Path Construction

Drawing (describing) a polygon

This example attempts to mimic the behavior of the built-in path construction operators like arc.

If there is a current point, poly first draws a line to (x,y)+(r,0), otherwise it starts by moving to that point.

Instead of gsave … grestore (which has the undesirable effect of discarding the very changes to the current path which we want), it saves a copy of the current transformation matrix (CTM) as it exists when the function starts.

Then it does lineto to each succeeding point, which by scaling and rotating the matrix is always at (0,1). Finally, it calls closepath and then restores the saved matrix as the CTM.

% x y n radius  poly  -
% construct a path of a closed n-polygon
/poly {
    matrix currentmatrix 5 1 roll  % matrix x y n radius
    4 2 roll translate             % matrix n radius
    dup scale                      % matrix n
    360 1 index div exch           % matrix 360/n n
    0 1 {lineto currentpoint moveto}stopped{moveto}if   % start or re-start subpath
    {                              % matrix 360/n
        dup rotate                 % matrix 360/n
        0 1 lineto                 % matrix 360/n
    } repeat                 % matrix 360/n
    pop                      % matrix
    closepath                % matrix
    setmatrix                %
} def

Iterating through a path

This snippet dumps the contents of the current path to stdout. It uses the ghostscript procedure =only which may not be available on all interpreters. An equivalent procedure on Adobe interpreters is called =print.

pathforall is a looping operator which takes 4 procedure bodies as arguments which are called for the specific types of path elements, the result of moveto, lineto, curveto, closepath, and all other path-contruction operators which boil-down to these elements.

{ exch =only ( ) print =only ( ) print /moveto =} 
{ exch =only ( ) print =only ( ) print /lineto =}
{ 6 -2 roll exch =only ( ) print =only ( ) print
  4 2 roll exch =only ( ) print =only ( ) print
  exch =only ( ) print =only ( ) print /curveto =}
{ /closepath = }
pathforall

Graph Paper

/in {72 mul} def
/delta {1 in 10 div} def
/X 612 def
/Y 792 def
0 delta Y {
    0 1 index X exch  % i 0 X i
    moveto exch       % 0 i
    lineto
    stroke
} for
0 delta X {
    0 1 index Y  % i 0 i Y
    moveto       % i 0
    lineto
    stroke
} for
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