PanoTools NG:
Re: Use of (central) cylindrical projection?
Roger D. Williams 2007-Mar-07 09:01:55
On Wed, 07 Mar 2007 16:12:33 +0900, dmgalpha <#removed#> wrote:
> I am curious, does anybody use central cylindrical projection? I know
> it is one of the oldest projections in panotools, but I can't think of
> a good reason why it exists. Perhaps I am missing something.
>
> Can anybody think of a good reason why central cylindrical should be
> used instead of equirectangular?
>
> Also, how about sinusoidal? It is almost a year since it was added to
> panotools, but I don't see it being used.
If you use a rotary camera to create a panorama, as I sometimes do and
used to do almost exclusively, then the native format is cylindrical.
There is no "warping" involved and the two ends dovetail together
beautifully. No cylindrical camera gets anywhere near a 180-degree FOV
vertically (my 120-film camera which uses a Nikon 28mm lens designed
for 35mm lenses has a usable 90-degree vertical FOV). The panoramas are
surprisingly free from any sense of loss at not having full vertical
FOV. Probably this is because empty foreground and empty sky are
mostly what you get at top and bottom anyway... (this is not to deny
the strong impression an all-round and full-up-and-down image can
create, nor to imply that I prefer rotary cameras now I've learnt how
to stitch fisheye images!)
If you use a viewer designed for equiangular projections the tops and
bottoms get noticeably distorted. Some viewers look for the typical
2:1 aspect ratio of the 360 x 180-degree image, and assume a cylindrical
projection if they find a larger one, as it will usually imply a cylin-
drical image. When it comes from an equiangular image that has been
trimmed top and bottom, perhaps to eliminate holes at zenith and nadir,
then this is inappropriate, and you get the reverse distortion. I've
run into this problem from both directions. <grin> There's even an
account of this in the archives somewhere, as people put me straight
on why my images were sometimes distorted one way, and other times in
another.
Roger
--
Work: www.adex-japan.com
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