Here is a file which contains a block that has two ParticleEmitters, and a script that will emit particles from these emitters in an interval: (Hit the Play button to see the problem I will explain)
EmissionDirectionRepro.rbxl (11.3 KB)
The only difference between the emitters is their EmissionDirection, namely one is Top and the other is Front (and the particle colors are different for demonstration purposes). When I run this place, I see that the spread of the Top emitter (in red) is as expected:
However, that of the Front emitter (in blue) does not seem to be correct. For low VelocitySpread it has the shape of a line, and then for higher VelocitySpread it turns into a cross-like shape:
This only happens to Front/Back EmissionDirections, the other directions are fine. The expected behaviour is that the spread pattern is the same regardless of the EmissionDirection.
Related to:
http://devforum.roblox.com/t/is-there-a-reason-my-particles-are-coming-out-in-rectangle-form/29110