DxInputSelector

DxInputSelector is a GStreamer element designed for multi-channel video streaming. It merges frames from multiple input streams into a single synchronized output stream, selecting frames based on Presentation Timestamp (PTS) ordering.

It is the entry boundary of the multi-stream domain — its src caps are application/x-dxvideoraw. See Multi-Stream Domain for the overall model.

Key Features

Stream Selection

• Among N input streams, DxInputSelector selects the buffer with the smallest PTS and forwards it downstream.
• This approach ensures that the output stream maintains temporal consistency across input channels.

Event Handling

• After all input sink pads have received their first sticky events, DxInputSelector emits a single domain STREAM_START, a CAPS event with application/x-dxvideoraw, and a SEGMENT event downstream — these are the L1A events that drive the merged chain.
• The original per-stream sticky events (STREAM_START, CAPS=video/x-raw, SEGMENT, TAG, GAP) and per-stream EOS are wrapped as application/x-dx-wrapped-event (a CUSTOM_DOWNSTREAM event) tagged with the stream-id, and emitted alongside the L1A events. Downstream elements that need per-stream lifecycle information (e.g., DxOsd, DxOutputSelector) read them from the wrap.
• EOS received on a single sink does not propagate downstream immediately. It is wrapped as a per-stream EOS. Only after every sink has received EOS is a single global EOS emitted downstream.
• Upstream QoS/RECONFIGURE events arriving as CUSTOM_UPSTREAM wraps from DxOutputSelector are unwrapped and routed to the corresponding sink pad's upstream peer.

Hierarchy

GObject
 +----GInitiallyUnowned
       +----GstObject
             +----GstElement
                   +----GstAggregator
                         +----GstDxInputSelector

Pad Templates

Sink (input, request pad sink_%u)

Property	Value
Format	video/x-raw

Src (output, always pad src)

Property	Value
Format	application/x-dxvideoraw

Properties

Name	Description	Type	Default Value
name	Sets the unique name of the DxInputSelector element.	String	"dxinputselector0"
max-queue-size	Maximum number of buffers to queue per input stream.	Unsigned Integer	2

NOTE

• If an incoming buffer does not contain DXFrameMeta, the element creates a new DXFrameMeta using dx_create_frame_meta() and assigns the sink pad index as the stream_id.
• This metadata tagging is essential for downstream elements that rely on stream identification, such as DxOutputSelector.
• When two input streams share the same framerate, their buffers can interleave by up to 50% in the merged output (i.e., a buffer from one stream may arrive several PTS positions ahead or behind its strict interleave position before the other stream catches up). This is expected behavior of PTS-based selection.

Downstream Element Restrictions

GstBaseTransform-based elements that depend on stable caps negotiation (e.g., DxScale, DxConvert) must not be placed directly downstream of DxInputSelector. The reason is that the src caps of DxInputSelector are application/x-dxvideoraw, not video/x-raw — standard GStreamer elements (videoconvert, videoscale, compositor, …) will fail caps negotiation.
Elements designed for the domain (DxPreprocess, DxInfer, DxPostprocess, DxTracker, DxOsd, DxRate, DxMsgConv, DxMsgBroker) accept application/x-dxvideoraw and read per-buffer dimensions/stream-id from DXFrameMeta.
See Multi-Stream Domain for the placement matrix.

---