Is there a fisheye or dual fisheye to equirectangular filter for ffmpeg?

Question

Or a way to do it with an existing filter? So that you could take in video from a fisheye or dual fisheye camera (such as the Ricoh Theta) and directly output equirectangular,

Answer 1

The Remap filter does just this:

This filter copies pixel by pixel a source frame to a target frame. It remaps the pixels to a new x,y destination based on two files ymap/xmap.

Basic command syntax is

ffmpeg -i fisheye_grid_input.jpg -i fisheye_grid_xmap.pgm -i fisheye_grid_ymap.pgm -filter_complex remap out.png 

Also included at that link are the mapping files for

Ricoh Theta S camera: input files for resolution 1920x960(1080)