int osd_firstpatch(<geometry>geometry, int face_id)
When running in the context of a node (such as a wrangle SOP), this argument can be an integer representing the input number (starting at 0) to read the geometry from.
Alternatively, the argument can be a string specifying a geometry file (for example, a
.bgeo) to read from. When running inside Houdini, this can be an
For a given face in the coarse mesh, this returns the number of the first patch associated with the face. Since each face in a hull may generate multiple patches, this function will return the first of the patches generated by the face. See also osd_patchcount to find out the number of patches generated by the face.