driving a Clip sop with prim attributes

[odToad]

I have some prims that I need to drive through a Clip SOP. Each prim has got its own clipping plane origin and direction, previously computed and stored in prim vector attributes, eg v@clipOrigin and v@clipDirection

I would like to connect these attribute values to the inputs of a Clip SOP: Origin & Direction parameters but cannot figure out how to do it.

I tried (just showing the Origin.x parameter of the Clip here, but the y/z and the Direction.x/y/z would be analogous:

1. @clipOrigin.x                                              no error, but wrong result (alway zero)
2. v@clipOrigin.x                                            error: unable to evaluate expression - extra tokens detected in expression
3. ch("../nodeJustAboveClip/clipOrigin/x")          error (bad parameter reference)
4. hou.ch("../nodeJustAboveClip/clipOrigin/x")    error (unknown function in expression)
5. also tried with a spare parameter

I realize the clipOrigin is not a parameter on my nodeJustAboveClip SOP, but rather an attribute on its prim output. So 3-5 are likely just plain wrong. 

Just in case Clip assumes it is running over points rather than prims, I also inserted a Attribute Promote SOP to copy the prim v@clipOrigin to each point.

With version 1 above, the result is the same - no error but always zero.

If you know how to do this, thank you in advance !

This is in Houdini 16.5 Indie

--------

SOLVED: I was using the wrong syntax in the parameter expressions. You can pick up prim or detail attributes in SOP (not just Clip SOP) parameters like this:

• prim(surface_node, prim_num, attrib_name, attrib_index)
• detail(surface_node, attrib_name, attrib_index)
• details(surface_node, attribute)
• point(surface_node, point_number, attribute, index)

Note these are VEX expression functions used in parameter expression one-liners, not regular VEX functions used in wrangles. The signatures of the functions are slightly different.

Edited December 16, 2017 by odToad
solution found, updating for future readers

[Noobini]

look at mestela's file for clues ?