[MPlayer-users] converting 25 fps -> 23.976 fps?

[D Richard Felker III]

On Tue, Dec 16, 2003 at 03:49:30PM +0100, Andres Heinloo wrote:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> 
> Hi,
> 
> I have converted NTSC movies (23.976 fps) to PAL by playing the movie a
> little bit faster and resampling audio at 48kHz with sox. I think even in
> case of commercial DVDs, movies (24fps) are converted to PAL (25fps) by
> simply playing them a bit faster. Full interpolation of frames probably
> takes a huge amount of CPU power.

"Full interpolation" isn't even meaningful by itself. How do you
interpolate? Any linear filter you apply will just give horrible
blurring, not the desired effect.

It may be possible to interpolate from 24 fps to 25 using motion
estimation and rescaling the motion vectors, but I'm doubtful of the
quality this will give. Also it's very slow.

In principle there should also be an approach using optimal transport
theory (actually more simple, just a linear programming problem since
a sampled image only has finitely many points) but this could take
years to encode a movie without some very clever tricks and
shortcuts...

> I guess downconverting from 29.976 fps
> to 23.976 fps (with inverse telecine) makes sense if it is an NTSC movie
> that has been upconverted with 3:2 pulldown (telecine).

Yes, but that doesn't require any special process.

BTW, one case where a linear filter might be appropriate for dropping
the framerate to 4/5 is post-inverse-telecine. Most of the 29.97 fps
sequences left over will presumably be (near-)stills, where no
telecine pattern could be caught, so the linear filter will do fine on
them. If you also have a limited number of 29.97 fps progressive
sequences, you might be able to put up with them getting blurred. And
if there happen to be 59.94 fps sequences, you're probably already
going to want to apply a linear blend filter, so a little more
blurring couldn't hurt...

Rich