Advanced Programming Guide

236 • Chapter 6: Programming with Maple Graphicsthe sum plot example from section 6.3 by adding local information to theobjects. This section provides examples that describe how to program atthis lower level.Writing Graphic PrimitivesYou can write procedures that allow you to work with plot objects ata more fundamental level. For example, the line and disk commandsin the plottools package provide a model for programming primitivessuch as points, lines, curves, circles, rectangles, and arbitrary polygons inboth two and three dimensions. In all cases, you can specify options, suchas line or patch style and color, in the same format as in other plottingprocedures in Maple.Example 1 Inside the following procedure, nargs is the number ofarguments and args is the actual argument sequence. Thus, in line,args[3..nargs] is the sequence of arguments that follow x and y.> line := proc(x::list, y::list)> # x and y represent points in either 2-D or 3-D> local opts;> opts := [ args[3..nargs] ];> opts := convert( opts, PLOToptions );> CURVES( evalf( [x, y] ), op(opts) );> end proc:The convert(..., PLOToptions) command converts user-level optionsto the format that PLOT requires.> convert( [axes=boxed, color=red], PLOToptions );[AXESSTYLE(BOX ), COLOUR(RGB, 1.00000000, 0., 0.)]Example 2 The disk procedure is similar to the previous line procedureexcept that you can specify the number of points that disk usesto generate the disk. Therefore disk must handle the option, numpoints,separately. The hasoption command determines whether a certain optionis present.> disk := proc(x::list, r::algebraic)> # draw a disk of radius r centered at x in 2-D.> local i, n, opts, vertices;> opts := [ args[3..nargs] ] ;> if not hasoption( opts, numpoints, n, ’opts’ )> then n := 50;> end if;> opts := convert(opts, PLOToptions);