Houdini 20.5 Nodes Geometry nodes

FLIP Volume Combine geometry node

Combine FLIP fields of different resolutions.

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This node allows you to blend the fields (surface, vel, and pressure) resulting from two FLIP simulations of different resolutions into a single set of fields. This is particularly useful when using the FLIP up-res workflow, where a subdomain of the low-resolution simulation is re-simulated at a higher resolution. The node requires the fields of both simulations as well as the container of the high-resolution simulation to use as a mask with a smooth transition between the two sets of fields.

Note

Not all fields are required. You could optionally use this node to blend just the surface field.

Inputs

Low-Resolution Fields

The FLIP fields of the low-resolution simulation (surface, vel, and pressure).

High-Resolution Fields

The FLIP fields of the high-resolution simulation (surface, vel, and pressure).

High-Resolution Container

A surface field or geometry hull of the high-resolution simulation container.

Clipping Bounding Box

Optional geometry to use as a bounding box to limit computation to a specific region.

Outputs

Combined Fields

The resulting fields from the merge.

Parameters

Surface Resolution

The resolution to use for the combined surface field.

Low Resolution

Use the resolution of the low-resolution surface field.

High Resolution

Use the resolution of the high-resolution surface field.

Activate to Low-Resolution Half Width

Low-resolution fields will likely have a wider narrow band in world space than high-resolution fields. This option will extend the narrow band of the high-resolution fields to avoid artifacts during blending.

Velocity Resolution

The resolution to use for the combined velocity field.

Low Resolution

Use the resolution of the low-resolution velocity field.

High Resolution

Use the resolution of the high-resolution velocity field.

Pressure Resolution

The resolution to use for the combined pressure field.

Low Resolution

Use the resolution of the low-resolution pressure field.

High Resolution

Use the resolution of the high-resolution pressure field.

Blending

Blend Range

The range of distances to the high-resolution container that will be used as the blending region to combine the two sets of fields. Negative values are inside the container, while a value of 0 corresponds to the container’s boundary.

Blend Ramp

By default, blending is mapped linearly to a (0, 1) range. This behavior can be changed by specifying a custom mapping.

Extrapolate Surfaces

Enable Extrapolate Surfaces

In cases where the low and high-resolution simulations present important differences in the blending region, you might have to fully activate the gaps between the two surface fields. This option can be expensive, but should be used when visual artifacts are present in the blending region of the combined fields.

Mode

Define the method used to extrapolate the surfaces.

Manual (fast)

Increase the narrow band of both surfaces to twice the World Space Distance. This method is faster but might require more memory and adjustments.

Automatic (slow)

Construct a mask representing the gap between the two surfaces before extrapolating them into it. This method is slower because of the mask building, but it only activates the minimum amount of voxels and does not require any parameter tuning.

World Space Distance

The world space distance to extrapolate both surfaces in Manual Mode.

Smooth Inner Seams

Smooth Inner Seams

Enable a band in the blending region to blur the combined surface field.

Smoothing Band

Half width of the mask to perform the smoothing operation. The mask starts at the surface of the high-resolution container and goes inside the container.

Filter Radius

The size of the smoothing stencil in world space.

Iterations

Number of times to repeat the smoothing operation.

To...Do this

Smooth out artifacts in the interpolation between high and low res sims

There are two ways to do this. The first is to turn on the Extrapolate Surfaces checkbox and manually increase the World Space Distance. Alternatively, you can change the Mode to Automatic. This approach is more reliable, but slower.

Speed up the preview in the viewport

Use the 4th input to isolate the region you are working on.

See also

Geometry nodes