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A frame where motion had been detected in zones 1 and 3 will include the motion zone identifiers in the
pragma text.
nal_length = 0x<nnnn> (length depends on data)
nal_code = 0x06 (SEI)
sei_payload_type = 0x05 (User data)
sei_payload_length = 0x3d
uuid = 0x<nnnn> (16 bytes, camera specific)
msg_type = 0x01 Pragma trigger message)
msg_length = 0x001a
msg_data = "Pragma: trigger=motion 1 3"
msg_type = 0x02 (Timestamp message)
msg_length = 0x000d
msg_data = "1285697899840" (milliseconds since epoch)
rbsp_trailer = 0x80
The motion detection may occur and the pragma information be inserted in a frame 0, 1 or 2 frames before the
actual image data reflects motion. In other words, the SEI information may be considered a pre-trigger, but
the number of pre-triggered frames is not deterministic.
