New article on color management

Started Apr 26, 2013 | Discussions thread
OP gollywop Veteran Member • Posts: 8,284
Re: Camera-->sRGB versus Camera-->ProPhoto-->sRGB

Detail Man wrote:

gollywop wrote:

Jack Hogan wrote:

gollywop wrote:  I'm wondering, however, where the XYZ came from unless you're talking about OOC jpegs.

Hi gollywop.  As I understand it, all conversions (including from camera space, if not short circuited by vendors' proprietary Raw converters) need to go through Profile Connection Space (XYZ D50) .  Come to think of it, the two color space flows should probably look like this:

1) Camera Space --> PCS --> ProPhotoRGB --> PCS --> sRGB
2) Camera Space --> PCS --> sRGB

If my understanding is incorrect I'd be interested to know.

It's my understanding that the camera's jpeg process begins with a trip to XYZ.  I wouldn't take bets, but I don't believe ACR does and I'm of an even stronger belief that RPP doesn't.

Regarding white balancing processes specifically:

Iliah Borg wrote:

The usual way to achieve WB is to equalize response of all colour channels over a grey (or synthetic grey, as in the grey world model) and is performed in RGGB domain. This is based on the theory of equalizing colour channel sensitivities using achromatic colours as a reference.

The "chromatic adaptation" way of achieving white balance is acting over XYZ, that is the ACR/LR way.

All cameramakers who do not adopt DNG stay with the first method.


Regarding RAW processor internal operations:

All raw converters ultimately have to take the demosaiced raw camera coordinates (RGB or GMCY) and map them to XYZ or Lab, though it may be only an intermediate step. This is especially true for raw converters that use ICC profiles, where either XYZ or Lab D50 is the working space. The final mapping from XYZ to any of the standard RGB spaces is just a linear transform plus an encoding curve. The lookup tables provide a mechanism for non-linear corrections (e.g., if you need to tweak deep saturated reds differently than less-saturated reds, something the earlier matrix-only profiles obviously could not do).

- Eric Chan, Reply #51 on: July 28, 2008, 08:31:26 PM

exdeejjjaaaa has a slew of Eric Chan quotes (relating to ACR and LR) here:

Fantastic resource.  Thank you DM and exdeejjjaaaa (and, of course, Eric Chan).

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