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Subject: Re: ACR Calibration

Rags Gardner wrote:

> Chris was refuting a statement from Bruce Fraser. Bruce’s comment was
similar to mine. Chris referred to some colorimeter filters. Colorimeters
can and do use XYZ metrics. But to imply they use optical filters that
match XYZ is a bit of a stretch. I don’t think we need to go into the
fundamental differences between a colorimeter and a camera, so I will
still simply agree to disagree.

I do find Thomas Knoll’s comments to be on target. If I understood him,
the math and matrices he refers to would be the CIE color matching functions
that depend on spectral data about the sensor. Mapping the sensor data
to XYZ coordinates (chromacity) is the fundamental objective of color
matching. The data can be vendor provided, measured by another vendor
(or the consumer), or simply a WAG. This is what could be in the ISO TIFF/EP
tag or simply published. <<


I like to see it quite simple: IF the absorption spectra of Bayer R/G/B filters just have the same shape as any linear combination of the XYZ matching functions, then any RGB combo released from Bayer interpolation automatically refers to a matrix space. No rendering tables and no further effort would be required as just to determine the right matrix primaries via profiling.

In agreement with Thomas Knoll’s statement, that’s not perfectly given with today’s cameras. Using a matrix space to describe the input gamut will leave some rest-errors across all colors. Using a Lut-type input profile, like other Raw converter do, bears its own set of problems; but that’s a different discussion.

> I am disappointed when I experience the effort required on my part to
calibrate something that logically should already be close at least. <<

Provided that unit-to-unit deviations play a subordinate role, ACR calibration cannot outperform the original profiling – ref. to the sum of deviations across all colors (*). A redistribution of said rest-errors with a focus on memory colors can make sense; however, if a camera’s gamut is too far away from ‘matrix’ it doesn’t rectify the situation in principle.

(*) Famous comparisons “before vs. after / calibrated state vs. simulated target” can create the polite fiction of an improvement via calibration. It ignores the side effect from tonal changes on color saturation. IMO, it’s more reasonable to assume that Adobe’s profiling software aims to balance the whole coloring – hue and saturation - under consideration of the default tone curve from Shadows, Brightness and Contrast settings.

In short: where can I buy a “matrix camera” :)? … Seriously, it would be kind if Adobe would give us a hint – concerning the cameras which behaved best when both generic profiles were built (lowest rest-error).

Cheers, Peter



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