[FFmpeg-user] Dimensions of the input video as a variable

[Wolfgang Hugemann]

I would like to superimpose the audiograph of the video's audio on the 
video itself using 'blend'. I sucseeded in doing so with the line:

c:\programme\ffmpeg\bin\ffmpeg -y -i test.avi -filter_complex 
"[0:a]showwaves=s=640x480:mode=line[waves];[0:v]setsar=1:1[video];[waves][video]blend=all_mode='lighten'" 
-c:v libxvid -c:a copy ovl.avi

As you see, I supplied the dimensions of the input video (640x480) in 
the command line. I would like to turn this into a more general script, 
determining the dimensions of the input video on the fly, but how do I 
do that?

I tried "s=wxh" and alike, by that doesn't seem to work.

Wolfgang