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• Simply beyond ridiculous

  7 mai 2010, par Dark Shikari — H.265, speed

  For the past few years, various improvements on H.264 have been periodically proposed, ranging from larger transforms to better intra prediction. These finally came together in the JCT-VC meeting this past April, where over two dozen proposals were made for a next-generation video coding standard. Of course, all of these were in very rough-draft form ; it will likely take years to filter it down into a usable standard. In the process, they’ll pick the most useful features (hopefully) from each proposal and combine them into something a bit more sane. But, of course, it all has to start somewhere.

  A number of features were common : larger block sizes, larger transform sizes, fancier interpolation filters, improved intra prediction schemes, improved motion vector prediction, increased internal bit depth, new entropy coding schemes, and so forth. A lot of these are potentially quite promising and resolve a lot of complaints I’ve had about H.264, so I decided to try out the proposal that appeared the most interesting : the Samsung+BBC proposal (A124), which claims compression improvements of around 40%.

  The proposal combines a bouillabaisse of new features, ranging from a 12-tap interpolation filter to 12thpel motion compensation and transforms as large as 64×64. Overall, I would say it’s a good proposal and I don’t doubt their results given the sheer volume of useful features they’ve dumped into it. I was a bit worried about complexity, however, as 12-tap interpolation filters don’t exactly scream “fast”.

  I prepared myself for the slowness of an unoptimized encoder implementation, compiled their tool, and started a test encode with their recommended settings.

  I waited. The first frame, an I-frame, completed.

  I took a nap.

  I waited. The second frame, a P-frame, was done.

  I played a game of Settlers.

  I waited. The third frame, a B-frame, was done.

  I worked on a term paper.

  I waited. The fourth frame, a B-frame, was done.

  After a full 6 hours, 8 frames had encoded. Yes, at this rate, it would take a full two weeks to encode 10 seconds of HD video. On a Core i7. This is not merely slow ; this is over 1000 times slower than x264 on “placebo” mode. This is so slow that it is not merely impractical ; it is impossible to even test. This encoder is apparently designed for some sort of hypothetical future computer from space. And word from other developers is that the Intel proposal is even slower.

  This has led me to suspect that there is a great deal of cheating going on in the H.265 proposals. The goal of the proposals, of course, is to pick the best feature set for the next generation video compression standard. But there is an extra motivation : organizations whose features get accepted get patents on the resulting standard, and thus income. With such large sums of money in the picture, dishonesty becomes all the more profitable.

  There is a set of rules, of course, to limit how the proposals can optimize their encoders. If different encoders use different optimization techniques, the results will no longer be comparable — remember, they are trying to compare compression features, not methods of optimizing encoder-side decisions. Thus all encoders are required to use a constant quantizer, specified frame types, and so forth. But there are no limits on how slow an encoder can be or what algorithms it can use.

  It would be one thing if the proposed encoder was a mere 10 times slower than the current reference ; that would be reasonable, given the low level of optimization and higher complexity of the new standard. But this is beyond ridiculous. With the prize given to whoever can eke out the most PSNR at a given quantizer at the lowest bitrate (with no limits on speed), we’re just going to get an arms race of slow encoders, with every company trying to use the most ridiculous optimizations possible, even if they involve encoding the frame 100,000 times over to choose the optimal parameters. And the end result will be as I encountered here : encoders so slow that they are simply impossible to even test.

  Such an arms race certainly does little good in optimizing for reality where we don’t have 30 years to encode an HD movie : a feature that gives great compression improvements is useless if it’s impossible to optimize for in a reasonable amount of time. Certainly once the standard is finalized practical encoders will be written — but it makes no sense to optimize the standard for a use-case that doesn’t exist. And even attempting to “optimize” anything is difficult when encoding a few seconds of video takes weeks.

  Update : The people involved have contacted me and insist that there was in fact no cheating going on. This is probably correct ; the problem appears to be that the rules that were set out were simply not strict enough, making many changes that I would intuitively consider “cheating” to be perfectly allowed, and thus everyone can do it.

  I would like to apologize if I implied that the results weren’t valid ; they are — the Samsung-BBC proposal is definitely one of the best, which is why I picked it to test with. It’s just that I think any situation in which it’s impossible to test your own software is unreasonable, and thus the entire situation is an inherently broken one, given the lax rules, slow baseline encoder, and no restrictions on compute time.

• Anomalie #1697 : Redirections des anciennes vers les nouvelles URL inopérante avec URL arbo

  4 juillet 2011, par nicolas -

  cedric - a écrit : Je ne demandais pas une réponse technique qui présuppose d’un choix d’implémentation, mais bien une réponse fonctionnelle :) Arf ! ;-) Donc, l’idéal fonctionnel, c’est que la dernière adresse "valable" est la dernière adresse "à montrer", celle où toute la hiérarchie est (...)

• Inside WebM Technology : The VP8 Alternate Reference Frame

  15 juin 2010, par noreply@blogger.com (John Luther) — inside webm, vp8

  Since the WebM project was open-sourced just a week ago, we’ve seen blog posts and articles about its capabilities. As an open project, we welcome technical scrutiny and contributions that improve the codec. We know from our extensive testing that VP8 can match or exceed other leading codecs, but to get the best results, it helps to understand more about how the codec works. In this first of a series of blog posts, I’ll explain some of the fundamental techniques in VP8, along with examples and metrics.

  The alternative reference frame is one of the most exciting quality innovations in VP8. Let’s delve into how VP8 uses these frames to improve prediction and thereby overall video quality.

  Alternate Reference Frames in VP8

  VP8 uses three types of reference frames for inter prediction : the last frame, a "golden" frame (one frame worth of decompressed data from the arbitrarily distant past) and an alternate reference frame. Overall, this design has a much smaller memory footprint on both encoders and decoders than designs with many more reference frames. In video compression, it is very rare for more than three reference frames to provide significant quality benefit, but the undesirable increase in memory footprint from the extra frames is substantial.

  Unlike other types of reference frames used in video compression, which are displayed to the user by the decoder, the VP8 alternate reference frame is decoded normally but is never shown to the user. It is used solely as a reference to improve inter prediction for other coded frames. Because alternate reference frames are not displayed, VP8 encoders can use them to transmit any data that are helpful to compression. For example, a VP8 encoder can construct one alternate reference frame from multiple source frames, or it can create an alternate reference frame using different macroblocks from hundreds of different video frames.

  The current VP8 implementation enables two different types of usage for the alternate reference frame : noise-reduced prediction and past/future directional prediction.

  Noise-Reduced Prediction

  The alternate reference frame is transmitted and decoded similar to other frames, hence its usage does not add extra computation in decoding. The VP8 encoder however is free to use more sophisticated processing to create them in off-line encoding. One application of the alternate reference frame is for noise-reduced prediction. In this application, the VP8 encoder uses multiple input source frames to construct one reference frame through temporal or spatial noise filtering. This "noise-free" alternate reference frame is then used to improve prediction for encoding subsequent frames.

  You can make use of this feature by setting ARNR parameters in VP8 encoding, where ARNR stands for "Alternate Reference Noise Reduction." A sample two-pass encoding setting with the parameters :

  --arnr-maxframes=5 --arnr-strength=3

  enables the encoder to use "5" consecutive input source frames to produce one alternate reference frame using a filtering strength of "3". Here is an example showing the quality benefit of using this experimental "ARNR" feature on the standard test clip "Hall Monitor." (Each line on the graph represents the quality of an encoded stream on a given clip at multiple datarates. The higher points on the Y axis (PSNR) indicates the stream with the better quality.)

  
  

  The only difference between the two curves in the graph is that VP8_ARNR was produced by encodings with ARNR parameters and VP8_NO_ARNR was not. As we can see from the graph, noise reduced prediction is very helpful to compression quality when encoding noisy sources. We’ve just started to explore this idea but have already seen strong improvements on noisy input clips similar to this "Hall Monitor." We feel there’s a lot more we can do in this area.

  Improving Prediction without B Frames

  The lack of B frames in VP8 has sparked some discussion about its ability to achieve competitive compression efficiency. VP8 encoders, however, can make intelligent use of the golden reference and the alternate reference frames to compensate for this. The VP8 encoder can choose to transmit an alternate reference frame similar to a "future" frame, and encoding of subsequent frames can make use of information from the past (last frame and golden frame) and from the future (alternate reference frame). Effectively, this helps the encoder to achieve results similar to bidirectional (B frame) prediction without requiring frame reordering in the decoder. Running in two-pass encoding mode, compression can be improved in the VP8 encoder by using encoding parameters that enable lagged encoding and automatic placement of alternate reference frames :

  --auto-alt-ref=1 --lag-in-frames=16

  Used this way, the VP8 encoder can achieve improved prediction and compression efficiency without increasing the decoder’s complexity :

  
  

  In the video compression community, "Mobile and calendar" is known as a clip that benefits significantly from the usage of B frames. The graph above illustrates that the use of alternate reference frame benefits VP8 significantly without using B frames.

  Keep an eye on this blog for more posts about VP8 encoding. You can find more information on above encoding parameters or other detailed instructions to use with our VP8 encoders on our site, or join our discussion list.

  Yaowu Xu, Ph.D. is a codec engineer at Google.