Tuesday, March 5, 2013

HEVC: is it really twice as good as H.264?

The new standard for video compression, High Efficiency Video Coding or HEVC, is claimed to require "half the bandwidth for high quality video transmission", compared with the older H.264/AVC standard. What does this mean? If the same video clip is encoded with H.264 at a particular bitrate, and with HEVC at half the bitrate, then the quality of the decoded HEVC video should be at least as good as the decoded H.264 video.

Here's an example. This is a close-up of a frame from the sequence "Kristen and Sara", 720p resolution. On the left, the sequence has been encoded at 800kbps using the x264 video encoder, a popular H.264 implementation. On the right is the same frame from the sequence encoded at 420kbps using the HEVC Test Model encoder, HM10.0. The frame on the right was decoded from one of the HEVC Anchor Sequences (Kristen and Sara, QP=32) which you can download here: ftp://ftp.kw.bbc.co.uk/hevc/hm-10.0-anchors/bitstreams/ra_main/. The original YUV test sequences are available here: ftp://hvc:US88Hula@ftp.tnt.uni-hannover.de/testsequences.

Left: x264 at 800kbps. Right: HM10 at 420kbps.
The two frames look very similar. However, some detail is lost in the HEVC version. You can see this more clearly here:

x264 at 800kbps: extreme close-up
HM10 at 420kbps: extreme close-up
Neither of the images is perfect, but the x264 version retains more detail than the HEVC version. The HEVC output looks smoother, but this actually means that some important detail has been removed during encoding.

Here's another example, this time a close-up from a frame of the Racehorses sequence, originally 480p resolution:
x264 at 1.8Mbps : extreme close-up
HM10 at 960kbps: extreme close-up
Once again the x264 version retains more detail than the HEVC version. Notice the over-smoothed appearance of the horse's face in the HEVC / HM10 close-up.

HEVC will give a clearer, higher quality image than H.264 at the same bitrate. But is it really twice as good as H.264, i.e. does it give the same quality at half the bitrate? These still frames imply that HM10 does not perform as well as H.264 at half the bitrate. However, recent research by Wang et al indicates that HEVC has bigger performance gains for moving image sequences than for still images taken out of a sequence.

It's also worth noting that x264 is a highly efficient implementation of H.264 that has been optimized over a period of years. As HEVC and its implementations mature, we can expect improvements in performance and image quality.

I'll be posting more results soon and explaining exactly how to test this for yourself. To download the HM reference software, visit this page.


Unknown said...

Thank you for an interesting comparison. With reference to Sara, I agree that the HEVC frame looks smoother in comparison to the H.264 frame, but it's also lacking some of the artifacts that the H.264 frame suffers from. For example, the blue background to the left of the face looks a bit "noisy", and ringing around the edges appears stronger for H.264.

I'm not sure what's worse: more blur or stronger artifacts. It'd be interesting to see how they stand out in an actual moving picture. I'm looking forward to seeing more results.

Unknown said...

Thanks Misha, good point. I agree about the visible artefacts in the H.264 frame. However, what concerns me about the HEVC frame is the loss of information. Comparison of moving images requires a real time decoder or playing back the full raw decoded files. I'll be posting more results and examples soon.
- Iain

Markus said...

Could you please add the uncompressed input images a reference to compare against? How can you compare the output of two lossy codecs without looking at the input?

Unknown said...

Hello Markus, thanks for the comment. If you follow the link you can download the original YUV sequences yourself - you will need software such as GLYUVPlay to play them.

It's common practice to compare the output of different processes without the original (reference). For example, Absolute Category Rating and Pair Comparison (ITU-T P.910) and Single Stimulus Continuous Quality Evaluation (ITU-T BT.500) are all methods of no-reference subjective quality testing.
- Iain.

Anonymous said...

Zhou Wang had a paper on this: https://ece.uwaterloo.ca/~z70wang/publications/vpqm13.pdf

Long story short, the magic of HEVC is in its massive reduction of temporal artifacts, not necessarily spatial ones.

In fact, the paper continues to detail tests showing how subjective analysis of stillframes will significantly underestimate the quality of a clip as a whole.

Unknown said...

Very interesting paper by Zhou Wang, I'll update the blog with a link. Thanks for the comment.

Anonymous said...

I'm going to suggest that you're looking at the issue from the wrong perspective, Ian. The codec isn't about using half the resources to be as good, it's about a fixed pipe capacity delivering more channels. Whether its fibre and the 'conversations' or its digital transmission, broadcasters have the key issue of more channels of content in a fixed volume of delivery channel. Now, add 4K, or yet another must have home device and the new iteration of the codec is essential. I'm guessing that even 6months of scrutiny and shaping will improve it greatly. Thanks for the great work, by the way.

Anonymous said...

Very interesting results!
I have also read the following papers, which compares HEVC and H264, without however noticing the details of your listing.
However, they report some interesting findings on the performance of the two codecs.



It seems that the performance of HEVC is doubled in comparison to AVC, but especially in the temporal domain. In the spatial domain, needs more investigation (in cases of half bit rate).

Please keep posting more results.

Anonymous said...

I think taking a still image out of a moving video which it has been optimized to is not a good way of judging stuff.

Most importantly dancing noise is impossible to see on a still but obvious on a video. I think I will trust the pros and vector encoding with more info in less space until I see it at NAB. Also, how about an example with modern 1080p instead of 720 and who shoots 480i?

Unknown said...

Nice to see the debate around this topic. I will look into generating some sample clips that can be downloaded and compared.

DSE said...

Great post, Iain.
Any chance of some downloadable short clips being forthcoming?

DSE said...

Great blog post, Iain,
Any movement on generating some short downloadable clips for evaluation/comparisons?

Unknown said...

Yes I will put together some short YUV clips...

Sean Duncan said...

Do you mind posting the .cfg file you used with the reference HEVC encoder...or did you just use one of the default .cfg files that come with HM10? (However, I'd venture a guess that you probably tweaked at least some of those numerous parameters) :) Also, Do you mind posting what x264 parameter values you used to create your h264 comparison?

Unknown said...

Sean - I used an HEVC anchor sequence, please see the link to these clips. The configuration settings are listed in a JCT-VC document, if I remember correctly. I'll add the x264 command line parameters shortly.
- Iain

Velda said...

This is cool!

Anonymous said...

Hi Iain,

I downloaded hm10 bitstream from site you gave in post, and tried to decode it with tappdecoder (from HM11) and got this erro:


Is that because newer HM can't decode older HM bitstreams?

If you have tappdecoder from HM10, can you upload it?

Unknown said...


Instead of trying to decode the HM10 bitstream, I would suggest downloading HM11 anchor bitstreams instead: ftp://ftp.kw.bbc.co.uk/hevc/hm-11.0-anchors/

- Iain

Anonymous said...

Hi All,

Where can i get test bit-streams for main-still picture profile? And yuv for test images.

Anonymous said...

Very Informative, Thanks a Lot Ian ..