Houdini 20.5 Nodes Geometry nodes

VDB Advect geometry node

Moves VDBs in the input geometry along a VDB velocity field.

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Since 12.5

See volumes for an explanation of standard volumes and OpenVDB volumes.

Overview

The VDB Advect operation will move both narrow band signed distance fields and other fields according to a velocity field defined in a vector VDB.

Animating advection

This node is not a feedback loop.

It moves the fields it finds in the input geometry. It cannot modify the fields over time. (That is, if you hook this node up to do advection, and press play, the fields will not animate.)

To set up a feedback loop, where the advection at each frame affects the advected field from the previous frame, do one of the following:

  • Do the advection inside a SOP Solver.

  • Set the Timestep to $T This will cause the node to recalculate, at every frame, the path of every particle through every previous frame to get the current one. This is obviously not very practical.

Parameters

Group

A subset of VDBs in the first input to move using the velocity field.

These must be narrow band signed distance fields.

Velocity

The name of a VDB primitive in the second input to use as the velocity field.

This must be a vector-valued VDB primitive. You can use the VDB Vector Merge SOP to turn a vel.[xyz] triple into a single primitive.

See specifying volumes.

Respect Grid Class

When this option is disabled, all VDBs will use a general numerical advection scheme, otherwise level set VDBs will be advected using a spatial finite-difference scheme.

Timestep

Number of seconds of movement to apply to the input points. The default is 1/$FPS (one frame’s worth of time). You can use negative values to move the points backwards through the velocity field.

General Advection

These control how non-SDF VDBs are transported by the velocity field.

Substeps

The VDBs have to be pre-dilated for how far they may be moved, so with large velocities the memory footprint can become large. Increasing substeps allows each dilating to be more reasonable.

Advection Scheme

Method to use for tracing through the velocity to find the initial location of each value. The methods are of increasing accuracy. The MacCormack and BFECC methods are error-correcting methods that try to avoid the smoothing caused by linear interpolation.

Limiter Scheme

Controls behavior of error correction when the error corrector results in a value outside of the original possibilities. Without and error corrector ringing and instabilities can occur.

Level Set Advection

These control how SDF VDBs are moved through the velocity field.

Spatial Scheme

How accurately the gradients of the signed distance field are computed. The later choices are more accurate but take more time.

Temporal Scheme

How accurately time is evolved within the timestep. Later choices are more accurate but take more time.

Renormalization

After moving the signed distance field, it will often no longer be a proper signed distance field. A number of renormalizaton passes can be performed to convert it back into a proper field.

Steps

The number of times to renormalize between every substep.

Spatial Scheme

How accurately the gradients of the signed distance field are computed. The later choices are more accurate but take more time.

Temporal Scheme

How accurately time is evolved within the renormalizaton stage. Later choices are more accurate but take more time.

See also

Geometry nodes