@scharfis
Just a warning, I did a lot of rambling and thinking while writing this reply so it kind of jumps arounds. Also I was not sure whether you were specifically talking about TDeint or also TIVTC.
Quote:
I mean fieldmatching should only be applied, if the algorithm is pretty sure that two fields are derived from one frame.
This is only the case with a reasonable amount of motion.
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I think it depends on the material type. If you know the source is primarily telecined then I don't think having it use a motion adaptive deinterlacer in low motion areas is preferable because even the best ones aren't perfect and will give inferior results compared to pure field matching. However, if the source is primarily interlaced then it would probably give better results. I see the problem as one of combed frame detection and not field matching (which are two completely separate processes in TIVTC/TDeint). In fact, TIVTC's field matching has no notion of a frame being combed or not, it just decides which of two fields fits best to another field.
TIVTC's field matcher can handle low motion scenes (mouthes moving) pretty well (assuming a good match exists) because it uses temporal differencing in combination with field differencing (if it wants to compare matching two top fields A/B with a bottom field C, it first compares A/B to find where they are different and then only applies the spatial metric to the created A/C and B/C frames in those areas). That said, the field matching built into TDeint is much simpler than that of TIVTC's (though at this point it probably runs slower than tfm w/ slow=2). It doesn't use temporal differencing at all and just runs the spatial metric at every point and sums the result... so I would expect it to be much more inaccurate in low motion areas. Too check if the chosen match is combed the created frame is then passed off to the combed frame detection routine.
The reason the combed frame detection routine has such a hard time detecting frames which have very little combing or 'weak' combing (falls under the cthresh level) is simply that it is a tough problem. The comb metric isn't perfect and causes false detections due to noise and high frequency detail. Because of this, it cannot be made that sensitive (low cthresh and low MI values) without causing a lot of false detections.
In TIVTC, which works under the assumption that a large majority of the clip should be matchable, but that you will occasionally get a frame that isn't, it is usually better to let a few combed frames slip through every now and than to have a lot of false detections. However, in TDeint, which w/ tryWeave=true should work under the assumption that a majority of the clip is interlaced, it should be better to have a some false detections than to let a lot of combed frames slip through. So the low-motion idea might help. Or it might help to simply adjust the combed frame detection based on the motion level (the defaults for cthresh/MI/blockx/blocky are 6/64/16/16, so maybe use 6/15/4/8 in low motion areas). The filter could then vary the settings between those two ranges based on the measured motion level.