Aim Constraint (Rotation Controller)
The Aim Constraint controller constrains an object's orientation so that it aims at another object, similar to the LookAt constraint. However, it builds the rotation using a quaternion method. This approach reduces flipping angles compared to the traditional LookAt constraint, resulting in only one flipping angle instead of two. You can build more robust rig setups with this controller.
For example, you can use the aim constraint to set up a twist bone in the upper arm of a character rig. Usually, you would use the traditional LookAt constraint, but as soon as the upper arm rotates too much, the twist bone will flip, creating undesirable deformations in the shoulder. This flipping occurs when the joint rotates more than 90 degrees or less than negative 90 degrees. With the Aim Constraint, the flip occurs only when the joint rotates beyond 180 degrees. At that point, the upper arm would clip through the clavicle, which is not a natural rotation in the first place.
Interface
Once you assign an Aim Constraint, you can access its properties on the Aim Constraint rollout in the Motion panel. (On the Motion panel's PRS Parameters rollout, click the Rotation button.)
Pick Target
Here, you can click the pick button and select the object that the controller is aiming at. With the "use" checkbox, you can enable or disable this target.
Aim Axis Group
In this group you can set which axis is used to aim the controller to the target, and whether the axis is flipped or not.
Align Axis Group
Here, you can set the reference axis used in the quaternion method to build the aim rotation and specify whether the axis is flipped. The flip occurs at the negative side of this axis and only in this angle, unlike the two flipping angles in the LookAt constraint.
Align Group
Here, you can set different types of reference systems used in the quaternion method to build the aim rotation. If you select the "Node" type, you can pick the node used for alignment in the button below.
Viewline Length
Defines the length of the main viewline drawn from the pivot of the constrained object to the pivot of its target (or the average position, in the case of multiple targets). A negative value draws the line from the constrained object in the opposite direction to the target or targets. The length of the viewline is determined by the distance between the constrained object and the target, as well as the Viewline Length setting. However, if "Viewline Length Absolute" is on, the distance between the two has no effect on the length.
Viewline Length Absolute
When enabled, 3ds Max uses only the Viewline Length setting for the length of the main viewline; the distance between the constrained object and the target(s) has no effect.