Dynamic textoverlay from network

uler3161 uler3161 at hotmail.com
Mon Jun 8 11:20:52 PDT 2015


I'm working on what is essentially a videoconferencing system using two
Raspberry Pi model 2 units. They communicate through a PC (which acts kind
of like a switchboard). Everything works pretty well, but we'd like to add
some text overlay and I haven't found a good way to do it. We'll most likely
need a countdown (Something like timeoverlay, but instead it would count
down from some selected value such as 20 minutes). It'd also be nice to put
a message onto the video at any point.

A couple ideas that I thought might work:

1. On the PC I currently forward the stream from one Pi to the other. I
thought about inserting here. What I found is that I need to decode the
stream (it's h264), add the overlay, then re-encode and transmit to the
destination. Unfortunately this appears to put in quite a lot of lag. Maybe
a second or two. Not so great for video conferencing. I'm pretty sure this
is due to the h264 decode and/or encode.

2. Do the overlay directly on the Pi. I can put this right after decoding
the stream into raw. I've tested with timeoverlay and it seems to work
without adding any delay. But, the problem is how to get the text overlay
that I want. I want the countdown to be exactly the same on both Pis, but
with something like timeoverlay that would require both Pis starting
gstreamer at exactly the same point. So my thought was to use the PC to
generate the countdown and transmit it over the network to the Pis. If I
could do this, then I could also probably use the same method to send any
message to both Pis to display.

I'm using gst-launch-1.0 version 1.4.5. I'm not afraid to write code, but
would prefer to have something I could just use without code. I see there is
a text_sink on the textoverlay. Can that be of use, maybe with the tcp

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