Sunday, 13 April 2014

A short story of interlacing in HEVC

And how it nearly ruined another video coding standard.

Everyone knows that interlacing was a trick in the '80s for pseudo motion compensation with analogue video. This more or less worked because it mimicked how television worked back then. This technique was preserved when flat panels for pc and tv were introduced, for a mix of backward compatibility and technical limitations, and video coding features interlacing in MPEG2 and H264 and similar.

However as with black and white, TACS and Gopher, old technology has to be replaced with modern and efficient technology, as a trade off of users' interests and technology providers' market prospects. In case you are not familiar, interlacing is a mess to support, makes decoding slower and heavily degrades quality. People saying that interlacing saves bandwidth do not know much about video coding and bad marketing claiming that higher resolution is better than higher framerate has an effect too.

So, when ITU and then MPEG set out to establish the mandates for a new video standard capable of superseding H264, it was decided that interlacing was old enough, did more harm than good and it was time for retirement: HEVC was going to be the first video codec to officially deprecate interlacing.

Things went pretty swell during its development, until a few months before the completion of the standard. A group of US companies complained that the proposed tools were not sufficient (a set of SEI messages and treating fields like progressive frames) and heavily protested with both standardisation bodies. ITU firmly rejected the idea (with the video group chair threatening to step down) while MPEG set out to understand the needs of the industry and see if there was anything that could be done.

An ad-hoc group was established to see if there was any evidence that interlaced coding tool would have improved the situation. Things looked really shady, the Requirements group even mentioned that it was the first time that an AhG was established to look for evidence, instead of establishing an AhG because there was evidence. Several liasons from EBU and other DVB members tried to point out this absurdity while the threat of adding interlacing back in HEVC became real. Luckily the first version of the specifications got published in the meantime, so this decision didn't slow down the standardisation process.

Why so much love towards interlacing? Well in the "rebellious" group defence, it is true that interlaced content in HEVC is less performant than in H264; however it is also true that such deinterlaced content in HEVC outperforms H264 in any configuration. Truth is that mass marketed deinterlacers (commonly found in televisions for example) bring a lot of royalty income, so it is normal that companies with vested interests would prefer to have interlacing in a soon-popular video standard like HEVC. Also in markets like US where the network operator (which has control on the encoding but not on the video source) might differ from the content provider, it could be politically difficult to act as a carrier only if you have to deinterlace a video.

However these problems are actually not enough for forcing every encoder, decoder, analyser to support a deprecated technology like interlacing. Technical problems can be solved with good deinterlacers at the top of the distribution chain, while political ones can be solved amending contracts. Plus having progressive only video will definitely improve quality and let the industry concentrate on other delicate subjects, like bit depth, both properties going in favour of users' interests.

At the last MPEG meeting, the "rebellious" group which had been working on reintroducing interlacing for a year provided no real evidence that interlaced coding tools would improve HEVC at all. The only sensible solution was to disband the group over this wasted effort and support progressive video only, which is what happened luckily. So now both ITU and MPEG support progressive video only and this has finally nailed it.

Interlacing is dead, long live progressive.

Written by Vittorio Giovara (
Published under a CC-BY-SA 3.0 license.

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