Frame index from GST_BUFFER_PTS.

[Mark Cox]

Hello GStreamer-devel,

I am trying to write a GStreamer element, embedfoo, which embeds data
in received GStreamer video frame buffers using the function
gst_buffer_add_meta. The data added to the buffer is retrieved from a
database using the frame index of the GstBuffer.

I am using the value of GST_BUFFER_PTS and the framerate to obtain the
frame index of the received GstBuffers. The issue I am having is that
the value of the PTS timestamps in the received GstBuffer objects
differs according to the source.

For example, if I output the value of GST_BUFFER_PTS for the first
buffer received in the following pipelines I get differing results:

Pipeline 1:
  videotestsrc ! video/x-raw,framerate=15/1 ! embedfoo
location=database.txt ! x265enc ! mpegtsmux ! filesink location=test.ts

Pipeline 1 First PTS value:
  0

Pipeline 2:
  filesrc location=test.ts ! decodebin ! embedfoo location=database.txt !
videoconvert ! ximagesink

Pipeline 2 First PTS value:
  125000000

The embedfoo element is implemented as a subclass of GstBaseTransform
and I am using GStreamer 1.14.5 as provided by Ubuntu 18.04.

I have tried using the methods gst_segment_position_from* and
gst_segment_to_stream_time using the segment member of
GstBaseTransform, but I am unable to correctly calculate the frame
index of a buffer from the PTS timestamp.

Any pointers would be awesome.

Thanks,
Mark
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