[argyllcms] Re: Camera Profiling using ArgyllCMS

  • From: "Pascal de Bruijn" <pmjdebruijn@xxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 27 Nov 2008 20:13:34 +0100

On Thu, Nov 27, 2008 at 3:08 AM, Graeme Gill <graeme@xxxxxxxxxxxxx> wrote:
> Hal V. Engel wrote:
>
>> The LProf profile also shows a reversal in the red channel near the black
>> point.  This could be because of a problem with the profiling algorithm
>> and/or because of too little smoothing.  I suspected that increasing the
>> smoothness
>
> There are many differences between the three profiles. The profile maker
> profile has XYZ PCS, and includes both matrix and CLUT tables. Given this,
> it's probably hard to know which of these a CMM or application
> is actually using.

I thought I noticed that as well. But aren't there any conventions
about this? My
guess is most software is written towards use with Adobe's CMM...

> I note that a profile made with Argyll using an XYZ PCS CLUT is a better
> fit to the data than Lab (ie. colprof -ax), and will not clip
> the gamut in the blue due to the Lab PCS encoding limitations.
> (Often an XYZ PCS is a good choice for input devices).
> A matrix shaper profile could also be created using -as.

A matrix+shaper profile gives worse results.

Using -a x and -r 2.5 seems to improve the profile quite a bit. Excessively high
values like -r 5 don't seem to make much of an impact either way.

>> There is also another difference between how LProf and Argyll do this
>> compared to ProfileMaker.  Specifically ProfileMaker appears to create the
>> same CLUT curve for all three channels which means that no correction is
>> done for response shifts in the individual channels at different luminosity
>> levels.  Where as LProf and Argyll create distinct curves for each channel.
>>  I know that the author of UFRAW believes that using a single curve for all
>> three channels is the correct approach and it appears that authors of
>> ProfileMaker agree him.  At some point I will add a switch to LProf to allow
>> users to

Well, I'd love for ArgyllCMS to be able to do this as well, optionally.


I've updated my testcase file (it now includes the spectral data (.hist):
http://files.pcode.nl/temp/argyll/canon_eos400d_testcase.zip

How can I determine the correct -r value from the spectral data?

My procedure for creating the profile is documented here:
http://blog.pcode.nl/2008/11/color-profiling-your-own-dslr-redux

The photos I'm using to subjectively "verify" the profile quality are taken in
unobscured sunlight as well.

Please note that I'm aware, a profile is only fully valid in the exact situation
it was made. However, considering the fact that the unobscured sunlight has
the most complete spectrum, I'm trying to create a "generic" profile. At least
for as far as that possible.

Regards,
Pascal de Bruijn

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