The Vellum Reference Frame ties points in a Vellum object to a reference frame that is defined by animated geometry.
Point attributes are added to the Geometry stream to inform the solver that the velocity of the points should be adjusted during solve to match the reference frame. The reference frame is specified by a point attribute that will point to a SOP. In this case, the SOP pointed to is internal to the Vellum Reference Frame SOP. It is a SOP that will compute the rigid change of motion of the third input.
The effect of a reference frame is allow the simulation to ignore the sudden acceleration of their container objects. For example, if the reference frame is a flying plane, the points will ignore the acceleration from the plane speeding up and slowing down. If angular velocity is computed, they will also ignore accelerations from the plane banking.
The points to update the reference frame attributes for. A point can have only one reference frame, so updating it will overwrite any previous reference frame on the point.
When the reference frame accelerates or decelerates, its change will also be applied directly to the points velocity. How much to apply its change is controlled by the velocity compensation.
Vel Comp Attrib
Attribute to scale the velocity compensation. Should be a 0..1 attribute.
The reference frame velocity is added directly to the
the drag value. It is first scaled by this drag compensation amount.
Drag Comp Attrib
Attribute to scale the drag compensation. Should be a 0..1 attribute.
This affects the velocity compensation. Only accelerations above this threshold will be applied. The intent is to not inherit small motions instead cap the maximum acceleration caused by the moving reference.
Accel Threshold Attrib
Attribute to scale the
How accurate a method to use for calculating velocity. First Order is the simplest but might not fully compensate for all motion in the reference frame. Second Order is slightly more expensive to compute but gives an estimate that more closely matches the Vellum Solver’s Second Order integration method. There might be discontinuities when collisions occur, however.
Compute Angular Velocity
Enable the computation of both rotations and accelerations in the reference frame. Including rotations can allow cloth to predict spinning characters, canceling the accelerations induced.
Computation of angular momentum assumes that the reference geeometry is moving rigidly, deforming characters can confuse this and result in spurious spins. It may be useful to isolate a torso that moves mostly rigidly for the reference.
For accurate computation of the velocity of the reference frame it is important that the timestep of the reference frame computation matches that of the solver. This parameter should be linked to your solver substeps.
To avoid evaluating the reference frame geometry out of bounds, the start frame is used to clamp the earliest evaluation.
The reference frame geometry must have smooth point positions within a frame or all its motion will be applied at one point in the frame, resulting in jerky motion. This option will blend the reference frame within frames, and should be turned off only if the reference frame has proper sub-frame positions.