Monthly Archives: June 2010

Multiprocess FATE Revisited

I thought I had brainstormed a simple, elegant, multithreaded, deadlock-free refactoring for FATE in a previous post. However, I sort of glossed over the test ordering logic which I had not yet prototyped. The grim, possibly deadlock-afflicted reality is that the main thread needs to be notified as tests are completed. So, the main thread sends test specs through a queue to be executed by n tester threads and those threads send results to a results aggregator thread. Additionally, the results aggregator will need to send completed test IDs back to the main thread.



But when I step back and look at the graph, I can’t rationalize why there should be a separate results aggregator thread. That was added to cut down on deadlock possibilities since the main thread and the tester threads would not be waiting for data from each other. Now that I’ve come to terms with the fact that the main and the testers need to exchange data in realtime, I think I can safely eliminate the result thread. Adding more threads is not the best way to guard against race conditions and deadlocks. Ask xine.



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FFmpeg Has A Native VP8 Decoder

Thanks to David Conrad and Ronald Bultje who committed their native VP8 video decoder to the FFmpeg codebase yesterday. At this point, it can decode 14/17 of the VP8 test vectors that Google released during the initial open sourcing event. Work is ongoing on those 3 non-passing samples (missing bilinear filter). Meanwhile, FFmpeg’s optimization-obsessive personalities are hard at work optimizing the native decoder. The current decoder is already profiled to be faster than Google/On2’s official libvpx.

Testing
So it falls to FATE to test this on the ridiculous diversity of platforms that FFmpeg supports. I staged individual test specs for each of the 17 test vectors: vp8-test-vector-001 … vp8-test-vector-017. After the samples have propagated through to the various FATE installations, I’ll activate the 14 test specs that are currently passing.

Initial Testing Methodology
Inspired by Ronald Bultje’s idea, I built the latest FFmpeg-SVN with libvpx enabled. Then I selected between the reference and native decoders as such:

$ for i in 001 002 003 004 005 006 007 008 009 \
 010 011 012 013 014 015 016 017
do
  echo vp80-00-comprehensive-${i}.ivf
  ffmpeg -vcodec libvpx -i \
    /path/to/vp8-test-vectors-r1/vp80-00-comprehensive-${i}.ivf \
    -f framemd5 - 2> /dev/null
done > refs.txt

$ for i in 001 002 003 004 005 006 007 008 009 \
 010 011 012 013 014 015 016 017
do
  echo vp80-00-comprehensive-${i}.ivf
  ffmpeg -vcodec vp8 -i \
    /path/to/vp8-test-vectors-r1/vp80-00-comprehensive-${i}.ivf \
    -f framemd5 - 2> /dev/null
done > native.txt

$ diff -u refs.txt native.txt

That reveals precisely which files differ.

libvpx 0.9.1 and FFmpeg 0.6

Great news: Hot on the heels of FFmpeg’s 0.6 release, the WebM project released version 0.9.1 of their libvpx. I can finally obsolete my last set of instructions on getting FFmpeg-svn working with libvpx 0.9.

Building libvpx 0.9.1
Do this to build libvpx 0.9.1 on Unix-like systems:

libvpx’s build system has been firmed up a bit since version 0.9. It’s now smart enough to install when said target is invoked and it also builds the assembly language optimizations. Be advised that on 32- and 64-bit x86 machines, Yasm must be present (install either from source or through your package manager).

Building FFmpeg 0.6
To build the newly-released FFmpeg 0.6:

  • Install Vorbis through your package manager if you care to encode WebM files with audio; e.g., ‘libvorbis-dev’ is the package you want on Ubuntu
  • Download FFmpeg 0.6 from the project’s download page
  • Configure FFmpeg with at least these options: ./configure --enable-libvpx --enable-libvorbis --enable-pthreads; the final link step still seems to fail on Linux if the pthreads option is disabled
  • ‘make’

Verifying
Check this out:

$ ./ffmpeg -formats 2> /dev/null | grep WebM
  E webm            WebM file format

$ ./ffmpeg -codecs 2> /dev/null | grep libvpx
 DEV    libvpx          libvpx VP8

That means that this FFmpeg binary can mux a WebM file and can both decode and encode VP8 video via libvpx. If you’re wondering why the WebM format does not list a ‘D’ indicating the ability to demux a WebM file, that’s because demuxing WebM is handled by the general Matroska demuxer.

Doing Work
Encode a WebM file:

ffmpeg -i <input_file> <output_file.webm>

FFmpeg just does the right thing when it seems that .webm extension on the output file. It’s almost magical.

For instant gratification that the encoded file is valid, you can view it immediately using ‘ffplay’, if that binary was built (done by default if the right support libraries are present). If ffplay is not present, you can always execute this command line to see some decode operation:

ffmpeg -i <output_file.webm> -f framecrc -

FFmpeg 0.6; Something About HTML5

The FFmpeg project made a formal release yesterday. You can download version 0.6 from the project’s download page.

I’ve actually been seeing more news items about this today than I would have expected. This is most likely because the 0.6 release is named “Works with HTML5”. Let it never be said that nerds don’t know marketing. The team really latched on to the hottest buzzword going right now.

The name of the release refers to FFmpeg’s new native support for Google’s WebM format. It can mux and demux the WebM container, and decode the Vorbis audio, all natively (FFmpeg’s Vorbis encoder has been demoted to “experimental” for this release and it is recommended to enable libvorbis for encoding). But the big news is that this release can support Google’s libvpx natively for VP8 encoding and decoding, without having to apply any other patches.

Getting libvpx to compile still might be a bit tricky. Fortunately, the first pass of a native, independent VP8 decoder is currently in review on the ffmpeg-devel list.