/***********************************************************
Copyright 1987 by Digital Equipment Corporation, Maynard, Massachusetts,
and the Massachusetts Institute of Technology, Cambridge, Massachusetts.
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Permission to use, copy, modify, and distribute this software and its
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DIGITAL DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING
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WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION,
ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
SOFTWARE.
******************************************************************/
/* $Header: mipolyclip.c,v 1.9 87/06/15 00:57:41 sue Exp $ */
/*
* clippoly.c (Sutherland and Hodgeman clipper)
*
* Written by Brian Kelleher; Jan. 1986.
*
* This is an implementation of Sutherland and Hodgeman's
* polygon clipping algorithm.
* The result of this routine is to fill in the array
* clippedpts with a new polygon which is the intersection
* of the original polygon with the clip rect. The number of
* vertices in the new polygon is returned.
* The routine will also clip properties associated with
* vertices (examples: r, g, b, i, z). These properties must be
* unsigned shorts and are linearly interpolated along an edge.
*
* I think this algorithm works fine for general (concave)
* polygons as well as convex. It leaves extraneous edges along
* the clip boundary; however, these shouldn't be a problem
* since they are sent directly to the scan-converter, which
* will do the right thing.
* One possible problem, however, is that when a vertex is
* outside the clip boundary, this algorithm just
* throws it away. This may cause problems when scan-converting,
* since we have potentially lost information along a scanline
* where edges existed outside of the clip boundary. If this turns
* out to be a problem, then all points outside the clip boundary
* can simply be truncated to the boundary (e.g. the point (10, 500)
* is truncated to (500, 500) at the clip boundary x=500). I can't think
* of any case where this causes trouble (I think all is OK), but
* just wanted to record my concerns here.
*
*/
#include "X.h"
#include "Xmd.h"
#include "misc.h"
#include "miscstruct.h"
#include "mistruct.h"
#include "gc.h"
#include "os.h"
#include "opaque.h"
/*
* Used to describe a clipping boundary.
* The edge field tells which side of the clip rect we are
* dealing with (ie. TOP), and the value field tells the value
* of that edge (ie. y = 100).
*/
typedef struct ←boundary {
int edge;
int value;
} boundary;
#define TOP 0
#define BOTTOM 1
#define LEFT 2
#define RIGHT 3
/*
* Call the clipper once for each boundary of the
* clipping rectangle.
* Return the points in the resulting polygon.
* The ptsOut buffer is a temporary buffer allocated by the
* client of this routine which we can use to put the new
* points into.
*/
int miClipPoly(nIn,ptsIn,nOut,ptsOut,clipRect)
int nIn; /* number of points */
DDXPointPtr ptsIn; /* first point in vertex list */
int *nOut; /* number of points out */
DDXPointPtr ptsOut; /* temporary buffer of points */
RectangleRec clipRect; /* clipping rectangle */
{
DDXPointPtr localPts;
boundary bound;
RectangleRec bbox; /* polygon's bounding box */
if(!(localPts=(DDXPointPtr)ALLOCATE←LOCAL(sizeof(DDXPointRec)*(2*nIn+4))))
return(1);
getPgonBbox(nIn, ptsIn, &bbox);
/*
* Check trivial accept
*/
if ((bbox.x >= clipRect.x) &&
(bbox.y >= clipRect.y) &&
(bbox.x + bbox.width <= clipRect.x + clipRect.width) &&
(bbox.y + bbox.height <= clipRect.y + clipRect.height))
{
*nOut = nIn;
DEALLOCATE←LOCAL(localPts);
return(0);
}
/*
* Check trivial reject
*/
if ((bbox.x > clipRect.x + clipRect.width) ||
(bbox.y > clipRect.y + clipRect.height) ||
(bbox.x + bbox.width < clipRect.x) ||
(bbox.y + bbox.height < clipRect.y))
{
*nOut = 0;
DEALLOCATE←LOCAL(localPts);
return(1);
}
/*
* clip to the top, then bottom, left, right
*/
bound.edge = TOP; bound.value = clipRect.y;
nIn = SHclip(ptsIn,localPts,nIn,bound);
bound.edge = BOTTOM; bound.value = clipRect.y + clipRect.height;
nIn = SHclip(localPts,ptsOut,nIn,bound);
bound.edge = LEFT; bound.value = clipRect.x;
nIn = SHclip(ptsOut,localPts,nIn,bound);
bound.edge = RIGHT; bound.value = clipRect.x + clipRect.width;
nIn = SHclip(localPts,ptsOut,nIn,bound);
*nOut = nIn;
DEALLOCATE←LOCAL(localPts);
return(1);
}
/*
* return the bounding box of the polygon
*/
static getPgonBbox(n, pts, bbox)
register int n;
register DDXPointPtr pts;
RectanglePtr bbox;
{
register int l, r, t, b;
l = r = pts->x;
b = t = pts++->y;
while (--n)
{
if (pts->x < l)
l = pts->x;
if (pts->x > r)
r = pts->x;
if (pts->y < b)
b = pts->y;
if (pts->y > t)
t = pts->y;
pts++;
}
bbox->x = l;
bbox->y = b;
bbox->width = r - l;
bbox->height = t - b;
}
/*
* This is where the real work is done.
* Sutherland and Hodgeman are a couple of crazy guys.
*/
static int
SHclip(ptsIn,ptsOut,nIn,bound)
register DDXPointPtr ptsIn; /* first point in vertex list */
register DDXPointPtr ptsOut; /* resulting polygon */
register int nIn; /* number of points */
boundary bound; /* which edge of cliprect */
{
DDXPointRec s, p; /* start and end points of an edge */
register int nOut = 0; /* number of pts output by clipper */
register int pIn, sIn; /* boolean flags */
DDXPointRec ClipIntersect();
/*
* start at the top of the polygon and work around the poly
*/
s = ptsIn[nIn-1];
while (nIn--)
{
p = *ptsIn++;
/*
* determine the insideness of the points
*/
switch (bound.edge)
{
case TOP:
pIn = (p.y >= bound.value) ? TRUE : FALSE;
sIn = (s.y >= bound.value) ? TRUE : FALSE;
break;
case BOTTOM:
pIn = (p.y <= bound.value) ? TRUE : FALSE;
sIn = (s.y <= bound.value) ? TRUE : FALSE;
break;
case LEFT:
pIn = (p.x >= bound.value) ? TRUE : FALSE;
sIn = (s.x >= bound.value) ? TRUE : FALSE;
break;
case RIGHT:
pIn = (p.x <= bound.value) ? TRUE : FALSE;
sIn = (s.x <= bound.value) ? TRUE : FALSE;
break;
}
/*
* if p is inside
*/
if (pIn)
{
/*
* if s is inside
*/
if (sIn)
{
*(ptsOut++) = p;
nOut++;
}
else
{
*(ptsOut++) = ClipIntersect(s,p,bound);
*(ptsOut++) = p;
nOut += 2;
}
}
else
{
/*
* if s is inside
*/
if (sIn)
{
*(ptsOut++) = ClipIntersect(s,p,bound);
nOut++;
}
}
s = p;
}
return(nOut);
}
/*
* Calculate intersections with simple boundaries.
* The algorithm is simple -- we just use similar
* triangles.
* We also have to clip the pixel data to the boundary.
* This is done very similar to the coordinate data, except
* that we have to worry about different pixel types.
*
* In calculating intersections, we must make certain that
* the direction of the first point to the second point is
* always consistent. This will ensure that polygons sharing
* an edge will remain coincident. We will not worry about this
* for pixel data because the problem is not noticable.
*/
static DDXPointRec
ClipIntersect(s,p,bound)
DDXPointRec s, p; /* describe a line */
boundary bound;
{
DDXPointRec i;
register int dpts, dbound;
/*
* Note that we don't have to worry about divide by 0
* because this routine will never get called in such
* a case.
* Also, make sure we are always going in a consistent direction
* so we don't get hurt by numerical errors.
*/
if ((bound.edge == TOP) || (bound.edge == BOTTOM))
{
dbound = bound.value - p.y;
dpts = s.y - p.y;
if (dpts > 0)
{
i.x = p.x + ((s.x - p.x) * dbound) / dpts;
i.y = bound.value;
}
else
{
i.x = s.x + ((p.x - s.x) * (bound.value-s.y)) / -dpts;
i.y = bound.value;
}
}
else
{
dbound = bound.value - p.x;
dpts = s.x - p.x;
if (dpts > 0)
{
i.y = p.y + ((s.y - p.y) * dbound) / dpts;
i.x = bound.value;
}
else
{
i.y = s.y + ((p.y - s.y) * (bound.value-s.x)) / -dpts;
i.x = bound.value;
}
}
return(i);
}