Hmmm. For some reason, I never saw this original post, 2.5 years ago!
The easiest way to resample an equirectangular image is to use it in a colorpict entity to define a 360° environment map in Radiance, then render it with the desired view. Your scene description would look something like:
void colorpict equi_rect
7 red green blue equirect.hdr . ‘atan2(Dy,Dx)/PI+1’ ‘Acos(Dz)/PI’
0
0equi_rect glow env_glow
0
0
4 1 1 1 0env_glow source background
0
0
4 0 0 1 360
You can then render this into as many views as you like using rpict or whatever.
-Greg