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 ------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Do you grep through log files for problems? Stop! Download the new AJAX search engine that makes searching your log files as easy as surfing the web. DOWNLOAD SPLUNK! http://sel.as-us.falkag.net/sel?cmd=lnk&kid=103432&bid=230486&dat=121642 _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user
