Make a slow motion video at a constant rate

[luciofulci]

Hi All,

I have a H264 video file that is 23.976 fps and I need to keep it as H264 23.976 fps but I want to slow down the speed to about half. The file has to remain as blu ray compliant as I'm using the video as a background movie for a blu ray menu. I just wondered what the simplest way to do this is?

I've read about doing it in virtualdub and changing the frame rate but I'm not sure if this will result in a file that is no longer 23.976 FPS? I've also read about slowmovideo and I'm wondering if that is a bit overkill, as that seems cater for people who want to speed up then slow down different parts of the same video, I just want a constant slow down through the entire 60 second clip.

Any suggestions or things I am missing?

Many thanks

[jagabo]

In VirtualDub you can change the source frame rate to 11.99 fps, then convert the frame rate to 23.976. Every frame will be duplicated and it will still run at 23.976 fps. The duplicate frames means motion will jerky. In AviSynth you could use a sequence like AssumeFPS(11.99).ChangeFPS(23.976). That will get you the same jerkiness.

Getting smooth slow motion requires software that can interpolate motion between frames. AviSynth has such filters. Most of the big commercial video editors do too. It can work well for some types of material, not well with others.

[sneaker]

Do you have AviSynth?

ffvideosource("source_file.ext")
AssumeFPS(12000, 1001)
ChangeFPS(24000, 1001) # ConvertFPS() would do some simple interpolation

/edit:
ah, jagabo was faster

[LigH.de]

Even more complex in AviSynth{+}, but with much smoother interpolation, you could use e.g. MVTools v.2 with MFlowFPS. Not recommended for users with only little experience...

[luciofulci]

Thanks everyone who replied, I will try the virtualdub option first and see if I can live with the jerkiness.

[abolibibelot]

Getting smooth slow motion requires software that can interpolate motion between frames. AviSynth has such filters. Most of the big commercial video editors do too. It can work well for some types of material, not well with others.

Even more complex in AviSynth{+}, but with much smoother interpolation, you could use e.g. MVTools v.2 with MFlowFPS. Not recommended for users with only little experience...

In my experience it works very well when there's little movement, or slow movement, but it usually produces ugly artifacts when large objects (or people, which are really a particular kind of usually smelly and noisy objects) are moving fast.

Thanks everyone who replied, I will try the virtualdub option first and see if I can live with the jerkiness.

Indeed slow motion effects in older movies could only be done that way and have that sort of jerkiness which is not too disturbing if it's steady (which should be the case with a 2:1 ratio). I find it more disturbing when there's a stuttering effect on a camera panning of a footage converted from 29.97 FPS to 25 FPS, or something similar.

[Cornucopia]

Slomo is done the movies by shooting at 48fps (or higher, etc) and then slowing down to normal 24fps, so it is very smooth. That isn't possible with stuff shot at or under 24fps. Then, only interpolation methods or duplication/hold is possible, and that at best is good (no major motion) or smudgy/smeary/ghosty (most motion) when interpolating, or skippy/jerky when duplicating/holding.

Also, luciofulci, applying a type of processing such as slomo ALWAYS modifies the video to the point where if you want to save it that way, you will need to re-encode. With accompanying generational loss, as per the app & source.

Scott

[abolibibelot]

Slomo is done the movies by shooting at 48fps (or higher, etc) and then slowing down to normal 24fps, so it is very smooth. That isn't possible with stuff shot at or under 24fps.

This remark was prompted by the fact that I just rewatched a few days ago the first two Rocky movies, which have quite jerky slow motion effects at some points (most notably at the end of the iconic scene where Rocky climbs the steps alone in the first one, also at the end of a similar scene in the second one when he's surrounded by kids, and at the very end of the second movie when the motion is slowed down until it stops with the still frame of Rocky holding Mickey on his shoulder). The first one was a low budget movie, so perhaps the director didn't have access to expensive high motion cameras (although I heard recently that it was one of the very first movies to involve a prominent use the steadicam which was then a fancy novelty) but I suppose that for the second one the budget was high enough (based on the success of the first) so that they could use state-of-the-art equipments, so I wonder if that was indeed the intended effect.

Also, luciofulci, applying a type of processing such as slomo ALWAYS modifies the video to the point where if you want to save it that way, you will need to re-encode. With accompanying generational loss, as per the app & source.

Obviously, so using a lossless intermediate codec (like Lagarith / MagicYUV / UT Video) is good practice, or a virtual uncompressed file through AVFS if using Avisynth in the first place (but if it's for a menu background the lossless output shouldn't be huge).