Houdini 18.5 Nodes Dynamics nodes

Gas Field VOP dynamics node

Runs CVEX on a set of fields.

The Gas Field VOP operation runs CVEX over a set of fields. The CVEX shader can be defined as a .vfl file, a Shop, or by building a CVEX VOP network inside this node. The last is the most straightforward approach.

Binding to the CVEX script controls what values are passed to which parameters of the script. The default AutoBind will use the name of the fields to determine which parameter they should override. If that parameter is marked as exported in the CVEX script, the field will be written to. Otherwise, the field is just read to get the parameter’s values.

The CVEX script is applied once for each exported field. Locally defined ix, iy, iz, resx, resy, resz, orig, and P bindings are created that refer to the index, resolution, center, and current voxel position of the currently exported primitive.

The parameters list starts with parameters created by the CVEX script’s parameters. If the parameter is meant only to be bound to a field, it can be hidden by setting the invisible flag on the VOP parameter.


Vex Setup

Vex Source

Where to get the CVEX script from. Myself will refer to the VOP network built inside this node. SHOP will use the Shop Path and Script will load the .vex file specified in the script.

Shop Path

The CVEX Shop to apply.

This can also point to any node that contains a CVEX VOP Network.


Referenced nodes parameters will be used.


The CVEX script to apply.

Re-load VEX Functions

When in script mode, this reloads any .vex files to account for updates made outside of Houdini.


The command line used by VOPs to compile the inside of this node.

Force Compile

Trigger a recompile of the VOP network.

Evaluation Node Path

VEX functions like ch() usually evaluate with respect to this node. Providing a path here can override where the path search starts from. This is useful for embedding in a digital asset where you would like the top level digital asset to be the search root.

Export Parameters

This pattern can be used to override the export option on the VEX shader to avoid writing to certain fields. The pattern matches the VEX parameter, not the bound field. The field will still be bound for reading.

Data Bindings

Autobind by Name

Will use the name of the field to determine which field binds with which parameter.

A scalar field named "foo" will be bound to the float parameter "foo". A vector field "foo" will be bound to the vector (three float) parameter "foo". A matrix field "foo" will be bound to the matrix (three by three) parameter "foo". An index field named "foo" will be bound to the integer parameter "foo".

Primitive, Name

Manually specifies the bindings of each field.

Stencil Field

A scalar field to use as a stencil for where to evaluate the VOP network. Voxels that are strictly greater than 0.5 will be run through the VOP network, others will be left unchanged.

VEX Precision

VEX can evaluate at 32-bit or 64-bit precision. 64-bit provides higher accuracy, especially for transforms.

The auto mode will currently run in 32-bit mode as fields are always 32-bit.


Input 1, 2, 3, 4

These control the four virtual inputs accessible inside of VOPs.

They can be accessed with the OpInput1-4 wires from the VOP, or with the @OpInput1-4 string parameters when using VEXpressions.

They can also be accessed numerically as 0-3 with VEX functions that take an input number.


No geometry is wired to this input.


The SOP geometry will be cooked prior to running the VEX and the result wired to this input.

DOP Data

Data in the current simulation to reference. Specified as an object/data, for example, pyro1/vel to refer to the velocity field of the pyro1 object (which will show up as three volume primitives).

Nth Context Geometry

These refer to the SOPs wired into the parent DOP Network itself.

Use Timestep

Determines if the current solver timestep will be used to apply this node.

If set, the current timestep size will be multiplied by the scale and used for the time increment for this operation. Otherwise, the time scale will specify an absolute fictitious time to integrate by.

By disabling the link between the actual real time and the microsolver time, you can perform operations in a separate, fictitious, time.

Time Scale

The timestep used for this microsolver will be scaled by this amount. This allows one to achieve non-realistic effects, such as parts of the simulation operating at different speeds than other parts.

Similarly, it is useful if a solver needs to be evaluated independently of the main timestep.

See also

Dynamics nodes