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Vellum uses a Position Based Dynamics approach to cloth, hair, and grains. This new approach is fast, reliable, and produces acceptable results quickly.
There are several shelf tools that will create the basic node setup for you. The best example of a SOP setup using Vellum is the Simple Cloth tool, which is available on the Characters tab for Houdini Core users, and on the Vellum tab for Houdini FX users.
It asks you to select a geometry object to solve as cloth, and a geometry object to use as a collider. In this example, a grid is used as the cloth object and a torus is used as a collider. These get wired into the Vellum Constraints node, which creates constraints to act like different materials. The Simple Cloth tool uses a Vellum Configure Cloth node, which is a Vellum Constraints node with common cloth parameters already applied. There are also presets for Vellum Configure Balloon, Vellum Configure Grain, Vellum Configure Hair, and Vellum Configure Softbody that use this same node.
The Vellum Solver allows you to modify things like Substeps and Constraint Iterations to balance the speed and quality of your simulation. The node also has a Forces tab that allows you to set Gravity, Wind, Wind Drag, and Friction.
There is a default Wind Drag force that is set in SOPs. However, the default DOP network does not create a drag force.
The Vellum I/O node saves the output geometry as well as the constraint geometry to disk. There are some default deletes on the Save Filter tab to clean out the most common attributes that aren’t needed in order to cut down on the file size.
The Vellum Post-Process allows you to visualize components like False Color Mode for stretch or bend stress. You can also visualize constraints and thickness, which is useful for troubleshooting your simulation.
Vellum Constraints nodes:
Vellum Attach to Geometry
Vellum Configure Balloon
Vellum Configure Cloth
Vellum Configure Hair
Vellum Configure Strut Softbody
Vellum Configure Tetrahedral Softbody
Vellum Stitch Points
Vellum Weld Points
Using Vellum in DOPs
There are shelf tools for each type of Vellum effect, which are all able to interact with each other. When you use multiple Vellum effects on multiple objects, all objects will be merged into one geometry object and one constraint geometry object in the DOP network.
For more information on each effect, see the following shelf help pages.
You can apply POP forces in Vellum, since it is particle based. It also respects Particle Streams, so a force applied to one object doesn’t have to be applied to all objects.
For example, if the grid is a cloth object and the torus is a softbody object, you can apply a POP Wind force to the cloth by placing the node in between the
However, if you want to apply the wind to the softbody instead, you can move the POP Wind force in between the
merge2 nodes. Similarly, if you want to apply the wind to both objects, you can place it between the
Vellum Solver SOP and DOP differences
Gravity is on by default. This is the case in some DOP networks, but not all.
The ground plane is not on by default, but can be turned on with the Ground Position parameter. You have to explicitly add a ground plane in DOPs.
Using Dynamic Constraints
Typically constraints are set up before simulation. However, you can also set up constraints dynamically that occur during simulation. If you dive inside your Vellum Solver, you can create Vellum Constraints nodes and wire them into your
force_output to create all kinds of effects.
For example, you create a Vellum glue constraint that will glue points once they get close together.
Set the Constraint Type to Glue.
Change the Create Constraint parameter to Each Frame so it will check to see if there are any nearby points every frame.
There is a maximum Constraints Per Point parameter that is set to 1 by default, so there will only ever be 1 constraint created.
You can optionally turn on Breaking and set a Threshold where the glue constraints will separate.