Re: [rtcweb] More H.264 vs VP8 tests

Harald Alvestrand <harald@alvestrand.no> Tue, 25 June 2013 10:17 UTC

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Date: Tue, 25 Jun 2013 12:17:26 +0200
From: Harald Alvestrand <harald@alvestrand.no>
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To: Bo Burman <bo.burman@ericsson.com>
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Subject: Re: [rtcweb] More H.264 vs VP8 tests
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Again - thanks for releasing this openly!

I ran the scripts (with a few tweaks; you run on a system where sh is 
bash, not dash, for instance), and got the same numbers within +/- 0.5% 
(probably some binary version skew); we may have disagreements on the 
parameters to use, but we agree on the numbers those parameters produce.

(I have since modified the Google framework to include a script that 
pulls in the sources for the needed binaries and compiles them - if you 
want to make 100% sure people are working from the same sources, you may 
want to rebase to a newer version of the comparision toolkit.)

On 06/22/2013 03:41 PM, Bo Burman wrote:
> Hi all,
>
> We have had a look at Google's comparison between VP8 and H.264 constrained baseline that was posted on April 3rd (http://www.ietf.org/mail-archive/web/rtcweb/current/msg07028.html). This post contains, as the one mentioned above (and if the attachments make it to the list), information on the exact tools and options used for encoding and should thus be repeatable by anyone interested.
>
> As was already stated by others on this list, one major problem is that Google's test involves the rate control mechanism. Typically codecs are measured with rate control turned off, since it acts as a huge noise on the measurement. Instead we propose to compare the codecs using fixed qp-levels. The qp-level is the quantization parameter that affects the rate/distortion tradeoff. Comparing using fixed qp-levels is what has been used when benchmarking HEVC against H.264 in the JCT-VC standardization, for instance. We are going to select a codec (essentially bit stream format), not a rate control mechanism: Once the codec is selected you can choose whatever rate control mechanism you wish.
>
> We used Google's excellent framework as the baseline and changed the parameter settings in order to make it possible to measure using fixed qp. We used the same sequences, but limited them to the first 10 seconds since they varied from 10 seconds to minutes; this also eased computation time.
>
> We used two H.264 encoder implementations: X264, which is an open-source codec that can operate in everything from real-time to slow, and JM which is the reference implementation that was used to develop H.264. JM is very slow but attempts to be very efficient in terms of bits per quality. The results were as follows:
>
> X264 baseline vs VP8: H.264 wins with 1%
> JM baseline vs VP8: H.264 wins with 4%
>
> Running times:
> X264: 1 hour 3 minutes
> VP8: 2 hours 0 minutes
> JM: order of magnitude slower
>
> It is interesting to note that the measurements are more stable in the new test; the variance of the percentages for the sequences is now around 70, down from around 700 in Google's test of April 3rd.  We believe this is due to the removal of the rate controller, which acts like noise on the measurements.
>
> We also tried setting H.264 to constrained high (no interlace and no B-pictures, compared to high). The results were then:
>
> X264 constrained high vs VP8: H.264 wins with 25%
> JM constrained high vs VP8: H.264 wins with 24%
>
> We also note that the script that Google provided to calculate the rate differences ("BD-rate") does not give exactly the same numbers as the JCT-VC-way of calculating BD-rate. The main difference is that the JM score for constrained high is better (around 29%) if the JCT-VC way of calculating BD-rate is used.
>
> In summary we think that proper testing can conclude that there is no clear performance advantage to any codec between VP8 and H.264 baseline. When comparing VP8 against H.264 constrained high on the other hand, it seems like there is an advantage for H.264 constrained high.
>
> The attached file includes the files necessary to reproduce the test.
>
> Best Regards,
>
> Bo Burman
>
>
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