Devlog

As part of our ongoing efforts to extend GStreamer's support for ancillary data, I've recently improved the ancillary data handling in the Blackmagic DeckLink plugin. This plugin can be used to capture or output SDI/HDMI/ST2110 streams with Blackmagic DeckLink capture/output cards.

Previously only CEA 608/708 closed captions and AFD/Bar ancillary data was handled in that plugin. Now it can also additionally handle any other kind of ancillary data via GstAncillaryMeta and leave interpretation or handling of the concrete payload to the application or other elements.

This new behaviour was added in this MR, which is part of git main now, and can be enabled via the output-vanc properties on the video source / sink elements.

The same was already supported before by the plugin for AJA capture/output cards.

For example the following pipeline can be used to forward an SDI stream from an one DeckLink card to an AJA card

gst-launch-1.0 decklinkvideosrc output-vanc=true ! queue ! combiner.video \
  decklinkaudiosrc ! queue ! combiner.audio \
  ajasinkcombiner name=combiner ! ajasink handle-ancillary-meta=true

With both the AJA and DeckLink sink elements, special care is needed to not e.g. output closed captions twice. Both sinks can retrieve them from GstVideoClosedCaptionMeta and GstAncillaryMeta, and outputting from both will likely lead to problems at the consumer of the output.

A couple of weeks ago I implemented support for static HDR10 metadata in the decklinkvideosink and decklinkvideosrc elements for Blackmagic video capture and playout devices. The culmination of this work is available from MR 7124 - decklink: add support for HDR output and input

This adds support for both PQ and HLG HDR alongside some improvements in colorimetry negotiation. Static HDR metadata in GStreamer is conveyed through caps.

The first part of this is the 'colorimetry' field in video/x-raw caps. decklinkvideosink and decklinkvideosrc now support the colorimetry values 'bt601', 'bt709', 'bt2020', 'bt2100-hlg', and 'bt2100-pq' for any resolution. Previously the colorimetry used was fixed based on the resolution of the video frames being sent or received. With some glue code, the colorimetry is now retrieved from the Decklink API and the Decklink API can ask us for the colorimetry of the submitted video frame. Arbitrary colorimetry support is not supported on all Decklink devices and we fallback to the previous fixed list based on frame resolution when not supported.

Support for HDR metadata is a separate feature flag in the Decklink API and may or may not be present independent of Decklink's arbitrary colour space support. If the Decklink device does not support HDR metadata, then the colorimetry values 'bt2100-hlg', and 'bt2100-pq' are not supported.

In the case of HLG, all that is necessary is to provide information that the HLG gamma transfer function is being used. Nothing else is required.

In the case of PQ HDR, in addition to providing Decklink with the correct gamma transfer function, Decklink also needs some other metadata conveyed in the caps in the form of the 'mastering-display-info' and 'light-content-level' fields. With some support from GstVideoMasteringDisplayInfo, and GstVideoContentLightLevel the relevant information signalled to Decklink and can be retrieved from each individual video frame.