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 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

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