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

  • From: Stephen T <stwebvanuatu@xxxxxxxxxxxx>
  • To: "argyllcms@xxxxxxxxxxxxx" <argyllcms@xxxxxxxxxxxxx>
  • Date: Fri, 1 Aug 2014 05:58:07 -0700

OK folks,

Here are my findings for linear matrix camera input profiles:

1) For -am profiles and linear device RGB, the wtpt and bkpt tags are 
irrelevant in practise. However, we still might check the colour of the 
estimated wtpt (xyZ) to confirm it is near to D50, but don't worry if L* is not 

2) The rXYZ, gXYZ, bXYZ define the matrix transform from linear device RGB to 
D50 XYZ. The magnitudes of the rXYZ, gXYZ, bXYZ depend on any optional scaling 
that can be applied to prevent clipping.

3) Normally, we scale linear device RGB so that the white patch = 100 (maximum 
device response). This can be done in the raw photo software (linear scaling) 
or in Argyll CMS using -u (automatic scaling) or -U (manual scaling). Both raw 
photo and Argyll CMS scaling are equivalent. I suggest to use manual scaling in 
Argyll CMS and then check that no individual RGB exceeds 100 in an xicclu -g - 
fb plot.

Happy profiling.

On Wednesday, 30 July 2014 10:38 PM, Graeme Gill <graeme@xxxxxxxxxxxxx> wrote:

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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