decklinksrc: Use a more accurate capture time

Use the hardware reference clock time when the frame was finished being captured
instead of a time much further down the road.

This improves the stability/accuracy of buffer times.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2040>

diff --git a/sys/decklink/gstdecklink.cpp b/sys/decklink/gstdecklink.cpp
index 2cef6b9..7916e8b 100644 (file)
--- a/sys/decklink/gstdecklink.cpp
+++ b/sys/decklink/gstdecklink.cpp
@@ -1023,6 +1023,41 @@ public:
 
     if (clock) {
       capture_time = gst_clock_get_time (clock);
+      if (video_frame) {
+       // If we have the actual capture time for the frame, compensate the
+       // capture time accordingly.
+       //
+       // We do this by subtracting the belay between "now" in hardware
+       // reference clock and the time when the frame was finished being
+       // capture based on the same hardware reference clock.
+       //
+       // We then subtract that difference from the "now" on the gst clock.
+       //
+       // *Technically* we should be compensating that difference for the
+       // difference in clock rate between the "hardware reference clock" and
+       // the GStreamer clock. But since the values are quite small this has
+       // very little impact.
+       BMDTimeValue hardware_now;
+       res = m_input->input->GetHardwareReferenceClock (GST_SECOND, &hardware_now, NULL, NULL);
+       if (res == S_OK) {
+         res =
+           video_frame->GetHardwareReferenceTimestamp (GST_SECOND,
+                                                       &hardware_time, &hardware_duration);
+         if (res != S_OK) {
+           GST_ERROR ("Failed to get hardware time: 0x%08lx", (unsigned long) res);
+           hardware_time = GST_CLOCK_TIME_NONE;
+           hardware_duration = GST_CLOCK_TIME_NONE;
+         } else {
+           GstClockTime hardware_diff = hardware_now - hardware_time;
+           GST_LOG ("Compensating capture time by %" GST_TIME_FORMAT,
+                    GST_TIME_ARGS (hardware_diff));
+           if (capture_time > hardware_diff)
+             capture_time -= hardware_diff;
+           else
+             capture_time = 0;
+         }
+       }
+      }
       if (capture_time > base_time)
         capture_time -= base_time;
       else