:30:05 +0100

Am 26.11.2012 16:23, schrieb Iliah Borg:
On Nov 26, 2012, at 9:20 AM, Gerhard Fuernkranz wrote:

> But my feeling is that obviously many people want to
> build their camera profiles from pre-processed raw files,
> which have already been white-balanced by the raw converter.

That is indeed the case; ...

Hi Iliah,

one of the problems with applying the profile after white-balancing is of 
course that the profile is not applied to the full raw RGB cube, but only to 
the subset of raw RGB space which has been selected by the white-balancing 
operation, and for each shot this is a different subset. This makes non-linear 
device models virtually useless; except one would build a separate profile for 
each white point selected by the white-balancing operation.

In the general case (arbitrarily non-linear device behaviour)
This is always the case. Extreme highlights and shadows are non-linear for any 
sensor, and raw converters apply linearization before applying white balance; 
or white balance won't work well.

Actually I had expected that the profile would/should do this task, i.e. 
convert non-linear raw RGB data to a well-defined color space (e.g. XYZ, or 
better Bradford cone space) where the white-balancing can be done. If 
demosaicing requires however prior white balancing, then this leads indeed to a 
chicken-egg problem...

Profiles are best applied to demosaicked data, and demosaicking needs to be run 
after white balance - given we want quality of course, not just speed.

Best Regards,

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