Lower FPS to save bitrate, reasonable?

[huang_ch]

For those 29.xx/30 fps videos, could we convert the FPS to 24, so that the total frames are less, and could use less bitrate to encode, is this method correct? And I've seen the description of ConvertFPS in avisynth, it mentions that the two methods used in avisynth is not always perfect, so I have the concern that using ConvertFPS would result in a quality lost, would it?

[foxyshadis]

ConvertFPS will require significantly higher bitrate because blends usually compress badly (and ugly). However, it could work for mvflowfps (or a simple changefps(framerate/2.0)) and probably would save some, at the cost of fluidity.

If you want to do that, bob from original interlaced source first and downconvert from 60fps, you'll end up with smoother motion.

[DryFire]

If it is anime, odds are it will have a lot of duplicates. You could try something like dedup. I'm not quite sure how well it works for real life footage, tdecimate as also an option.

[DarkZell666]

I have another question regarding this :

Having less frames means having more differences between the frames ok ? And more differences means more keyframes in some places, and longer motion vectors for instance (and less SKIP-blocks for low-motion, which don't use that much bits ;)). So is there really any use in having a lower framerate, if it isn't for the per-frame overhead ?

*wondering, awaiting a solid answer :p*

[foxyshadis]

As you might expect, the answer is a resounding "it depends", but you usually save more on texture bits than you spend in extra mv bits, so overall quality is higher. Animation expecially, since texture and mv bits on what should be dup frames are wasted anyway.

Dropping the framerate is a little like going from 22khz->11khz, though, it's definitely noticeable. (Whereas 44->22khz, like 60->30fps, is only noticeable on some content or with keen senses.)

[akupenguin]

Animation expecially, since texture and mv bits on what should be dup frames are wasted anyway.
OTOH, that only holds if they should be dup frames but aren't (i.e. there's noise). If they are really digitally identical, then it doesn't cost any noticeable amount of bits to say so.
Thus, if you do decide to use dedup, it doesn't really matter whether you drop the frames, or leave them as duplicates at the original framerate.

[huang_ch]

Thanks for your posts, I'd like to clarify that what I'm working on is not Anime, for Anime, I think just use something like DeDup is OK, right? But for non-Anime, for it's complexity, I'm just afraid that using ConvertFPS would cause quality loss.

[Awatef]

It's definitely not recommended to drop frames to improve compression quality.
The better way to achieve this, is to reduce the resolution. Like if you have 640x480, you could go down to 448x336 and save about half the bitrate.

Only if you intend to put your videos available for download in the internet, you may want to apply a decimation. Like if you have 30fps, you decimate by 2 and get 15fps, or decimate by 3 to get 10fps. This has the advantage to keep the camera movements regular, as if the video was originally shot with such a low framerate ;)

[DarkZell666]

silly question : does OTOH mean "On Top Of the Hat" ? XD
*has been wondering for a couple of days now*

[imcold]

OTOH = On The Other Hand

[Chainmax]

AcronymFinder (http://acronymfinder.com/) is your friend :).