Keyframes, InterFrame and Video Compression (2016)

https://blog.video.ibm.com/streaming-video-tips/keyframes-interframe-video-compression/

by woranl • 6 years ago

17 9 6 years ago

blog.video.ibm.com

nitrogen• 6 years ago

Long ago I read something on HN about encoding x264 without any keyframes at all, and instead there are equivalent blocks that are swept across the image over many frames. But I can't find that reference. Does anyone know what that technique is called and/or where to read more about it?

londons_explore3• 6 years ago

I'm not sure what the benefit would be... Sounds like a cool proof of concept though.

In fact, with very very tiny modifications (ie. modifying the bit of the spec that says 'frame 1 must be a keyframe' and replacing it with 'the first frame is initialized with solid black'), I believe any modern video compression scheme could be used with no key frames. Compression ratio wouldn't suffer, but you'd loose the ability to seek the video.

nullc3• 6 years ago

The benefit is minimizing latency on a bandwidth constrained channel-- it also has some robustness benefits.

Imagine we have a 1mbps link between us. We'd like have fluent video conferencing so we really want to have under 200ms latency. If keyframes are 10x the size of non-keyframes we'll be stuck either massively short changing the bitrate on keyframes, underutilizing our link (say using a 100kbps video), or taking a large amount of delay so that the keyframe's usage could be averaged out over the next $key_int frames.

Instead, if part of the frame is constantly being updated you'll still recover from loss in finite time, but the bitrate will be more constant.

Compression ratio wouldn't suffer, but you'd loose the ability to seek the video.

Just having none means you can't recover from loss-- and in any case where you care about latency, you'll likely have some loss because retransmission creates delay (and forward error correction consumes bandwidth).

nullc3• 6 years ago

I called it "rolling intra" when I implemented it with libtheora.

In x264 land it's apparently called "periodic intra refresh":

https://web.archive.org/web/20101017210542/http://git.videol...

barbegal• 6 years ago

This isn't possible H.264 is required to start with a key frame. Having said that you can have an H.264 stream with a single key frame.

mdonahoe• 6 years ago

Implementations I’ve seen use a gray I-frame to bootstrap the stream, then the p-frames will completely replace it after a complete N frame cycle (typically N is 5)

The frames are divided into slices where one strip of the frame contains complete blocks, but the rest is motion predicted.

Keyframes, InterFrame and Video Compression (2016)

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