Diary Of An x264 Developer

Update: post now contains a Theora comparison as well; see below.

JPEG is a very old lossy image format.  By today’s standards, it’s awful compression-wise: practically every video format since the days of MPEG-2 has been able to tie or beat JPEG at its own game.  The reasons people haven’t switched to something more modern practically always boil down to a simple one — it’s just not worth the hassle.  Even if JPEG can be beaten by a factor of 2, convincing the entire world to change image formats after 20 years is nigh impossible.  Furthermore, JPEG is fast, simple, and practically guaranteed to be free of any intellectual property worries.  It’s been tried before: JPEG-2000 first, then Microsoft’s JPEG XR, both tried to unseat JPEG.  Neither got much of anywhere.

Now Google is trying to dump yet another image format on us, “WebP”.  But really, it’s just a VP8 intra frame.  There are some obvious practical problems with this new image format in comparison to JPEG; it doesn’t even support all of JPEG’s features, let alone many of the much-wanted features JPEG was missing (alpha channel support, lossless support).  It only supports 4:2:0 chroma subsampling, while JPEG can handle 4:2:2 and 4:4:4.  Google doesn’t seem interested in adding any of these features either.

But let’s get to the meat and see how these encoders stack up on compressing still images.  As I explained in my original analysis, VP8 has the advantage of H.264′s intra prediction, which is one of the primary reasons why H.264 has such an advantage in intra compression.  It only has i4x4 and i16x16 modes, not i8x8, so it’s not quite as fancy as H.264′s, but it comes close.

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Back when I originally reviewed VP8, I noted that the official decoder, libvpx, was rather slow.  While there was no particular reason that it should be much faster than a good H.264 decoder, it shouldn’t have been that much slower either!  So, I set out with Ronald Bultje and David Conrad to make a better one in FFmpeg.  This one would be community-developed and free from the beginning, rather than the proprietary code-dump that was libvpx.  A few weeks ago the decoder was complete enough to be bit-exact with libvpx, making it the first independent free implementation of a VP8 decoder.  Now, with the first round of optimizations complete, it should be ready for primetime.  I’ll go into some detail about the development process, but first, let’s get to the real meat of this post: the benchmarks.

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I’ve been working the past few weeks to help finish up the ffmpeg VP8 decoder, the first community implementation of On2′s VP8 video format.  Now that I’ve written a thousand or two lines of assembly code and optimized a good bit of the C code, I’d like to look back at VP8 and comment on a variety of things — both good and bad — that slipped the net the first time, along with things that have changed since the time of that blog post.

These are less-so issues related to compression — that issue has been beaten to death, particularly in MSU’s recent comparison, where x264 beat the crap out of VP8 and the VP8 developers pulled a Pinocchio in the developer comments.  But that was expected and isn’t particularly interesting, so I won’t go into that.  VP8 doesn’t have to be the best in the world in order to be useful.

When the ffmpeg VP8 decoder is complete (just a few more asm functions to go), we’ll hopefully be able to post some benchmarks comparing it to libvpx.

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Back in my original post about Internet video, I made some initial comments on the hope that VP8 would solve the problems of web video by providing a supposed patent-free video format with significantly better compression than the current options of Theora and Dirac. Fortunately, it seems I was able to acquire access to the VP8 spec, software, and source a good few days before the official release and so was able to perform a detailed technical analysis in time for the official release.

The questions I will try to answer here are:

1. How good is VP8? Is the file format actually better than H.264 in terms of compression, and could a good VP8 encoder beat x264? On2 claimed 50% better than H.264, but On2 has always made absurd claims that they were never able to back up with results, so such a number is almost surely wrong. VP7, for example, was claimed to be 15% better than H.264 while being much faster, but was in reality neither faster nor higher quality.

2. How good is On2′s VP8 implementation? Irrespective of how good the spec is, is the implementation good, or is this going to be just like VP3, where On2 releases an unusably bad implementation with the hope that the community will fix it for them? Let’s hope not; it took 6 years to fix Theora!

3. How likely is VP8 to actually be free of patents? Even if VP8 is worse than H.264, being patent-free is still a useful attribute for obvious reasons. But as noted in my previous post, merely being published by Google doesn’t guarantee that it is. Microsoft did similar a few years ago with the release of VC-1, which was claimed to be patent-free — but within mere months after release, a whole bunch of companies claimed patents on it and soon enough a patent pool was formed.

We’ll start by going through the core features of VP8. We’ll primarily analyze them by comparing to existing video formats.  Keep in mind that an encoder and a spec are two different things: it’s possible for good encoder to be written for a bad spec or vice versa! Hence why a really good MPEG-1 encoder can beat a horrific H.264 encoder.

But first, a comment on the spec itself.

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This is going to be a much longer post than usual, as it’s going to cover a lot of ground.

The internet has been filled for quite some time with an enormous number of blog posts complaining about how Flash sucks–so much that it’s sounding as if the entire internet is crying wolf.  But, of course, despite the incessant complaining, they’re right: Flash has terrible performance on anything other than Windows x86 and Adobe doesn’t seem to care at all.  But rather than repeat this ad nauseum, let’s be a bit more intellectual and try to figure out what happened.

Flash became popular because of its power and flexibility.  At the time it was the only option for animated vector graphics and interactive content (stuff like VRML hardly counts).  Furthermore, before Flash, the primary video options were Windows Media, Real, and Quicktime: all of which were proprietary, had no free software encoders or decoders, and (except for Windows Media) required the user to install a clunky external application, not merely a plugin.  Given all this, it’s clear why Flash won: it supported open multimedia formats like H.263 and MP3, used an ultra-simple container format that anyone could write (FLV), and worked far more easily and reliably than any alternative.

Thus, Adobe (actually, at the time, Macromedia) got their 98% install base.  And with that, they began to become complacent.  Any suggestion of a competitor was immediately shrugged off; how could anyone possibly compete with Adobe, given their install base?  It’d be insane, nobody would be able to do it.  They committed the cardinal sin of software development: believing that a competitor being better is excusable.  At x264, if we find a competitor that does something better, we immediately look into trying to put ourselves back on top.  This is why x264 is the best video encoder in the world.  But at Adobe, this attitude clearly faded after they became the monopoly.  This is the true danger of monopolies: they stymie development because the monpolist has no incentive to improve their product.

In short, they drank their own Kool-aid.  But they were wrong about a few critical points.

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