Bug 282227 – [WebCodec] Test incorrectly assumes that no frame returned should be an error

• Status: NEW
• Component: WebRTC

[Jean-Yves Avenard [:jya]]

Both imported/w3c/web-platform-tests/webcodecs/videoDecoder-codec-specific.https.any.html and 
imported/w3c/web-platform-tests/webcodecs/videoDecoder-codec-specific.https.any.worker.html

assume that when decoding some content that yields no frame should be considered an error.

This is incorrect.

Both VP9 and VP8 may require to decode multiple frames before a decoded frame is returned.

Both VP8 and VP9 have the concept of alternative reference frames, and to properly decode those you need to input multiple frames to get one out.
When this occurs it shouldn't be considered as an error.

[Jean-Yves Avenard [:jya]]

The file ./LayoutTests/imported/w3c/web-platform-tests/webcodecs/vp8.webm contains such alternative reference frame

http://blog.webmproject.org/2010/05/inside-webm-technology-vp8-alternate.html
```
VP8 uses three types of reference frames for inter prediction: the last frame, a "golden" frame (one frame worth of decompressed data from the arbitrarily distant past) and an alternate reference frame. Overall, this design has a much smaller memory footprint on both encoders and decoders than designs with many more reference frames. In video compression, it is very rare for more than three reference frames to provide significant quality benefit, but the undesirable increase in memory footprint from the extra frames is substantial.
Unlike other types of reference frames used in video compression, which are displayed to the user by the decoder, the VP8 alternate reference frame is decoded normally but is never shown to the user. 
```

[Radar WebKit Bug Importer]

<rdar://problem/138819149>