I've uploaded a short tutorial on the Ray SOP.

A look at some uses for the Ray SOP - a SOP that allows you to project the points of one object onto the surface of another. In this video we look at four uses for the SOP - to transform the shape of one object to match another, to detect when one object is below another, and to transfer UV attributes from one object to another. Finally we look at how to use the Ray SOP to project some ‘ivy’ geometry onto a pillar.

https://vimeo.com/60937947 [vimeo.com]

I attach the project files.

Hope it is useful.

Thank you Peter, love your work.

Cool … Don't forget the famous pinboard effect is basically the Ray SOP as well:

http://varomix.net/mixblog/2011/01/pinboard-effect-done-in-houdini/ [varomix.net]