[FFmpeg-user] Overlay and varied framerates

[Alexandre Ferrieux]

Hi,

I'm using the 'overlay' vfilter, along with the 'movie' source (as a patch), with great success. (Thanks again guys).


Now when I do this on live streams (from webcams), I see a differential latency settle: cam A is rendered realtime, camB 
a constant 2 seconds behind. Permutations show this is not linked to the order in the overlay graph, but to the cameras 
themselves. I suspect this is due to a slight but systematic difference in the cams' effective framerates: an intrinsic 
delta between the 2 clocks gives a linear drift, and the delay saturates when overrun occurs (ie all various buffers are 
filled)


Q1: how is time handled in the case of a multiple-input vfilter like overlay ?

Assuming the following timeline:

	A:     ....F1..F2.......F3.....
	B:     ..G1.G2.G3.G4.G5........

am I right to think that we get:

	A+B:   .....H1.H2.......H3.....

with: H1=F1+G1, H2=F2+G2, H3=F3+G3
?



Q2: If yes, what can I do so that, instead, overlay just "samples" the inputs in a nonblocking manner at regular 
intervals set by the target output framerate, thus:

	A+B:   ....J1..J2..J3..J4..J5..

with: J1=F1+G1, J2=F2+G3, J3=F2+G4, J4=F2+G5, J5=F3+G5
?

Thanks in advance,

-Alex