VDB Collisions for Vellum

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Advantages
Limitations
Collider types
Collision behavior
Fluids and grains

The VDB Collider SOP defines VDB colliders to bounce or kill dynamic objects, such as particles. It has fast substepping on the GPU and has an easy SOP-based setup. It can be used by the MPM Solver and Vellum Solver in the context of a simulation and can either create a VDB from a mesh or be provided with one directly.

Advantages ¶

SOP based for easy viewing, manipulation, and merging of colliders.
Fast and accurate (GPU) interpolation using velocity fields.
Controls for other effects such as Friction, Sticky, Bounce, Delete, etc.
Merge multiple VDB Colliders into one stream before feeding them into the Vellum Solver.

Limitations ¶

Computing this creates a huge chunk of OpenCL code that takes a very long time to compile. This results in very slow compile times the first time you run the simulation (sometimes several minutes).

Note

You will see a notification at the bottom of the status bar that says Compiling OpenCL kernels along with a timer to indicate something is happening and Houdini isn’t frozen.

Tip

You can press ⎋ Esc to stop the compile at any time.
Potentially high GPU memory use for very high resolution deforming objects.

Collider types ¶

There is a Collider Type parameter that allows you to choose between 3 collider types: static, transforming, or deforming.

To... Do this

To...	Do this
Set up a static collider	Set the Collider Type is set to Static. This is the default VDB Collider, as it is the most efficient. It is represented by a single VDB `surface` in memory throughout the simulation.
Set up a transforming collider	Use a Transform SOP between your object and the VDB Collider and keyframe some transformations. For example, you can use the expression `$FF10` in the Y value of the Rotate* parameter to cause your collider to rotate around the Y-axis. Set the Collider Type to Animated (Rigid). This is the most efficient of the animated colliders and it is extremely accurate as the transformation is interpolated between frames for each substep. It is represented by a single VDB along with a transformation matrix. Only the rigid transformation is updated per frame as the VDB `surface` never changes.
Set up a deforming collider	Connect some deforming geometry to the VDB Collider. Set the Collider Type to Animated (Deforming). This should only be used over the Animated (Rigid) type when actual deformation is taking place, as it is the most expensive of the two. It is represented by a pair of `surface` and `vel` VDBs that must be updated on each frame. The fields are also interpolated between frames to yield better results, which requires two pairs of VDB to be uploaded on the OpenCL device per frame.

Set up a static collider

Set the Collider Type is set to Static.

This is the default VDB Collider, as it is the most efficient. It is represented by a single VDB surface in memory throughout the simulation.

Set up a transforming collider

Use a Transform SOP between your object and the VDB Collider and keyframe some transformations. For example, you can use the expression $FF*10 in the Y value of the Rotate parameter to cause your collider to rotate around the Y-axis.
Set the Collider Type to Animated (Rigid).

This is the most efficient of the animated colliders and it is extremely accurate as the transformation is interpolated between frames for each substep. It is represented by a single VDB along with a transformation matrix. Only the rigid transformation is updated per frame as the VDB surface never changes.

Set up a deforming collider

Connect some deforming geometry to the VDB Collider.
Set the Collider Type to Animated (Deforming).

This should only be used over the Animated (Rigid) type when actual deformation is taking place, as it is the most expensive of the two. It is represented by a pair of surface and vel VDBs that must be updated on each frame. The fields are also interpolated between frames to yield better results, which requires two pairs of VDB to be uploaded on the OpenCL device per frame.

Collision behavior ¶

The Collision Response parameter allows you to control what happens when the dynamic object makes contact with the collider. Bounce will bounce or stop the dynamic object depending on its material type, and Delete will remove the dynamic object’s points. This can be used to dynamically cut the material as the simulation runs without resistance.

Turing on the Inverted SDF checkbox allows you to invert the sign of the VDB surface to simulate the dynamic object trapped inside the collider.

Video courtesy of Martin Winkler

The Sticky parameter allows projection of the dynamic object’s velocity even when moving away from the collider surface. This prevents the dynamic object from detaching from the collider, making it sticky if Friction is high enough. The following images show a sticky sphere dipped into a bed of particles and then lifted up with particles sticking to it.

Fluids and grains ¶

VDB Collisions are intended for use with the minimal solver, and is well suited for grain and fluid simulations.

Vellum Fluid simulations can take VBD collisions into account when solving for viscosity and surface tension, giving improved wetting effects when used with this style of collision. Vellum Grain simulations can take VBD collisions into account when solving for friction and stickiness, giving improved sticking and sliding effects when used with this style of collision.

For more information, see Minimal mode.

VDB Collisions for Vellum

Advantages ¶

Limitations ¶

Collider types ¶

Collision behavior ¶

Fluids and grains ¶

Vellum

Cloth ¶

Softbody ¶

Fluid ¶

Constraints ¶

Grains ¶

Collisions ¶

Other ¶

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