[MEncoder-users] Inverse telecine? framerate from 120000/1001 to 24000/1001

Munzuk munzuk at ono.com
Sun Aug 27 10:17:08 CEST 2006

Corey Hickey wrote:
> I think I know what's going on here.
> Your source video is 12000/1001 fps. The video is composed mostly of
> duplicate frames (use the '.' key to step through frames and see what
> I'm seeing). For the reasons Loren described, some of the duplicates are
> in sets of four and some are in sets of five, depending on the scene.
> Like this:
> ---sets of four---
> frames in file:
> frames visible:
> visible framerate: 30000/1001 (12000/1001 divided by 4)
> ---sets of five---
> frames in file:
> frames visible:
> visible framerate: 24000/1001 (12000/1001 divided by 5)
> When you watch the original file, the motion is reasonably smooth--not
> really like 120000/1001 fps, of course, but like 30000/1001 and 24000/1001.
> You want to reduce the framerate to 24000/1001, but you really can't do
> that without skipping some frames. 120000 is the least common multiple
> of 30000 and 24000. Picking either of the original "visual framerates"
> and trying to force the whole movie into that framerate is bound to
> produce either skipped frames or duplicate frames, leading to some
> jerkiness.
> Actually, now that I've written all that, I'm going to tell you that
> what I've just described isn't the real problem. I wanted to give you
> all that first, though, because most of it is prerequisite for
> understanding what's going wrong.
> The actual problem is that mencoder's A-V sync code (where it describes
> whether it needs to skip or duplicate a frame) just isn't cut out to
> handle these repetitive patterns of frames that need to be skipped in
> order to get the desired output framerate. As far as I understand it
> (not well, but I might be correct), mencoder allows A-V desync up to a
> certain threshold, at which point it skips or duplicates only enough
> frames to get back below that threshold. Rather than skipping three out
> of four frames (or four out of five), the actual pattern is somewhat
> scattered. Therefore, often pairs (or triples, etc.) of frames from the
> original video that look identical get encoded into the destination
> file. Also, mencoder will skip/duplicate more aggressively when it
> detects that A-V sync is way off. So, sometimes more than three or four
> consecutive frames get skipped and one of the original visible frames is
> lost entirely.
> This kind of A-V sync handling works decently well for most mencoder
> usage. Your case is somewhat unusual, and it falls outside the purview
> of what mencoder normally does. Is there a clean way to do what you're
> trying to do? Probably, but I could only speculate. You may have to
> describe exactly why you're re-encoding, and you may have to consider
> just leaving the original file alone.
> Someone will probably correct me if I've analyzed this wrong.
> -Corey

:'( I think I have understood.

I wanted to re-encode just because the files sizes are too high, also I'm 
planning to add a second audio stream. I'm not sure, but maybe an 120fps 
video will made it even harder.

(didn't know that of the '.' key, it's cool)

Thanks, anyways I'll keep trying... maybe uncompressing all... 

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