#### 4.2.8 SPHERE Files

The conventional suffix for `SPHERE` files is `.sph`.

         [ST][E|H|S]SPHERE # Keyword
         # auto-generated texture co-ordinates, only allowed with STSPHERE objects
         [SINUSOIDAL|CYLINDRICAL|RECTANGULAR|STEREOGRAPHIC|ONEFACE]
         # next four fields are required
         Radius
         Xcenter Ycenter Zcenter

The key word is `[ST][E|H|S]SPHERE`. The optional prefix characters mean:

‘`ST`’
  The sphere carries automatically generated texture co-ordinates. See below.
‘`E`’
  The sphere lives in Euclidean space.
‘`H`’
  The sphere lives in Hyperbolic space. See Non-Euclidean Geometry.
‘`S`’
  The sphere lives in spherical space. See Non-Euclidean Geometry.

Sphere objects are drawn using meshes which are rectangular in a polar co-ordinate system, with the equatorial plane parallel to the `x,y`-plane. Their smoothness, and the time taken to draw them, depends on the setting of the dicing level, 10x10 by default. From Geomview, the Appearance panel, the `<N>ad` keyboard command, or a `dice nu nv` Appearance attribute sets this.

Texture co-ordinates are generated for `STSPHERE` objects; the keyword following the initial `STSPHERE` keyword defines the way this is done. It follows the conventions of the `mktxmesh` Perl-script which comes with the Orrery.

SINUSOIDAL
  sinusoidal equal-area projection
CYLINDRICAL
  cylindrical proj: s is the longitude, t is the latitude
RECTANGULAR
  rectangular proj: s is the longitude, t is `sin(latitude)` (i.e. `z` co-ordinate in the sphere's co-ordinate system)
STEREOGRAPHIC
  stereographic projection from the south (`z=-1`) pole
ONEFACE
  stretch orthographic view of `+y` hemisphere over both, mirroring