Octant (solid geometry)

An octant in solid geometry is one of the eight divisions of a Euclidean three-dimensional coordinate system defined by the signs of the coordinates. It is analogous to the two-dimensional quadrant and the one-dimensional ray. The generalization of an octant is called orthant or hyperoctant.

Naming and numbering

A convention for naming an octant is to give its list of signs, e.g. (+,−,−) or (−,+,−). Octant (+,+,+) is sometimes referred to as the first octant, although similar ordinal name descriptors are not defined for the other seven octants. The advantages of using the (±,±,±) notation are its unambiguousness, and extensibility for higher dimensions.

The following table shows the sign tuples together with likely ways to enumerate them. A binary enumeration with − as 1 can be easily generalized across dimensions. A binary enumeration with + as 1 defines the same order as balanced ternary. The Roman enumeration of the quadrants is in Gray code order, so the corresponding Gray code is also shown for the octants.

+ Octants Graycode x y z Binary Balancedternary − as 1 + as 1 < > < > < > 0 + + + 0 0 7 7 13 13 1 − + + 1 4 6 3 11 −5 3 + − + 2 2 5 5 7 7 2 − − + 3 6 4 1 5 −11 7 + + − 4 1 3 6 −5 11 6 − + − 5 5 2 2 −7 −7 4 + − − 6 3 1 4 −11 5 5 − − − 7 7 0 0 −13 −13+ Quadrants for comparison Roman x y Binary Balancedternary − as 1 + as 1 < > < > < > I + + 0 0 3 3 4 4 II − + 1 2 2 1 2 −2 IV + − 2 1 1 2 −2 2 III − − 3 3 0 0 −4 −4

Little- and big-endian are marked by "<" and ">", respectively.

Verbal descriptions are ambiguous, because they depend on the representation of the coordinate system. In the two depicted representations of a right-hand coordinate system, the first octant could be called right-back-top or right-top-front respectively.

Octant (solid geometry)

Bron en licentie