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The Volume VOP operation runs CVEX over a set of volume primitives. 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 look at the name attribute of the volumes to determine which parameter they should override. If that parameter is marked as exported in the CVEX script, the volume will be written to. Otherwise, the volume is just read to get the parameter’s values. If a name attribute is not present, the first volume is bound to the "density" parameter which is provided in the default output and global parameters.
The CVEX script is applied once for each exported volume primitive.
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 volume, it can be hidden by setting the invisible flag on the VOP parameter.
Adding user-defined colors to volumes
You can only have one volume per attribute, which means you need three volumes to represent r, g, and b in the render. Then you can process each volume individually, all together as a single Volume VOP network, or use the Volume Mix SOP in user mode and modify
$V which is the voxel value at the current sample voxel point.
Pay close attention to what is happening in the Volume VOP. You index in to the various volumes via a Bind VOP set up to the exact same name as the name attribute present with the associated volume primitive. To export it back out, you need the same named parameter in a Bind Export VOP.
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.
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.
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.
This pattern can be used to override the export option on the VEX shader to avoid writing to certain volumes. The pattern matches the VEX parameter, not the bound volume. The volume will still be bound for reading.
There should be no threading issues with VEX. In the unlikely event that there are, this will allow you to determine which node is guilty, and work around it without turning off threading on a global scale.
Only Output Created Geometry
Don’t output any of the incoming volumes. Instead, the output will consiste of only geometry created by the VOPs. This is useful if generating points from volumes. Note you will still have to do a dummy write to any volume you wish to iterate over.
Signed-Flood Fill Output SDF VDBs
When narrow-band SDFs are processed the sense of the interior cells can be lost. This results in the interior being falsely output with the default background value. This option will do the sweep to reset these tiles to the correct sense. It only affects VDBs which are of grid class SDF.
Autobind by Name
Will use the name primitive attribute to determine which volume binds with which parameter. If the name attribute isn’t present, the first volume is bound to density.
A name like "foo" will be bound to the float parameter "foo". A name of "foo.x" will be bound to the x component of the vector (three float) parameter "foo". A name like "foo.zx" will be bound to the x,z component of the matrix (three by three) parameter "foo".
Bind Each to Density
The VOP network will be applied to each volume in turn, with the volume bound to density.
Primitive, Primitive Name, VEX Parameter
Manually specifies the bindings of each primitive. This is equivalent to those primitives having a name attribute with the given name. If Primitive Name is not empty, the primitive with the matching name will be bound to the given vex parameter.
This example shows how to import multiple volumes into a Volume VOP SOP.
The following examples include this node.