Simply Maya User Community - Instanced Particle Aim Direction

Well, in the Gnomon video that talked about RGBSpeed, it uses the same idea. ( sortof :) )

But, are you applying the field to the whole particle shape node? ..or PerParticle?

Check this page in the docs for reference on aim vector:
file:///C:/Program Files/AliasWavefront/Maya4.5/docs/en_US/html/UserGuide/Dynamics/Particles43.html#191045

Basicly, in the Particle shape settings, here are a few attributes that you can play with to help you:

Quote:

AimDirection:

Sets the orientation of the instanced objects by specifying the direction along which each instanced object points relative to the original position of its local origin. The default seting is None, which use the value of 1, 0, 0. See also RotationType.

Note that you can make an object point in the direction the particles are moving by selecting velocity. See Aiming instanced geometry for more information.

AimPosition:

Sets the orientation of the instanced objects by specifying the location where each instanced object points relative to the original position of its local origin. The default settins is None, which uses the value of 0, 0, 0. See RotationType for important details.

AimAxis:

Specifies the object axis that points directly at the AimDirection or AimPosition.

AimAxis is a vector attribute that works only with AimDirection and AimPosition rotation types. The default setting is None, which uses the value of 1, 0, 0.

AimUpAxis:

Specifies the object axis that points up (as much as possible) relative to how the AimAxis points at the AimDirection or AimPosition.

In this context, up is the direction the world up-axis points (see AimWorldUp). AimUpAxis is a vector attribute that works only with AimDirection and AimPosition rotation types. The default setting is None, which uses the value of 0, 1, 0.

For example, suppose the AimPosition is set to the origin of the workspace, and the world space Y-axis (0, 1, 0) is the AimWorldUp value. An instanced object's X-axis (1, 0, 0) is the AimAxis value, and the object's Y-axis (0, 1, 0) is the AimUpAxis value.

The object's X-axis points directly at the origin wherever the object moves in the workspace. The object is rolled around its X-axis as necessary to keep its Y-axis aiming up as much as possible. Up is defined by the AimWorldUp setting, which in this case is set to the world's Y-axis.

AimWorldUp:

Sets, in world coordinates, the axis that indicates the up direction used by the AimUpAxis. AimWorldUp is a vector attribute that works only with AimDirection and AimPosition rotation types. The default setting is None, which uses the value of 0, 1, 0. This attribute is unaffected by the setting for the World Coordinate System's Up Axis found in the Window > Settings/Preferences > Preferences > Settings window.

PS, why might I ask are you using instances for rain?

Are you going to need to see individual details of a given drop?
If so, cool:)

If not, might I suggest the typical rain solution using the simple streak(HW) particle type with, or without, multi-pass rendering?

Just FYI:)