// publicdomain code by Darel Rex Finley, 2007
// See diagrams at http://alienryderflex.com/polygon_inset
void insetPolygon(double *x, double *y, int corners, double insetDist) {
double startX=x[0], startY=y[0], a, b, c, d, e, f ;
int i ;
// Polygon must have at least three corners to be inset.
if (corners<3) return;
// Inset the polygon.
c=x[corners1]; d=y[corners1]; e=x[0]; f=y[0];
for (i=0; i<corners1; i++) {
a=c; b=d; c=e; d=f; e=x[i+1]; f=y[i+1];
insetCorner(a,b,c,d,e,f,&x[i],&y[i],insetDist); }
insetCorner(c,d,e,f,startX,startY,&x[i],&y[i],insetDist); }
// Given the sequentially connected points (a,b), (c,d), and (e,f), this
// function returns, in (C,D), a bevelinset replacement for point (c,d).
//
// Note: If vectors (a,b)>(c,d) and (c,d)>(e,f) are exactly 180° opposed,
// or if either segment is zerolength, this function will do
// nothing; i.e. point (C,D) will not be set.
void insetCorner(
double a, double b, // previous point
double c, double d, // current point that needs to be inset
double e, double f, // next point
double *C, double *D, // storage location for new, inset point
double insetDist) { // amount of inset (perpendicular to each line segment)
double c1=c, d1=d, c2=c, d2=d, dx1, dy1, dist1, dx2, dy2, dist2, insetX, insetY ;
// Calculate length of line segments.
dx1=ca; dy1=db; dist1=sqrt(dx1*dx1+dy1*dy1);
dx2=ec; dy2=fd; dist2=sqrt(dx2*dx2+dy2*dy2);
// Exit if either segment is zerolength.
if (dist1==0.  dist2==0.) return;
// Inset each of the two line segments.
insetX= dy1/dist1*insetDist; a+=insetX; c1+=insetX;
insetY=dx1/dist1*insetDist; b+=insetY; d1+=insetY;
insetX= dy2/dist2*insetDist; e+=insetX; c2+=insetX;
insetY=dx2/dist2*insetDist; f+=insetY; d2+=insetY;
// If inset segments connect perfectly, return the connection point.
if (c1==c2 && d1==d2) {
*C=c1; *D=d1; return; }
// Return the intersection point of the two inset segments (if any).
if (lineIntersection(a,b,c1,d1,c2,d2,e,f,&insetX,&insetY)) {
*C=insetX; *D=insetY; }}
