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Overview ¶
This tool lets you pick a piece of geometry and configure it as vellum balloon. It will then be added to the active simulation and merged with any other existing vellum objects.
A balloon consists of two types of constraints. A stretchable cloth constraint is built for the outer surface. A pressure constraint is added to keep the cloth from collapsing. The pressure constraint uniformly pushes the cloth outwards to maintain a global volume, so squeezing one area will bulge distant areas.
This is more expensive than a 
Vellum Strut Softbody since instead of creating struts between opposite interior points, it computes the volume of the entire model and inflates or shrinks the model to keep the same volume. The Vellum Strut Softbody is a better choice for soft bodies as it is faster to solve.
Note
The topology of your object could affect how pressure is applied. You may have to simplify the topology by using PolyReduce or increase the Stiffness parameter in the pressure_constraints node to properly inflate your object.
Using Vellum Balloon ¶
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Select the geometry object to configure as vellum balloon.
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Click the
Vellum Balloon tool on the Vellum tab.
Note
There is a 
Surface Collider tool on the Vellum shelf that you can use for objects that will be colliding with your balloon. It is essentially the same as the 
Deforming Object shelf tool on the Collisions tab, as they are both designed for deforming objects. However the Surface Collider does not try to generate a VDB collision object, which is less expensive.
For specific parameter information, see the 
Vellum Constraints and 
Vellum Solver help.
Working with Vellum Balloon ¶
By default the Stretch Stiffness is very low, since balloons have to be able to stretch.
There is an Output Group parameter on the pressure_constraints Vellum Constraints node which adds the generated stretch constraints to this primitive group. This is useful so you can edit them later with a 
Vellum Constraint Property DOP. For example, you can output the pressure constraints and animate them using the Rest Length Scale parameter.