[argyllcms] Re: White point and matrix device input profile?

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Wed, 30 Jul 2014 22:38:15 +1000

Stephen T wrote:


> A matrix profile is simply a linear transform from device RGB to linear XYZ.

Mathematically it's not linear, since the per channel (gamma) curves are 
a power like function.

> However, there also is a wtpt tag in the ICC profile from Argyll CMS. Where 
> does that
> come from?

ICC profiles are stored as PCS white (D50) normalized. The white point tag
represents the native white point of the device. (Not true for V4 display
profiles though, which force the white point tag to be D50 as well).

> Is the profile wtpt tag used to convert from profile white point to the white 
> point of
> the PCS (a second computation step after applying the matrix perhaps)?

I'd say "device white point to PCS white point".

Graeme Gill.

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