[argyllcms] Re: CLUT input profiles and extrapolation (blackpoint, ...)

  • From: Klaus Karcher <lists@xxxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 21 Oct 2010 14:32:54 +0200

Hi Graeme,

Thanks a million for struggling with my half-baked ideas.

Graeme Gill wrote:
Hi Klaus,
I notice that the XYZ cLUT extrapolation is better than Lab, but not quite
as smooth (perhaps the smoothness needs bumping up for XYZ cLUTs for
some reason.)

I haven't tried this yet. I had some sobering experience with XYZ CLUT profiles (distracting artifacts in spite of low self-fit errors), but maybe I should revisit them again.

I had a fiddle with the code, and this is the result:

great :-)

For cLUT input profiles with the -u flag, it creates a device black and
white point,
with PCS value derived from a gamma/matrix model.

See what you think.

I've compared both version with another data set (same sensor and light source as before, but L*-like TRC instead of gamma 1.5), see

I used gradients from the center to the cusps (in device space) and plotted the resulting Lab values (Profiles created with "colprof -qh -u"), see <http://digitalproof.info/argyll/virtualScanner/center2cusps.pdf>

My observations:

* I was surprised to see significant differences also in the gray->cyan gradient

* I was surprised how straight the curves to the colored cusps were in both cases (didn't yet examine where the training set gamut boundary is located and the extrapolation starts)

* The most significant difference between the profiles is in the gray-black gradient (as expected). Above all, the a* curve becomes much better with 1.3.1 beta, but the L* curve get sightly worse.


Other related posts: