[ptx] difference between rectangular and cylindrycal pano

[Rik Littlefield]

Marek,

PTviewer assumes that its input image is equirectangular projection, 
possibly less than full spherical 360x180 degrees.  For partial panos 
with small fov, there is not much difference between equirectangular and 
rectilinear or cylindrical, so visually it works OK to use those with 
PTviewer also.

Whether you should use rectilinear or cylindrical for website and 
printing depends on what you are trying to accomplish.  With 
rectilinear, all lines that are straight in the world are also straight 
in your picture, but you get severe distortion if you try to go beyond 
roughly 120 degrees fov.  With cylindrical, the horizon and verticals 
are the only straight lines in the world that end up straight in your 
picture, but you can go up to full 360 degrees horizontal.

Cylindrical and equirectangular have many of the same characteristics -- 
up to a full circle around, vertical lines in the world are also 
vertical in your picture, things near the poles get badly distorted.  
The difference is that in equirectangular, you can go clear to the poles 
and things near the poles get "squashed" vertically, where in 
cylindrical they get "stretched" and you can go only about +-60 degrees 
vertically before distortion gets too much.  (PTViewer remaps from 
equirectangular input to rectilinear on the screen, so when you look up, 
you see an undistorted view of the stuff above you.)

For technical discussion and pictures using maps of the Earth, see 
http://mathworld.wolfram.com/EquirectangularProjection.html , 
http://mathworld.wolfram.com/CylindricalProjection.html, and 
http://mathworld.wolfram.com/RectilinearProjection.html , which links to 
http://mathworld.wolfram.com/GnomonicProjection.html .  (What 
photographers call "rectilinear" projection, geographers call 
"gnomonic".  It's not immediately obvious that gnomonic projection maps 
lines that are straight in 3D to lines that are straight in the map, but 
it does.)

--Rik

spec wrote:

> Hello,
>
> What exatly is the difference if I stitch pano as rectangular and 
> cylindrical? As I understand it both are good to present a result as 
> an image on a website or sth., and both are good to present pano as 
> cylindrical pano in ptviewer. With equirectangular it's a whole 
> different story - that I understand.
>
> -- 
> Thanks,
> Marek
>
>