Project curves onto a surface in Houdini.

In this quick tip for Houdini, we show you how to use the Ray SOP to project curves onto geometry.  The key to this is to set the normals of the curve to point towards the surface of the geometry, then use the Ray SOP to project the curve onto the geo. Once this is complete, you can use the Polyframe SOP to reset the curve normals so they are suitable for use with a Sweep.

In this example the surface is a simple deformed grid, however not all geometry is equal and it may not be so flat.  Another approach would be to use an Attribute Transfer to transfer the Normals from your surface onto the Curve, then use the Reverse SOP to invert the normal so it points back at the surface, then use the Ray SOP to project onto the geometry.