TSN streams are expected to send packets to the network in a well
defined "pace", which is arbitrarily defined for each stream. This pace
is defined by the "measurement interval" property of a stream.
When the AVTP CVF payloader element - avtpcvfpay - fragments a video
frame that is too big to be sent to the network, it currently defines
that all fragments should be transmitted at the same time (via DTS
property of GstBuffers generated, as sink will use those to time the
transmission of the AVTPDU). This doesn't comply with stream definition,
which also has a limit on how many packets can be sent on a given
measurement interval.
This patch solves that by spreading in time the DTS of the GstBuffers
containing the AVTPDUs. Two new properties, "measurement-interval" and
"max-interval-frames", added to avptcvfpay element so that it knows
stream measurement interval and how many AVTPDUs it can send on any of
them. More details on the method used to proper spread DTS/PTS according
to measurement interval can be found in a code commentary inside this patch.
Tests also added for the new property and behaviour.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1004>
This patch introduces the AVTP Compressed Video Format (CVF) payloader
specified in IEEE 1722-2016 section 8. Currently, this payload only
supports H.264 encapsulation described in section 8.5.
Is also worth noting that only single NAL units are encapsulated: no
aggregation or fragmentation is performed by the payloader.
An interesting characteristic of CVF H.264 spec is that it defines an
H264_TIMESTAMP, in addition to the AVTP timestamp. The later is
translated to the GST_BUFFER_DTS while the former is translated to the
GST_BUFFER_PTS. From AVTP CVF H.264 spec, it is clear that the AVTP
timestamp is related to the decoding order, while the H264_TIMESTAMP is
an ancillary information to the H.264 decoder.
Upon receiving a buffer containing a group of NAL units, the avtpcvfpay
element will extract each NAL unit and payload them into individual AVTP
packets. The last AVTP packet generated for a group of NAL units will
have the M bit set, so the depayloader is able to properly regroup them.
The exact format of the buffer of NAL units is described on the
'codec_data' capability, which is parsed by the avtpcvfpay, in the same
way done in rtph264pay.
This patch reuses the infra provided by gstavtpbasepayload.c.
