I'm trying to create a shelf tool where the user can click anywhere on viewport and will create a ray that shoots into the screen and detects any intersection with primitives.

I'm using sceneViewer().selectPositions() (to get a point in 3d space) and sceneViewer().curViewport().viewTransform() to get a normal from the viewport camera but the math doesn't work out. are there any functions in the HOM that allow me to more easily do this?

Have you tried queryPrimAtPixel() ? However, I think you might need to be at the SOP level to do this.

http://www.sidefx.com/docs/houdini12.5/hom/hou/GeometryViewport#queryPrimAtPixel [sidefx.com]

edward
Have you tried queryPrimAtPixel() ? However, I think you might need to be at the SOP level to do this.

http://www.sidefx.com/docs/houdini12.5/hom/hou/GeometryViewport#queryPrimAtPixel [sidefx.com]

i already solved my problem, but thanks that looks interesting.