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Hacking on PyAV

The Real Goal

The goal here is to not only wrap ffmpeg in Python and provide nearly complete access to the library, but to make it easier to use without the need to understand the full library.

For example:

  • we don't need to mimick the underlying project structure as much as we do;
  • we shouldn't be exposing audio attributes on a video codec, and vise-versa;
  • the concept of packets could be abtracted away to yielding frames from streams;
  • time should be a real number, instead of a rational;
  • ...

FFmpeg vs Libav

We test compile and link tiny executables to determine what functions and structure members are availible, and some very small shim headers to smooth out the differences.

We continually test multiple versions of FFmpeg/Libav, as well as Linux/OS X, and Python 2/3.

You can use the same build system as Travis for local development:

source scripts/activate.sh ffmpeg 2.7
./scripts/build-deps
make
nosetests

Library Detection

Macros will be defined for the libraries we link against. In particular, you will have either PYAV_HAVE_LIBSWRESAMPLE or PYAV_HAVE_LIBAVRESAMPLE.

Function Detection

Macros will be defined for a few functions that are only in one of FFmpeg or LibAV. For example, there may be a PYAV_HAVE_AVFORMAT_CLOSE_INPUT macro. See the reflect command in setup.py to add more.

Struct Member Detection

Macros will be defined for structure members that are not garunteed to exist (usually because LibAV deprecated and removed them, while FFmpeg did not). For example, there may be a PYAV_HAVE_AVFRAME__MB_TYPE macro if the AVFrame.mb_type member exists.

Class Relationships

  • Context.streams is a list of Stream.
  • Packet.stream is the Stream that it is from.
  • Frame has no relationships in Python space.

Time in Libraries

Time is usually represented as fractions; there is often a uint64_t pts or dts variable in AVRational time_base units.

Both AVStream and AVCodecContext have a time_base. While encoding, they are for the AVPacket and AVFrame times respectively. However, while decoding all times are in AVStream.time_base.

When there is no time_base (such as on AVFormatContext), there is an implicit time_base of 1/AV_TIME_BASE.