alphacombine element is a simple element that assumes buffers are always
paired, or at least that missing buffers are signalled with a GAP. The QoS
implementation in the GstVideoDecoder base class allow decoders dropping
frames independently and that could lead to stall in alphacombine.

Part-of: <!2326>

Daniel Almeida authored 3 years ago and Nicolas Dufresne committed 3 years ago

codecalpha is a new plugin introduced to support VP8/VP9 alpha as
defined in the WebM and Matroska specifications. It splits the stream
into two streams, one for the alpha and one for the actual content,
then it decodes them separately with vpxdec and finally combine the
results as A420 or AV12 (i.e. YUV + an extra alpha plane).

The workflow above is setup by means of a bin, gstcodecalphabin.

This patch simulates the same workflow into the v4l2codecs namespace,
thus using the new v4l2 stateless decoders for hardware acceleration.

This is so we can register the new alpha decode elements only if the
hardware produces formats we support, i.e. I420 or NV12 for now.

Part-of: <!2272>

Nicolas Dufresne authored 3 years ago and GStreamer Marge Bot committed 3 years ago

The multiqueue was too flexible for our need, allowing to queue passed
the configured threshold. It also didn't work well when trying to
propagate EOS flow return.

Part-of: <!2238>

This includes base class with wrappers bin that will create a static
pipeline capable of handling the VP8/VP9 alpha channel decoding
using two instances of vp8/vp9dec element each.

Part-of: <!2199>