[argyllcms] Re: Capture One Profiles
- From: Ben Goren <ben@xxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Mon, 8 Jul 2013 15:58:53 -0700
On Jul 7, 2013, at 6:50 PM, Graeme Gill <graeme@xxxxxxxxxxxxx> wrote:
> I suspect from
> the discussion that few photo processing systems actually work this way ?
Graeme, I think you may have just pointed to a rather large elephant in the
room.
White balancing in digital cameras serves two functions.
One is the conversion of the scene illuminant to the output profile's
illuminant, such as what you described.
The other is to remedy an artifact resulting from the physical design of the
cameras.
A typical camera's sensor has as many green pixels as red and blue combined,
and each channel has a different native level of sensitivity. If you were to
photograph an Illuminant E light source, you would *NOT* get the same values
recorded in each channel, though the output of said light source *would* scale
linearly in each channel with changing exposure.
In today's raw development engines, both operations are done with three
different linear multipliers applied independently to each channel. And that,
truly, is all that there is to white balancing behind the scenes, regardless of
what sort of user interface is covering it up.
While that works well enough in practice, I'm really starting to think that
it's not the right way to do it.
What I think should happen is that each camera should first have its own
constant normalization factor to align the three channels (and, perhaps, to
linearize the data -- though sensors are remarkably linear in practice). This
factor would presumably be reasonably consistent for each camera of the same
model, but would optimally be tweaked for each individual camera. Then, a
separate profile-based operation would convert the scene illuminant to the
desired working illuminant, just as you suggested.
Iliah, does this make sense? And any chance for something like this in RPP?
Cheers,
b&
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