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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
     # next four fields are required
     Xcenter Ycenter Zcenter

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

The sphere carries automatically generated texture co-ordinates. See below.
The sphere lives in Euclidean space.
The sphere lives in Hyperbolic space. See Non-Euclidean Geometry.
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 objectes; 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 equal-area projection
cylindrical proj: s is the longitude, t is the latitude
rectangular proj: s is the longitude, t is sin(latitude) (i.e. z co-ordinate in the sphere's co-ordinate system)
stereographic projection from the south (z=-1) pole
stretch orthographic view of +y hemisphere over both, mirroring