FreeLists / argyllcms / [argyllcms] B2A mapping of out-of-gamut colors

[Thomas Baumann <thomas@xxxxxxxxxxx>]

Hello list,

when I print a Lab test target through any of my Argyll generated printer profiles there is a 
peculiarity, most pronounced with relative colorimetric intent: Dark out-of-gamut colors with high b 
values turn into a light yellow - see file Screenshot_Softproof_Argyll.tif.
Example (B2A1, L = 9, a = 0, b = 80):

  $ xicclu -fb -ir SP3800_PSPP_2880_ARG2.icc
  9 0 80
  9.000000 0.000000 80.000000 [Lab] -> Lut -> 0.982462 1.000000 0.000000 [RGB]

This corresponds to the inverted A2B table with CAM Jab mapping (switch -b):

  $ xicclu -fif -ir -a -b SP3800_PSPP_2880_ARG2.icc
  9 0 80
  9.000000 0.000000 80.000000 [Lab] -> Lut -> 0.982441 1.000000 0.000000 [RGB] 
(clip)
  [Actual 90.854331 -3.834409 115.588589, deltaE 89.338580]

So the problem seems to lie in the gamut mapping 9 0 80 [Lab] -> 90.854331 -3.834409 115.588589 
[Lab], from a very dark to a very light yellow. When I open the profile in ColorSync, I can see that 
for positive b, the B2A1 curves for both R and G over L change slope and rise up to maximum with 
decreasing L, thus producing a tonal inversion. What could be the reason for this mapping behaviour? 
It does not seem right to me.
Another detail I do not understand: When I specify absolute instead of relative colorimetric intent 
in Photoshop CS Soft Proof, the effect vanishes, although xicclu -ia ... indicates it should be present.
For reference, I used these command line parameters with 'profile' version 0.70 
Beta 7 on Mac OS 10.5:

  profile -qh -i D50 -f -S /usr/local/etc/argyll/AdobeRGB1998.icc -cmt -dpp 
SP3800_PSPP_2880_ARG2

All relevant files can be found here:
  http://www.tbaumann.de/public/argyll/SP3800_PSPP_2880_ARG2.ti3 (1593 patches, 
measured with DTP20)
  http://www.tbaumann.de/public/argyll/SP3800_PSPP_2880_ARG2.icc
  http://www.tbaumann.de/public/argyll/Lab_Test_Image.tif (smaller variant of 
Bill Atkinson's target)
  http://www.tbaumann.de/public/argyll/Screenshot_Lab.tif (for easier viewing)
  http://www.tbaumann.de/public/argyll/Screenshot_Softproof_Argyll.tif (relcol, 
with BPC)
  http://www.tbaumann.de/public/argyll/Screenshot_Softproof_Argyll_abscol.tif 
(no problem here)

For comparison:
  http://www.tbaumann.de/public/argyll/Pro38_PSPP.icc (Epson's canned profile)
  http://www.tbaumann.de/public/argyll/Screenshot_Softproof_Epson.tif

Regards
Thomas