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Encoding And Muxing FATE

February 21st, 2009 by Multimedia Mike

One weak spot in FATE‘s architecture has to do with encoding and muxing formats. So far, I have been content to allow the master regression suite handle the encode/mux tests for the most important formats that FFmpeg supports. But the suite doesn’t handle everything. Plus, I still have the goal of eventually breaking up all of the regression suite’s functionality into individual FATE test specs.

At first, the brainstorm was to encode things directly to stdout so that nothing ever really has to be written to disk. The biggest problem with this approach is that stdout is non-seekable. For formats that require seeking backwards, this is a non-starter (e.g., a QuickTime muxer will always have to seek back to the start of the file and fill in the total size of the mdat atom that was just laid down on disk).

So it’s clear that an encode/mux test needs to commit bytes to some seekable media. Where is it okay to write to disk? I think $BUILD_PATH should be okay, since the build step already writes data there.

The natural flow of the master regression suite is to encode/mux a test stream, run a hash of the resulting stream, then demux/decode the encoded stream and run a hash on the result. In FATE, I imagine these 2 operations would be split into 2 separate tests. But how can I guarantee that one will run before the other? Right now, there’s no official guarantee of the order in which the test specs run. But I can — and plan to — change the policy via fate-script.py so that the tests are always run in order of their database IDs. That way, I can always guarantee that the encode/mux test is run and the correct sample file is waiting on disk before the corresponding demux/decode test executes.

I also need a way to verify the contents of the encoded file on disk. I think this can be handled via a new special directive along the lines of:

{MD5FILE,$BUILD_PATH/output} $BUILD_PATH/ffmpeg -i $SAMPLES_PATH/input/input -f format -y $BUILD_PATH/output

This will read the bytes of the file ‘output’ and compute the MD5 hash. This seems simple enough in a local environment. But it is another item that may pose challenges in the cross-FATE architecture I am working on with Mans which will support automated testing on less powerful/differently-targeted platforms.

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