[argyllcms] False colors with shaper/matrix display profiles

Hello all,

I calibrated and measured two displays (a CRT and an LCD),
with argyll (dispcal and dispread), using an Xrite DTP-94.
I have the following problem with both displays:

When i generate LUT profiles for the displays, everything
is fine.
When i generate shaper/matrix profiles, however, and then
use these profiles (with Scribus, or with tifficc to manually
correct images for the displays), i get bright
false colors in some very dark image areas. Using
-qh with 'profile' reduces these areas, but the problem
itself persists.
It's as if the color transformation leads to slightly
negative values, which then get wrapped around into
high positive unsigned 8-bit values in the output image.
However shouldn't that be prevented by some clipping
mechanism?

# dispread -a -i94
# dispcal -gs -b 80 DisplayA   # (only for the CRT)
# targen -v -d3 -f500 DisplayA
# dispread -v -i94 -yc -k DisplayA.cal DisplayA
# profile -v -D"My Display" -qm -as DisplayA

'profile' report for the CRT:
: profile check complete, peak err = 5.396173, avg err = 0.697510
'profile' report for the LCD:
: profile check complete, peak err = 8.735311, avg err = 2.043373

I'm using argyll-0.60 and liblcms-1.14.


I would be grateful for advice. Is this a bug in argyll
or lcms? Am I doing something wrong?


Thanks,

Oliver

Other related posts: