Sun Path Overlay for Radiance Fisheye View
This form assumes that the Radiance rendering for which you are generating the sun path overlay has been generated with the new stereographic view type (-vts), and that the view direction is up (-vd 0 0 1).
To overlay the diagram sp_overlay.png onto the rendering rendering.jpg, use this command line which relies on ImageMagick's composite program. The new image will be named final.png:
$ composite -compose Dst_Over rendering.jpg sp_overlay.png final.png