H.264 scaled decoding?

[Sidi]

Does anybody know if is it possible to improve H.264 decoding performance in simmilar manner than JPEG? I mean scaling during decode (for example to 1/8), which speeds up at least two times on IJL. Is there any decoder that supports anything like this?
Or is there any other mean of making this process faster (other than I-Frame decoding only), even with lower quality? Lets say it is Baseline H.264.

[Adub]

One way that I know of is to turn in-loop deblocking off.

[LoRd_MuldeR]

It should be noted that turning off in-loop deblocking for a H.264 stream that was encoded with in-loop deblocking enabled, will not only cause more visible blocks. It will cause corrupted output! That's because deblocking is part of H.264 encoding process. P-Frames and B-Frames reference deblocked frames. Skipping the deblocking causes "wrong" references frames, so the error sums up from frame to frame...

[Shevach]

More idea.

You can discard pictures which are not used for reference (provided that an input stream contains non-reference pictures).

[LoRd_MuldeR]

You can discard pictures which are not used for reference (provided that an input stream contains non-reference pictures).

Why should a stream contain pictures that are neither referenced nor displayed?

I think any picture, referenced or not, will be displayed at least. So you can't simply "discard" any pictures.

That would result in missing pictures and produce jumpy output!

What you can do: Skip the deblocking part on non-referenced pictures (all b-frames, unless b-pyramid is used).

[Shevach]

Why should a stream contain pictures that are neither referenced nor displayed?


Perhaps I was mis-understood.
I suggested to discard pictures which are not used for reference despite these pictures are displayed. Thus the decoding process can be accelerated.
I think this is a popular action implemented in many decoders to cope with performance peaks.

Frankly speaking a stream can contain dangling fields which some decoders do not display.

[LoRd_MuldeR]

So you want to "discard" pictures that are not used for reference. But you still want to display these pictures :confused:

How do you want to display a picture that has been discarded ???

As said in my previous post: I believe the only the thing you can do is skipping the deblocking on non-referenced frames, but not discard those frames entirely!

Furthermore I think skipping the deblocking on non-referenced frames won't give that much speed-up. The only frames that aren't referenced are b-frames.

And even that is only the case with b-pyramid disabled. With b-pyramid b-frames may be referenced by other b-frames as well.

Last but not least deblocking is not the most compute-intensive task. CABAC decoding is, given that CABAC is used...

[nm]

So you want to "discard" pictures that are not used for reference. But you still want to display these pictures :confused:

How do you want to display a picture that has been discarded ???
No, he suggested skipping those frames completely, i.e. dropping them within the decoder. As you said, this causes jumpiness, but one could argue that it's still "improved decoding performance at lower quality".

Edit: By the way, libavcodec already does have this option. Try -lavdopts skipframe=nonref with MPlayer.

Last but not least deblocking is not the most compute-intensive task. CABAC decoding is, given that CABAC is used...
The relative time required for these tasks depends a lot on the frame resolution and bitrate though. I'd guess that at low bitrates deblocking would be harder than CABAC with current software implementations.

[LoRd_MuldeR]

No, he suggested skipping those frames completely, i.e. dropping them within the decoder. As you said, this causes jumpiness, but one could argue that it's still "improved decoding performance at lower quality".

That's true. Some dropped frame here and there may still be better than a complete desync.

The relative time required for these tasks depends a lot on the frame resolution and bitrate though. I'd guess that at low bitrates deblocking would be harder than CABAC with current software implementations.

Yeah, but I think decoding performance hardly becomes a problem for SD content. More for HD content (720p or 1080p).

And then the bitrate probably is high enough to make CABAC the performance bottleneck...

[Shevach]

Nm is right. I suggested to discard non-reference pictures and consequently don't display them.
Before discarding of pictures a decoder should care the very particular case when in a single complementary field pair one field is used for reference while another not. Indeed, in such case how the frame can be displayed if one of the two fields is absent.

As usual a removal of non-reference frames causes jerkiness artifacts. Although in case of significant motion or a scene cut the removal several frames is not noticeable.