> have limited control over their encode pipeline.

Frustratingly this seems common in many video encoding technologies. The code is opaque, often has special kernel, GPU and hardware interfaces which are often closed source, and by the time you get to the user API (native or browser) it seems all knobs have been abstracted away and simple things like choosing which frame to use as a keyframe are impossible to do.

I had what I thought was a simple usecase for a video codec - I needed to encode two 30 frame videos as small as possible, and I knew the first 15 frames were common between the videos so I wouldn't need to encode that twice.

I couldn't find a single video codec which could do that without extensive internal surgery to save all internal state after the 15th frame.

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orisho 5 hours ago | parent | next [-]

A 15 frame min anf max GOP size would do the trick, then you'd get two 15 frame GOPs. Each GOP can be concatenated with another GOP with the same properties (resolution, format, etc) as if they were independent streams. So there is actually a way to do this. This is how video splitting and joining without re encoding works, at GOP boundary.

	▲	londons_explore 5 hours ago \| parent [-]
		In my case, bandwidth really mattered, so I wanted all one GOP. Ended up making a bunch of patches o libx264 to do it, but the compute cost of all the encoding on CPU is crazy high. On the decode side (which runs on consumer devices), we just make the user decode the prefix many times.

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6r17 2 hours ago | parent | prev | next [-]

I wonder if we could scan / test / dig these hidden features somehow ; like in a scrapping / fuzz fashion

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Sesse__ 6 hours ago | parent | prev [-]

> I couldn't find a single video codec which could do that without extensive internal surgery to save all internal state after the 15th frame.

fork()? :-)

But most software, video codec or not, simply isn't written to serialize its state at arbitrary points. Why would it?

	▲	londons_explore 5 hours ago \| parent [-]
		A word processor can save it's state at an arbitrary point... That's what the save button is for, and it's functional at any point in the document writing process! In fact, nearly everything in computing is serializable - or if it isn't, there is some other project with a similar purpose which is. However this is not the case with video codecs - but this is just one of many examples of where the video codec landscape is limiting. Another for example is that on the internet lots of videos have a 'poster frame' - often the first frame of the video. That frame for nearly all usecases ends up downloaded twice - once as a jpeg, and again inside the video content. There is no reasonable way to avoid that - but doing so would reduce the latency to play videos by quite a lot!