There is a problem with local convergence extrapolation that results in local
reversals (abrupt localized changes in the curves) . There is a section in
the help files that describes the problem. There is also a bug report that
has been opened on this issue. Please see
http://sourceforge.net/tracker/index.php?func=detail&aid=1296658&group_id=146038&atid=764370
The problem was worse before 16 bit target image support was added but that
did not completely fix the problem. My testing indicates that it only
happens in the darkest parts of images and is worse in areas that have highly
saturated very dark colors (dark reds and yellows seem to be the worst). It
does not show up at all in images with lower contrast levels as far as I can
tell since these images will not have areas dark enough to show the problem.
I have not had a chance to dig into the code to figure out what is happening
but I think that this is related to the fact that digital cameras,
particularly those with larger sensor sites like DSLRs, have a gamut that is
considerably larger than what is represented on an IT8.7 chart. In other
words there are significant areas of the cameras gamut where there is little
or no data from the IT8.7 image and local convergence extrapolation in affect
tries too hard to make these areas "right" and the curves end up with
discontinuities.
So far most of my work on LPROF has been to consolidate the user interface and
to remove redundancies from the code. Little has been done to the
underlaying profile generation algorithms at his point. The current
recommendation is to not use local convergence extrapolation with wide gamut
devices as this will result in smoother curves that do not exhibit the
problem. I suspect that the problem is for the most part limited to those
who are working with raw images since raw processing is cabable of pulling
way more gamut/dynamic range out of images then would be typical for images
that are processed in camera.
Cory - When I look at the curves in the LPROF profile checker I can't really
see the discontinuities in the curves even though I know that these exist. I
have ICCExamin 0.38 and I don't see anyplace to inspect the curves as such.
Could you fill me in on how you are doing this?
Hal
Good to know that what I'm describing isn't just due to my
ignorance of how the whole system works. Like I said, it's only been
recently that I've been able to analyze the profiles enough to narrow it
down to the localized convergence parameter. The built-in LPROF profile
checker doesn't show any of the badness really... dE always looked good.
Just for the record, the problem is worse as you say with RAW files and
linear data... probably due to the reduced numerical contrast of linear
data.
As far as what I used to notice the problem... first it was the
little graphical curve in UFRAW's input histogram plot. What nominally
looks like a gamma curve ("fuzzy" due to the matrix) had large jumps in
the curve in the brightest area of the histogram. The resulting RAW
conversion had strange artifacts in the brightest areas of the image.
Yesterday, I figured out how to load up multiple profiles in
iccexamin (0.38) and compare the gamuts. You can see the discontinuities
there, but only if you turn on the Spektral line and show the 3D surface
in color (not gray). If you do that, there is a projection onto the Lab
plane that shows discontinuities.
Behrmann's answer to your question was another feature of
iccexamin I was unaware of. I didn't know there was a way to look into
the matrix before. Now that I've done that a bit, you can definately see
some funny stuff going on with the local-convergence-enabled profile.
Without it, all the slices are pretty much grescale cubes. With it, there
are some awfully strange discontinuous "blocky" shapes in all of the
different slices.
All of the curves (TRC, shaper, etc) have always looked good...
nominally linear or gamma as appropriate. Just FYI, I am using a Pentax
*ist-DS DSLR, so the sensor likely has the larger gamut than the IT8.7 can
adequately profile. I've even found the spectral response curves for the
Sony sensor inside... :)
-Cory
--
*************************************************************************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
*************************************************************************
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