www.pudn.com > CG2Programs.rar > clipSuthHodge.c

/* clipSuthHodge, Chapter 6, pp. 239-242 */

#include "graphics.h"

/* EXAMPLE STARTS HERE */
typedef enum { Left, Right, Bottom, Top } Edge;
#define N_EDGE 4

int inside (wcPt2 p, Edge b, dcPt wMin, dcPt wMax)
{
  switch (b) {
  case Left:   if (p.x < wMin.x) return (FALSE); break;
  case Right:  if (p.x > wMax.x) return (FALSE); break;
  case Bottom: if (p.y < wMin.y) return (FALSE); break;
  case Top:    if (p.y > wMax.y) return (FALSE); break;
  }
  return (TRUE);
}

int cross (wcPt2 p1, wcPt2 p2, Edge b, dcPt wMin, dcPt wMax)
{
  if (inside (p1, b, wMin, wMax) == inside (p2, b, wMin, wMax))
    return (FALSE);
  else return (TRUE);
}

wcPt2 intersect (wcPt2 p1, wcPt2 p2, Edge b, dcPt wMin, dcPt wMax)
{
  wcPt2 iPt;
  float m;

  if (p1.x != p2.x) m = (p1.y - p2.y) / (p1.x - p2.x);
  switch (b) {
  case Left:
    iPt.x = wMin.x;
    iPt.y = p2.y + (wMin.x - p2.x) * m;
    break;
  case Right:
    iPt.x = wMax.x;
    iPt.y = p2.y + (wMax.x - p2.x) * m;
    break;
  case Bottom:
    iPt.y = wMin.y;
    if (p1.x != p2.x) iPt.x = p2.x + (wMin.y - p2.y) / m;
    else iPt.x = p2.x;
    break;
  case Top:
    iPt.y = wMax.y;
    if (p1.x != p2.x) iPt.x = p2.x + (wMax.y - p2.y) / m;
    else iPt.x = p2.x;
    break;
  }
  return (iPt); 
}

void clipPoint (wcPt2 p, Edge b, dcPt wMin, dcPt wMax,
		wcPt2 * pOut, int * cnt, wcPt2 * first[], wcPt2 * s)
{
  wcPt2 iPt;
  
  /* If no previous point exists for this edge, save this point. */
  if (!first[b])
    first[b] = &p;
  else
    /* Previous point exists.  If 'p' and previous point cross edge,
       find intersection.  Clip against next boundary, if any.  If
       no more edges, add intersection to output list. */
    if (cross (p, s[b], b, wMin, wMax)) {
      iPt = intersect (p, s[b], b, wMin, wMax);
      if (b < Top)
	clipPoint (iPt, b+1, wMin, wMax, pOut, cnt, first, s);
      else {
	pOut[*cnt] = iPt;  (*cnt)++;
      }
    }

  s[b] = p;           /* Save 'p' as most recent point for this edge */

  /* For all, if point is 'inside' proceed to next clip edge, if any */
  if (inside (p, b, wMin, wMax))
    if (b < Top)
      clipPoint (p, b+1, wMin, wMax, pOut, cnt, first, s);
    else {
      pOut[*cnt] = p;  (*cnt)++;
    }
}

void closeClip (dcPt wMin, dcPt wMax, wcPt2 * pOut,
		int * cnt, wcPt2 * first[], wcPt2 * s)
{
  wcPt2 i;
  Edge b;

  for (b = Left; b <= Top; b++) {
    if (cross (s[b], *first[b], b, wMin, wMax)) {
      i = intersect (s[b], *first[b], b, wMin, wMax);
      if (b < Top)
	clipPoint (i, b+1, wMin, wMax, pOut, cnt, first, s);
      else {
        pOut[*cnt] = i;  (*cnt)++;
      }
    }
  }
}

int clipPolygon (dcPt wMin, dcPt wMax, int n, wcPt2 * pIn, wcPt2 * pOut)
{
  /* 'first' holds pointer to first point processed against a clip
     edge.  's' holds most recent point processed against an edge */
  wcPt2 * first[N_EDGE] = { 0, 0, 0, 0 }, s[N_EDGE];
  int i, cnt = 0;

  for (i=0; i