The GR_RenderHook class allows you to get inside the Houdini
rendering pipeline. The methods you override, renderWire and
renderShaded, are triggered before the normal Houdini rendering. You
can then also restrict further Houdini rendering through the use of
the getWireMask and getShadedMask.
The GR_RenderHook you create will be kept. Thus, cache information
can be stored as member data.