[vlc-devel] IVTC accuracy improvement idea

[Juha Jeronen]

Hi all,

I suddenly got one idea for the improvement of IVTC accuracy. Keep the 
frame composer running all the time (even when no telecine is detected), 
and only handle the framerate conversion (and frame dropping) with the 
cadence tracking mechanism.

If it loses the cadence, the output may stutter a bit (because of the 
duplicate frames that aren't getting dropped), but with this change the 
filter should keep outputting progressive frames in such a situation, 
too. This is assuming that the comb metric is good enough so that the 
frame composer won't incorrectly mix up fields when the input signal is 
progressive, but I think it seems pretty good so this is probably not a 
problem in practice.

As for the possibility that having the filter enabled may hurt playback 
of a true progressive signal - that's a hard problem. It already happens 
with the current version, and I think there's not much that can be done 
about it. The filter is, by design, rather aggressive about going into 
and staying in film mode - to the point of triggering false positives on 
some true progressive streams. I think that in an IVTC filter this is 
the preferable side to err on, rather than missing some telecined parts.

If anyone has ideas for more reliable autodetection of hard telecine vs. 
true progressive vs. true interlaced, I'm all ears ;)

I think any changes can wait until the first version passes review, though.

  -J