360 degree spherical panoramas and Little Planet imagery

introduction

360 degree panoramas can be:
- cylindrical - eg. not include detail high or low - as with most stitched panoramas
- spherical - includes everything - usually requires stitched images taken with fisheye lenses
- Little Planet style - converted into a circular image

generally start with 360 degree cylindrical panoramic image with at least 2:1 aspect ratio and left and right edges matching but can use any image where left and right edges match up (eg. islands)
ensure bottom quarter of image has little detail eg. sand
ensure upper quarter of image has little detail eg. blue sky
ensure horizon is exactly horizontal
then post-process:
- stretch image to 1:1 ratio so it is a perfect square
- rotate image 180deg so it is upside down
- in PS, apply polar filter (Filter > Distort > Polar Coordinates) and select Rectangular to Polar
  - or in Gimp, use Filters > Distorts > Polar Coords
- rotate as desired
- adjust contrast, etc

Image courtesy of PTgui