Add bit stream filter support

[leandromoreira]

Add support to Bit Stream Filter.

Bitstream filters transform encoded media data without decoding it. This allows e.g. manipulating various header values. Bitstream filters operate on AVPackets.

The bitstream filtering API is centered around two structures: AVBitStreamFilter and AVBSFContext. The former represents a bitstream filter in abstract, the latter a specific filtering process. Obtain an AVBitStreamFilter using av_bsf_get_by_name() or av_bsf_iterate(), then pass it to av_bsf_alloc() to create an AVBSFContext. Fill in the user-settable AVBSFContext fields, as described in its documentation, then call av_bsf_init() to prepare the filter context for use.

Submit packets for filtering using av_bsf_send_packet(), obtain filtered results with av_bsf_receive_packet(). When no more input packets will be sent, submit a NULL AVPacket to signal the end of the stream to the filter. av_bsf_receive_packet() will then return trailing packets, if any are produced by the filter.

Finally, free the filter context with av_bsf_free().

src: https://github.com/FFmpeg/FFmpeg/blob/release/5.1/libavcodec/bsf.h

[asticode]

Thanks for the PR ❤️

I've made some comments 👍

Also, could you add tests?

• for the bit stream filter, the best would be to add a Name() string function (and a String() string function that would return the result of .Name()) in the struct, and in the test find a filter available by default and assert its name.
• for the bit stream filter context, the best would be to test allocating a context, initializing it and freeing it. Before freeing it, that would be a good idea to test the getters and setters as well (for the Class you can use something similar to this). If you find a way to test Send/ReceivePacket that would be even better, but that's optionnal.

[leandromoreira]

done

[asticode]

Nice! Did you see the part of my comment mentioning tests as well?

[leandromoreira]

Thanks for the PR ❤️

I've made some comments 👍

Also, could you add tests?

* for the bit stream filter, the best would be to add a `Name() string` function (and a `String() string` function that would return the result of .Name()) in the struct, and in the test find a filter available by default and assert its name.

* for the bit stream filter context, the best would be to test allocating a context, initializing it and freeing it. Before freeing it, that would be a good idea to test the getters and setters as well (for the Class you can use something similar to [this](https://github.com/asticode/go-astiav/blob/master/format_context_test.go#L30)). If you find a way to test Send/ReceivePacket that would be even better, but that's optionnal.

Will add tests later :) please don't merge it yet.

[leandromoreira]

The tests I introduced are passing:

 === RUN   TestBitStreamFilterContext
--- PASS: TestBitStreamFilterContext (0.00s)
=== RUN   TestBitStreamFilter
--- PASS: TestBitStreamFilter (0.00s)

They shouldn't cause any side effect though.

[asticode]

The tests I introduced are passing:

 === RUN   TestBitStreamFilterContext
--- PASS: TestBitStreamFilterContext (0.00s)
=== RUN   TestBitStreamFilter
--- PASS: TestBitStreamFilter (0.00s)

They shouldn't cause any side effect though.

Yeah testing SendPacket and ReceivePacket like this doesn't feel right, but I have no alternative for now. Could add // TODO Test SendPacket and ReceivePacket at the end of TestBitStreamFilterContext in bit_stream_filter_context_test.go though?

[asticode]

Windows tests seem to be failing (without any error message, that would be too easy otherwise 🤦 ), I'll take a look

[leandromoreira]

Windows tests seem to be failing (without any error message, that would be too easy otherwise 🤦 ), I'll take a look

:( I don't have a windows to test, otherwise I'd try to help.

BTW, is it easy to run tests locally on macos?

[asticode]

:( I don't have a windows to test, otherwise I'd try to help.

I'm using Github actions to run tests on windows and linux

BTW, is it easy to run tests locally on macos?

Sure, as long as ffmpeg is installed and the proper environment variables are setup, a simple go test does the trick

[asticode]

Thanks again for the PR! ❤️

Let me know whether you need a tag 👍