The Blender Renderer has a panel for Texture influence, to make it possible to influence particle system parameters, like density, through a texture map. In Cycles it seems to be gone? Is there an alternative way, other than using vertex weights?
if you switch temporary to the blender render engine, you could setup your influence texture, then switch again to cycles and it could works.
I tried it in the past and it works… I hope it’s possible nowadays.
EDIT: I remember that to do it, you must set the particles as Newtonian physic…but I’m not sure…I’m really noob about blender