[FFmpeg-user] Using -re input option with UDP inputs

Marton Balint cus at passwd.hu
Mon Nov 25 23:40:04 EET 2019

On Mon, 25 Nov 2019, Dennis Mungai wrote:

> Hello there,
> Are there any known caveats to using -re with UDP streams as input, such as
> packet loss?
> I’ve seen the -re option being discouraged with the likes of RTP, does the
> same apply to mpegts UDP streams?

Yes. -re is widely misundetstood for some reason, despite that it is 
documented reasonably well:

-re (input)
Read input at native frame rate. Mainly used to simulate a grab device, or 
live input stream (e.g. when reading from a file). Should not be used with 
actual grab devices or live input streams (where it can cause packet 
loss). By default ffmpeg attempts to read the input(s) as fast as 
possible. This option will slow down the reading of the input(s) to the 
native frame rate of the input(s). It is useful for real-time output (e.g. 
live streaming).

So you only need -re if you want to limit reading your input to realtime 
speed in order to generate your output at realtime speed. If your input is 
already realtime, then you don't need it. If your output is inherently 
realtime (e.g. decklink), you don't need it either.


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