In various places in Houdini, nodes may have a parameter which expects a file name of a file containing a specific type of data. In these types of parameters you can usually use op:‹/path/to/node› instead of a file name to grab data from live nodes in the scene hierarchy instead of from a file.

For example, the Volume Sample from File VOP gets the value at a certain position in a volume. It has a Geometry file parameter for a volume file name, but you could sample a volume in a SOP network instead of a file on disk using something like:

op:/geo1/volume1

…as the value of the Geometry file parameter.

This works for (at least) the following common file types:

• Geometry file: surface node (SOP)

• Image file: Copernicus node (COP)

• Channel data file: channel node (CHOP)

• You can specify a frame number, by appending a frame number in square brackets, for example [23].

• You can specify one or two plane names by appending them in braces, for example {Cd} or {Cd,alpha}.

• You can override the resolution by appending the width and height in parentheses, for example (640,480).

When querying COPs, the path after op: must be an absolute path (that is, starting from the root of the scene hierarchy with /). However, you can work around this by converting a relative path to an absolute path using the opfullpath expression function inside backticks:

op:`opfullpath('../../volume1')`

It is possible to query SOPs using relative references, but care should be taking that where an op: path is evaluated may be distant from where it is written - for example, if stored on a geometry attribute. It is always better, where possible, to use inputs rather than op: references.