Patches

Bilinear and bicubic patches can be specified in AL. The basis function used for bicubic patches is a part of the graphics state and is specified by the basis gop.

AL also supports NURBS and trim curves.

(patch type paramlist)

type is a string and should be either "bilinear" or "bicubic." If a bilinear patch is being specified, 4 control points should be specified. If a bicubic patch is specified, 16 control points should be specified. In both cases, the control points are specified in the paramlist using a position parameter.

(patch-mesh type nu uwrap nv vwrap paramlist)

Specifies a mesh of surface patches. type (a string) must be "bilinear" or "bicubic." nu and nv are integers and specify the number of control points along the u and v parametric directions on the surface respectively. uwrap and vwrap are strings (either "periodic" or "nonperiodic") and specify whether or not the surface should "wrap" around in either parametric direction. nu * nv control points (vertex positions) must be specified in the parameter list using using a position parameter.

(nu-patch nu uorder uknot umin umax nv vorder vknot vmin vmax paramlist)

nu-patch creates a non-uniform rational B-Spline surface. The table below summarizes the required parameters. In addition, paramlist must contain an array of nu * nv control points using "P" parameter for a non-rational and "Pw" for a rational B-Spline surface.

name	type	description
nu (nv)	integer	number of control points in u (v) direction
uorder (vorder)	integer	the order of the polynomial (degree+1) along u (v)
unkot (vknot)	realist	the knot vector (non-decreasing and length = nu+uorder (nv+vorder)
umin (vmin) and umax (vmax)	real	the parametric limits of the surface in u (v).

Last updated: 11/9/95 / Steve May ( smay@cgrg.ohio-state.edu )

Any comments or suggestions appreciated.