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That is interesting context for this quote from the article: > "each encoder core can encode 2160p in realtime, up to 60 FPS (frames per second) using three re

by spuz 5y ago

That is interesting context for this quote from the article: > "each encoder core can encode 2160p in realtime, up to 60 FPS (frames per second) using three reference frames." Apparently reference frames are the frames that a codec scans for similarity in the next frame to be encoded. If it really is that expensive to reference a single frame then it puts into perspective how effective this VPU hardware must be to be able to do 3 reference frames of 4K at 60 fps.

daniellarusso 5y ago
I always thought of reference frames as like the sampling rate, so in that sense, is it how few reference frames can it get away with, without being noticeable? Would that also depend on the content? Aren’t panning shots more difficult to encode?
- izacus 5y ago
  > I always thought of reference frames as like the sampling rate, so in that sense, is it how few reference frames can it get away with, without being noticeable? Actually not quite - "reference frames" means how far back (or forward!) the encoded frame can reference other frames. In plain words, "max reference frames 3" means that frame 5 in a stream can say "here goes block 3 of frame 2" but isn't allowed to say "here goes block 3 of frame 1" because that's out of range. This has obvious consequences for decoders: they need to have enough memory to keep "reference frames" decoded uncompressed frames around in a chance that a future frame will reference them. It also has consequences for encoders: while they don't have to reference frames far back, it'll increase efficienty if they can reuse the same stored block of image data across as much frames as possible. This of course means that they need to scan more frames for each processed input frames to try to find as much reusable data as possible. You can easily get away with "1" reference frame (MPEG-2 has this limit for example), but it'll encode same data multiple times, lowering overall efficiency and leaving less space to store detail. > Would that also depend on the content? It does depend on the content - in my testing it works best for animated content because the visuals are static for a long time so referencing data from half a second ago makes a lot of sense. It doesn't add a lot for content where there's a lot of scenecuts and actions like a Michael Bay movie combat scene.

by spuz 5y ago

daniellarusso 5y ago

I always thought of reference frames as like the sampling rate, so in that sense, is it how few reference frames can it get away with, without being noticeable? Would that also depend on the content? Aren’t panning shots more difficult to encode?

izacus 5y ago
> I always thought of reference frames as like the sampling rate, so in that sense, is it how few reference frames can it get away with, without being noticeable? Actually not quite - "reference frames" means how far back (or forward!) the encoded frame can reference other frames. In plain words, "max reference frames 3" means that frame 5 in a stream can say "here goes block 3 of frame 2" but isn't allowed to say "here goes block 3 of frame 1" because that's out of range. This has obvious consequences for decoders: they need to have enough memory to keep "reference frames" decoded uncompressed frames around in a chance that a future frame will reference them. It also has consequences for encoders: while they don't have to reference frames far back, it'll increase efficienty if they can reuse the same stored block of image data across as much frames as possible. This of course means that they need to scan more frames for each processed input frames to try to find as much reusable data as possible. You can easily get away with "1" reference frame (MPEG-2 has this limit for example), but it'll encode same data multiple times, lowering overall efficiency and leaving less space to store detail. > Would that also depend on the content? It does depend on the content - in my testing it works best for animated content because the visuals are static for a long time so referencing data from half a second ago makes a lot of sense. It doesn't add a lot for content where there's a lot of scenecuts and actions like a Michael Bay movie combat scene.