What does 'num-buffers' do in gstreamer?

user_9230398 0
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I could only find few pages giving a one-liner explanation of num-buffers. Like this one.

Number of buffers to output before sending EOS (End of Stream). Default = -1 (unlimited)

I have a dummy pipeline using gst-launch-1.0 multifilesrc with default loop=False. The pipeline loops because of num-buffers=-1 as the default.

I don't want it to loop, which happens to be when I set num-buffers=1 or literally any other finite number.

What does it mean to be num-buffers=1 (or any value in that sense)?

Edit: Sample pipelines with a 10-second video

# 1. With loop=false and num-buffers=1
$> GST_DEBUG=3 gst-launch-1.0 multifilesrc location=preview.h264 loop=false num-buffers=1 ! h264parse ! avdec_h264 ! fakesink
...
Got EOS from element "pipeline0".
Execution ended after 0:00:00.425738029
...

# 2. With loop=false and num-buffers=10
$> GST_DEBUG=3 gst-launch-1.0 multifilesrc location=preview.h264 loop=false num-buffers=10 ! h264parse ! avdec_h264 ! fakesink
...
Got EOS from element "pipeline0".
Execution ended after 0:00:04.256451070
...

# 3. With neither loop flag (default=false) nor num-buffers (default=-1, unlimited)
$> GST_DEBUG=3 gst-launch-1.0 multifilesrc location=preview.h264 ! h264parse ! avdec_h264 ! fakesink
...This never ends because num-buffers=-1. Why?...

I didn't get any warnings in any case.

gstreamer
1 Answers

"num-buffers" defines how many frames will be published by a given element like videotestsrc. After sending "num-buffers", EOS event is published.

I find it useful in tests when you can define number of frames and framerate and then set expectations about how many frames shall be received during given time (e.g. using probe).

multifilesrc doesn't seem to support "num-buffers": it will read all files and exits (or start again when loop=True). You should see a warning when setting "num-buffers" on multifilesrc.

multifilesrc inherits from GstBaseSrc element and has num-buffers property. It should be used to replay a sequence of frames as video:

gst-launch-1.0 multifilesrc location="%08d.png" loop=true num-buffers=1000 ! decodebin ! videoconvert ! ximagesink

To replay images named 00000000.png to 99999999.png one after another.

For your purpose, just use filesrc element, not multifilesrc.

user_10761130
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