[FFmpeg-user] modify filter during runtime

[lucie Lemonnier]

Hello,
I use ffmpeg to stream video with rtp protocol, i use crop and overlay filter.
ffmpeg.exe
 -i cube.avi -loop 0 -vf "movie=lignes.png 
[mv1];movie=testsHDflvtransparent1.flv [mv2];[in][mv1] overlay=0:0 
[out];[out][mv2] overlay=0:0 [out2];[out2] crop=1000:680:200:100" rtp://ip:port
but i want to modify cropping parameters during runtime and change the overlay filter to another movies or png.
I searched in ffmepg.c but I have difficulty to modify it to have dynamic crop and overlay.
How do I do this?
Can you help me, please?
Lucie