[argyllcms] Re: Camera profile and non-neutral white patch
- From: "Gerhard Fürnkranz" <nospam456@xxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Wed, 31 Aug 2011 10:46:48 +0200
-------- Original-Nachricht --------
> Datum: Wed, 31 Aug 2011 00:53:09 -0700 (PDT)
> Von: Stephen T <stwebvanuatu@xxxxxxxxxxxx>
> An: "argyllcms@xxxxxxxxxxxxx" <argyllcms@xxxxxxxxxxxxx>
> Betreff: [argyllcms] Re: Camera profile and non-neutral white patch
> ??? if you want to use the profile for "general purpose shooting" (and not
> only for a very particular studio lighting condition), then the shots need
> to be white-balanced anyway after applying the profile ???
>
>
> I think one of the main benefits of shooting RAW is that we can white
> balance the RAW data. With a neutral profile, I think the white-balanced RAW
> output should be neutral.
The raw data are a priori not yet white-balanced, so each shot needs to be
white-balanced individually anyway. This eliminates however the need to care
about the media white point in the profile, because the subsequent white
balancing of each individual image will "fix" the WP of the image anyway.
The media white point in the profile would be only relevant, if the profile
should take the task of white-balancing the raw data (which is only possible
for one particular fixed lighting condition, of course).
Well, there may be of course a different understanding at which point in the
workflow the white balancing should happen...
White balancing is a conversion from a particular source WP to a particular
destination WP. So in fact this is a chromatic adaptation operation, and thus
it should be basically carried out in say Bradford or CAT02 cone space, while a
diagnonal scaling in device RGB space is rather suboptimal.
So my understanding is that 1) the profile should be applied to the (not yet
white-balanced) raw data, 2) the raw data should be converted to cone space, 3)
white balancing should now be done in cone space, and 4) the data should be
converted from cone space to the desired destination color space. Uing this
workflow, the media WP in the profile does indeed not matter, but cancels out
in the maths behind the overall transformation.
[ If you nevertheless would like to do the white-balancing as the first step,
by diagonal scaling in camera RGB space, and before applying the profile (which
is suboptimal), then you would need of course a profile with a media white
point located at camera RGB = [1,1,1], in order that the previous
white-balancing in RGB space is kept when applying the profile. ]
Regards,
Gerhard
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