Stitch constraints are now able to stitch a point to the closest position on a primitive, not just to another point. This is extremely useful for stitching cloth together. For example, now you can stich a pocket to a shirt anywhere on a triangle or quad, instead of being limited to only points. This ability is also very useful to create smooth and realistic looking sliding.
In order to create sliding constraints, you need to use Attach to Geometry or Stitch constraints, both of which have the ability to constrain points to the closest position on a primitive. To enable this, turn on the Use Closest Location on Primitive checkbox on the Vellum Constraints node. You also need to turn on Sliding Rate, which will allow the constraint to slide along the target geometry at this rate during the simulation. Set the Target Group Type to Primitives, and choose your target geometry as the Target Group.
Use Closest Location on Primitive is only available on polylines, triangles, and quads.
Tips and troubleshooting
The Sliding Rate controls how quickly the sliding will occur. A value of 1 matches the constrained point’s velocity. Decreasing this value will cause it to slide slower.
You can optionally use a Sliding Attribute for finer control. This attribute scales the Sliding Rate as the points move along the geometry. In the video above, the constraints in the blue area slide faster than the constraints in the red area.
Turning on Tangent Stiffness will slow down sliding, since the constraints are fighting with gravity to keep the points in place. You can work around this by increasing the Sliding Rate.
If your constraints are improperly jumping across concavities in the target geometry, you can change the Sliding Method on the Advanced tab of the Vellum Solver from Closest Point to Traverse Polygons. This method starts from the current target primitive and successively walks outwards, finding the closest point on the surrounding primitives. This approach is more expensive, but handles concave target geometry better. In the video above, Traverse Polygons is turned on so that the constraints don’t jump from the top of the circle to the bottom as the object slides downward.