Re: [Lcms-user] local convergence problems?

[Hal V. E. <hv...@as...>]

On Wednesday 25 January 2006 12:47 pm, Cory Papenfuss wrote:
>  	Hey all.  I've been playing with the new dev snapshot (with 16-bit
> support) and it's fixed one part of my problem, but not the other.  I've
> been looking at the generated profiles with iccexamin, and I think I've
> finally figured out what's causing my issues.  When I generate a profile
> with the local convergence enabled, there seems to be a relatively
> abrubtly changing areas.  It's difficult to describe, and I haven't
> completely convinced myself that it's not just a series of bad IT8.7 test
> target shots.  It's so weird that I haven't ruled out bugs either.
>
>  	Don't know if I can be anymore help, but I could at least provide
> the test target shot and see if anyone else could explain it.
>
> Thanks,
> -Cory

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