[MEncoder-users] Frame Rate and file Size

[bruno agostini]

I have a strange problem I quite don't understand here. I want to make a
divx with 2000 jpg. So I use the following command:

mencoder  mf://*.jpg -mf w=512:h=256:fps=30:type=jpg -ofps 30 -nosound
-ovc lavc -lavcopts vcodec=msmpeg4v2:vbitrate=500 -o movie.avi

Then I get a 5 MB movie of 67 sec and I am happy with that. But I also
want to make a version at 5 fps (it is a scientific movie so that I want
to have a slow version to see details without having to settle the speed
on the player everytime). So I use:

mencoder  mf://*.jpg -mf w=512:h=256:fps=5:type=jpg -ofps 5 -nosound
-ovc lavc -lavcopts vcodec=msmpeg4v2:vbitrate=500 -o movie.avi

I have the same number of frame (2000), the same effective bitrate (I
checked), only the fps (i.e. the movie is now 400 sec) has changed, but
I get a 25 MB movie! But there is the same amount of information to
encode in the second case isn't it? So why should the file size be
different?
 
thanks for your help!