Add ffmpeg-wasm-demuxer sample

[chcunningham]

Please see a live demo here https://chcunningham-samples.netlify.app/ffmpeg-wasm-demuxer/wasm_demuxer_demo.html

Partial fix for https://github.com/w3c/webcodecs/issues/78

[chcunningham]

@sandersdan, friendly ping :)

[dalecurtis]

Chris left Google and unfortunately won't be able to complete this PR. Chris you mentioned you might want to try to close this out still. Did you want me to leave this open for a bit longer?

[KevinBoeing]

Did somebody already tried to demux a file with a video and audiostream?

Currently I get the following warnings from FFmpeg:

• parser not found for codec h264, packets or times may be invalid.
• Could not find codec parameters for stream 0 (Video: h264, 1 reference frame (avc1 / 0x31637661), none, 1280x720 (0x0), 0/1, 2322 kb/s): unspecified pixel format
• Consider increasing the value for the 'analyzeduration' (0) and 'probesize' (5000000) options
• Could not find codec parameters for stream 1 (Audio: aac (mp4a / 0x6134706D), 48000 Hz, 1 channels, 69 kb/s): unspecified sample format
• Consider increasing the value for the 'analyzeduration' (0) and 'probesize' (5000000) options

When I try to read a Chunk, FFmpeg gives me the following error:

Invalid data found when processing input

[dalecurtis]

This copy of ffmpeg is built without proprietary codecs, otherwise we wouldn't be able to host/share it. You'll have to tweak the ffmpeg build options if you want other codecs enabled.

[dalecurtis]

Going to close this PR for now since we don't have anyone to finish it at this time. Others are welcome to clean it up and land it if they're so inclined.

[chrisn]

I'm interested to have a look at this if I can make some time.

[KevinBoeing]

Hey, sorry that I still comment under a closed PR but I am currently working on it. I am almost finished with this project, but have two thing I still dont understand / I am struggling with.

1. Is there an easy way to create a codec string using FFmpeg as required by the WebCodecs decoder configurations (e.g. mp4a.40.2)? I am already aware that I can get important codec information from the AVCodecParameters structure. The prefix (e.g. mp4a) can be determined by the codec_tag property and other important information is in my opinion the codec-profile and the codec-level. However, I'm still not sure how to get a mapping between the FFmpeg AVCodecParameters and the WebCodecs codec string.

2. In the FFmpegDemuxerBlockingHelper class, the AVIO_BUFFER_SIZE is set to 4,000 bytes. If I increase this value to 20,000 bytes, it works fine with a container that contains only a video stream or an audio stream. If it contains both, I need to set the AVIO_BUFFER_SIZE to the bytes of the container. Otherwise I get a "stream 0/1, offset 0x30 partial file" warning from ffmpeg and when I try to read the next frame (av_read_frame()), I get an error "Invalid data found when processing input". Is it correct that I always have to set the AVIO_BUFFER_SIZE to the size of the container or is there a workaround?