Erasing Polygon Overlay in Spatial Analysis - algorithm

Question

I am using Sutherland Hodgman's Algorithm inorder to clip the polygon overlay for clipping Spatial Query. And I want to implement erase overlay function as well. So suggest me some algorithm like Sutherland Hodgman Algorithm or Can anyone modify this Sutherland Hodgman Algorithm for me.
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
typedef struct { double x, y; } vec_t, *vec;
inline double dot(vec a, vec b)
{
return a->x * b->x + a->y * b->y;
}
inline double cross(vec a, vec b)
{
return a->x * b->y - a->y * b->x;
}
inline vec vsub(vec a, vec b, vec res)
{
res->x = a->x - b->x;
res->y = a->y - b->y;
return res;
}
/* tells if vec c lies on the left side of directed edge a->b
* 1 if left, -1 if right, 0 if colinear
*/
int left_of(vec a, vec b, vec c)
{
vec_t tmp1, tmp2;
double x;
vsub(b, a, &tmp1);
vsub(c, b, &tmp2);
x = cross(&tmp1, &tmp2);
return x < 0 ? -1 : x > 0;
}
int line_sect(vec x0, vec x1, vec y0, vec y1, vec res)
{
vec_t dx, dy, d;
vsub(x1, x0, &dx);
vsub(y1, y0, &dy);
vsub(x0, y0, &d);
/* x0 + a dx = y0 + b dy ->
x0 X dx = y0 X dx + b dy X dx ->
b = (x0 - y0) X dx / (dy X dx) */
double dyx = cross(&dy, &dx);
if (!dyx) return 0;
dyx = cross(&d, &dx) / dyx;
if (dyx <= 0 || dyx >= 1) return 0;
res->x = y0->x + dyx * dy.x;
res->y = y0->y + dyx * dy.y;
return 1;
}
/* === polygon stuff === */
typedef struct { int len, alloc; vec v; } poly_t, *poly;
poly poly_new()
{
poly p = (poly)malloc(sizeof(poly_t));
p->len = p->alloc = 0;
p->v = 0;
return p;
}
void poly_free(poly p)
{
if (p->alloc) {
free(p->v);
free(p);
}
}
void poly_append(poly p, vec v)
{
if (p->len >= p->alloc) {
p->alloc *= 2;
if (!p->alloc) p->alloc = 4;
p->v = (vec)realloc(p->v, sizeof(vec_t) * p->alloc);
}
p->v[p->len++] = *v;
}
/* this works only if all of the following are true:
* 1. poly has no colinear edges;
* 2. poly has no duplicate vertices;
* 3. poly has at least three vertices;
* 4. poly is convex (implying 3).
*/
int poly_winding(poly p)
{
return left_of(&p->v[0], &p->v[1], &p->v[2]);
}
void poly_edge_clip(poly sub, vec x0, vec x1, int left, poly res)
{
int i, side0, side1;
vec_t tmp;
vec v0 = &sub->v[sub->len - 1], v1;
res->len = 0;
side0 = left_of(x0, x1, v0);
if (side0 != -left) poly_append(res, v0);
for (i = 0; i < sub->len; i++) {
v1 = &sub->v[i];
side1 = left_of(x0, x1, v1);
if (side0 + side1 == 0 && side0)
/* last point and current straddle the edge */
if (line_sect(x0, x1, v0, v1, &tmp))
poly_append(res, &tmp);
if (i == sub->len - 1) break;
if (side1 != -left) poly_append(res, v1);
v0 = v1;
side0 = side1;
}
}
poly poly_clip(poly sub, poly clip)
{
int i;
poly p1 = poly_new(), p2 = poly_new(), tmp;
int dir = poly_winding(clip);
poly_edge_clip(sub, &clip->v[clip->len - 1], &clip->v[0], dir, p2);
for (i = 0; i < clip->len - 1; i++) {
tmp = p2; p2 = p1; p1 = tmp;
poly_edge_clip(p1, &clip->v[i], &clip->v[i + 1], dir, p2);
}
poly_free(p1);
return p2;
}
int main()
{
int i;
vec_t c[] = {{200,200}, {400,200}, {400,400}, {200,400}};
//vec_t c[] = {{100,300}, {300,300}, {300,100}, {100,100}};
vec_t s[] = { {50,150}, {200,50}, {350,150},
{350,300},{250,300},{200,250},
{150,350},{100,250},{100,200}};
#define clen (sizeof(c)/sizeof(vec_t))
#define slen (sizeof(s)/sizeof(vec_t))
poly_t clipper = {clen, 0, c};
poly_t subject = {slen, 0, s};
poly res = poly_clip(&subject, &clipper);
for (i = 0; i < res->len; i++)
printf("%g %g\n", res->v[i].x, res->v[i].y);
/* long and arduous EPS printout */
FILE * eps = fopen("test.eps", "w");
fprintf(eps, "%%!PS-Adobe-3.0\n%%%%BoundingBox: 40 40 360 360\n"
"/l {lineto} def /m{moveto} def /s{setrgbcolor} def"
"/c {closepath} def /gs {fill grestore stroke} def\n");
fprintf(eps, "0 setlinewidth %g %g m ", c[0].x, c[0].y);
for (i = 1; i < clen; i++)
fprintf(eps, "%g %g l ", c[i].x, c[i].y);
fprintf(eps, "c .5 0 0 s gsave 1 .7 .7 s gs\n");
fprintf(eps, "%g %g m ", s[0].x, s[0].y);
for (i = 1; i < slen; i++)
fprintf(eps, "%g %g l ", s[i].x, s[i].y);
fprintf(eps, "c 0 .2 .5 s gsave .4 .7 1 s gs\n");
fprintf(eps, "2 setlinewidth [10 8] 0 setdash %g %g m ",
res->v[0].x, res->v[0].y);
for (i = 1; i < res->len; i++)
fprintf(eps, "%g %g l ", res->v[i].x, res->v[i].y);
fprintf(eps, "c .5 0 .5 s gsave .7 .3 .8 s gs\n");
fprintf(eps, "%%%%EOF");
fclose(eps);
printf("test.eps written\n");
return 0;
}