# [Proj] Perspective view to a sphere (or similar)

Gerald I. Evenden geraldi.evenden at gmail.com
Wed Sep 10 21:45:05 EDT 2008

```On Wednesday 10 September 2008 9:07:29 pm support.mn at elisanet.fi wrote:
> Hello,
>
> there are situations when one would like to use a very basic
> projection: perspective view to a sphere. To use directly
> some photos from the space or airplanes. Or maybe a
> screen shot from Google Maps. The most obvious way
> would be to use them directly as they are. There are
> some spherical projections
> in proj-4. But is there exactly a direct view from a view point with
> forward and inverse algorithms?
>
> Here is more about that subject and how it should be calculated.
>
> http://en.wikipedia.org/wiki/Perspective_view
>
> http://en.wikipedia.org/wiki/Image:Perspectiva-1.svg
>
> http://en.wikipedia.org/wiki/Image:Perspectiva-2.svg
>
> What is needed:
> 1) view point coordinates
> 2) view direction (defaulting to the center of the earth maybe?)
> 3) distance to the earth surface and to the projection plane
> 4) rotation angles and zoom factors
> 5) more? maybe some most common distortion factors for cameras etc??
>
> What is best approximation projection in proj-4
> to get some results?
>
> Regards: Janne.

Try +proj=nsper   Near-sided perspective which is a view of the earth at a
height h.  There is the tilted perspective or tpers but it is a brute to use.
+lat_0=, +lon_0= determine the eye-sphere line intersect with the surface of
the sphere and +h= is the height above the surface in the same units as the
radius of the sphere. The central meridian (lon_0) is coincident with y=0.
Projection plane is normal to the eye-center line and tangent to the sphere.

Dunno what you mean by rotation angles an zoom factors other than simple
graphic scaling an rotation.  As for camera lens corrections:  LOL, lot'sa
luck friend.

nsper is one of the oldest projections in proj.

--
The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist
```