Particle attribute to density with a sequence ?

I tried to use a png sequence to map the density of particles, but it doesn’t work !

I tried to use Dvar as described in the doc, and also to set it to mutliply, which works with still frame.

So, has anybody here succesfully used seq/movies to control density of particles ?

Thx

Edit :

I could see particles, but very strange the way it’s distributed …