variable frame rates

[yawnmoth]

I don't think variable frame rates would have any application for DVD rips, but I think they would have a lot of benefit for people who make animation programs... for example, a friend of mine is making a program to load sprites from random games to make a sorta animated comic, of sorts. some of the objects, though, are intended to operate at different frame rates. So his whole comic has to be the frame rate of the thing with the highest frame rate, whereas instead, he could just have it be that frame rate only when that object appears... I think it'd be a good idea... perhapes a feature like this could be incorporated into the OGM container? There could be some sort of flag in the header - if it wasn't there, it would just assume a constant frame rate, but if it was, it would assume a variable frame rate? I dunno...

Also, would variable frame rates increase the overall quality of a movie? For example, you may not need to many frames per second in a really slow scene, but what about a scene with a lot of movement? Would their be any perceptual improvement with an increase in the frame rate for that particular scene?

[Asmodian]

Well this idea has been though of before (and RealPlayer 9 has variable frame rate). I don't think it is easy to code and isn't in the ogg container standard and would have to be supported by Xvid (AFAIK). Your friend could run the uncompressed (or huffyuv compressed) intermediate file through the dup plugin by Donald Graft in avisynth to get an effective variable frame-rate with a max frame rate of whatever the frame rate used to be (sort of - Dup detects motion and dupes the previous frame exactly if the motion is under the specified threshold, so the duped P-frame is very small - there are no differences between it and the previous frame to save!).
Variable frame rate should, of course, never go above the source frame rate - what is the point in making up frames?

[yawnmoth]

Heh - I didn't think about the dupped p-frame! That's certainly true!

As for my question about movies... I didn't mean it in the context of DVD rips - making up non existant frames hardly serves any point - but what if film studios filmed different scenes at different frame rates? Would variable frame rates even matter in movies? For example, does a rather slow scene need lots of frames per secon, and could action scenes benefit from more frames per second?

[Neo Neko]

MPEG4 has what I have come to know as N frames in the spec. It does not varry frame rates. But if frames are close enough then a null frame is written. Hence N frame. It is a teeny tiny frame that tells the decoder that nothing changed. And contrary to what has been said it has it's aplications for DVD as well. The latest dev versions of Xvid have it IIRC. And it was in some Divx4 builds as well.

AFAIK the N frame or drop frame method should be less resource intensive than true variable frame rate. And the file overhead is only slightly more.