knicol

I debugged some similar errors, for anyone else who encounters this in the future:

Invalid source /obj/geo1/whitewatersource1/vdbactivatesdf1
Error: Unable to evaluate expression (
Traceback (most recent call last):
File "<stdin>", line 5, in expression
....
/houdini/python2.7libs/hou.py", line 18395, in intrinsicValue
return _hou.Prim_intrinsicValue(self, intrinsic_name)
OperationFailed: The attempted operation failed.
Invalid intrinsic name
(/obj/geo1/whitewatersource1/vdbactivatesdf1/halfwidth)).

and

Invalid source /obj/geo1/whitewatersource3/vdbactivatesdf1
Error: Unable to evaluate expression (
Traceback (most recent call last):
File "<stdin>", line 4, in expression
IndexError: tuple index out of range
(/obj/geo1/whitewatersource3/vdbactivatesdf1/halfwidth)).

Since the main error was "Invalid source", I checked the inputs to the Whitewater source node and found no "surface" and "vel" VDBs.

Substituting a simple setup, wired into Input 1 of Whitewater source, eliminated the error message (though it's so simple that it doesn't produce a result) and confirmed that it was the lack of correct VDBs that caused this case:
Grid -> Scatter -> Particle Fluid Surface -> VDB from Particle Fluid (change the Output Name parm to "surface") -> Whitewater source.

Looking upstream in the network, another problem was found that caused the VDBs to not be generated for some frames and not give Whitewater source the "surface" and "vel" it was expecting.