[argyllcms] Re: Camera calibration: LUT only as good as matrix?
- From: Ben Goren <ben@xxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 4 Jul 2013 06:17:23 -0700
On Jul 4, 2013, at 12:56 AM, Torsten Bronger <bronger@xxxxxxxxxxxxxxxxxxxxx>
wrote:
> As far as I can see, even if you take into account that all other
> patches have to be fitted, too, this (purely mathematically) results
> in a matrix A * M, where A has only diagonal elements.
The math says that should be the case with ideal devices, yes.
But, though cameras are *close* to being ideal linear devices, in reality, they
aren't *actually* ideal linear devices. And errors in white balance will only
serve to multiply the nonlinearity.
Imagine an extreme example, such as profiling not the output of dcraw but the
output of a typical consumer raw development engine, such as Adobe Camera Raw
or Canon's Digital Photo Professional. Either development is going to have not
just white balance, but an S-curve and wacky color matrices and what-not all
baked in. And then there's gamma to boot. I'm not even sure that the total
transformation is theoretically reversible or invertible, and profiling
software doesn't pretend to try.
Even without that type of munging, the camera deviates from perfect linearity
in exactly the same type of chaotic manner, but just to a *much* lesser degree.
Think about it: if the camera actually *was* perfectly linear, especially if it
also met Luther-Ives, there'd be no need for profiling. You would just tag the
file with a simple profile in the style of a working space with the camera's
primaries and call it a day -- and, in this perfect world, for a given camera
model, the primaries would be the same to the Nth digit for all cameras.
In reality, the goal of profiling is to characterize how far the camera
deviates from ideal. And, to do that effectively, you want to eliminate, or at
least minimize, all other influences.
Cheers,
b&
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