PROBLEM:
When an object has a mask modifier and a particle system, most or all particles will be emitted from its center in a single line. This also happens with other modifiers in other situations, but I was not able to predict why and how they would happen. But with the mask modifier the problem is sure to happen.
TO REPLICATE:
Create a plane, subdivide it, create a vertex group and insert one or more faces on it, create a mask modifier with this vertex group selected, create the particle system and run the simulation.
WHAT WAS EXPECTED:
The particles should emit only from the non masked faces, and no particle should be emited from the object center.
SYSTEM:
blender 2.63 r46461
Linux Mint 64 bits (also tested on windows XP)
Geforce gtx 460 and other nvidia cards tested.
Description
Related Objects
Event Timeline
I tried recreate my own scene (haven't tried your blend) from your instructions alone here in linux and ran into the exact problem you mention.
While playing around, I noticed that if I mask off, say 2 or 3 faces on one side, and then use grid distribution for the psys, the particles seem to emit in lines that start on the mesh, but keep going past the part that should be masked, clearly creating more particles than it should.
I am thinking that for emitters with a mask mod before the psys, that the line going straight down in this case, are just all of these "extra" particles that should have been masked somehow shooting out of the origin since they don't know where they should be coming out from. Either way, sounds like a minor bug. :)
Also, I noticed that it doesn't appear to happen for hair particles (although in reality, it could just be drawing a few hundred hairs on the same exact origin point, and thus not visible).
Thanks =D
I would like to point out that this happens with many other modifiers, including situations where, just using the subsurf modifier on a mesh the same happened. But sometimes it does not happen.
Maybe solving the source of the problem with the mask modifier would solve it with all other modifiers.
don't think this is a bug, particles expect the original faces to exist.
while this could be supported - I think this is just limitation in design and not a bug.
Checked on code and TODO list -- it's known design limitation (see constructive modifiers section http://wiki.blender.org/index.php/Dev:2.5/Source/Development/Todo/Tools#Particles)
Would close the report as TODO.