Category : C Source Code
Archive   : GRAFGEM1.ZIP
Filename : CONCAVES.C

 
Output of file : CONCAVES.C contained in archive : GRAFGEM1.ZIP

/*
* Concave Polygon Scan Conversion
* by Paul Heckbert
* from "Graphics Gems", Academic Press, 1990
*/

/*
* concave: scan convert nvert-sided concave non-simple polygon with vertices at
* (point[i].x, point[i].y) for i in [0..nvert-1] within the window win by
* calling spanproc for each visible span of pixels.
* Polygon can be clockwise or counterclockwise.
* Algorithm does uniform point sampling at pixel centers.
* Inside-outside test done by Jordan's rule: a point is considered inside if
* an emanating ray intersects the polygon an odd number of times.
* drawproc should fill in pixels from xl to xr inclusive on scanline y,
* e.g:
* drawproc(y, xl, xr)
* int y, xl, xr;
* {
* int x;
* for (x=xl; x<=xr; x++)
* pixel_write(x, y, pixelvalue);
* }
*
* Paul Heckbert 30 June 81, 18 Dec 89
*/

#include
#include
#include "GraphicsGems.h"

#define ALLOC(ptr, type, n) ASSERT(ptr = (type *)malloc((n)*sizeof(type)))

typedef struct { /* window: a discrete 2-D rectangle */
int x0, y0; /* xmin and ymin */
int x1, y1; /* xmax and ymax (inclusive) */
} Window;

typedef struct { /* a polygon edge */
double x; /* x coordinate of edge's intersection with current scanline */
double dx; /* change in x with respect to y */
int i; /* edge number: edge i goes from pt[i] to pt[i+1] */
} Edge;

static int n; /* number of vertices */
static Point2 *pt; /* vertices */

static int nact; /* number of active edges */
static Edge *active; /* active edge list:edges crossing scanline y */

int compare_ind(), compare_active();

concave(nvert, point, win, spanproc)
int nvert; /* number of vertices */
Point2 *point; /* vertices of polygon */
Window *win; /* screen clipping window */
void (*spanproc)(); /* called for each span of pixels */
{
int k, y0, y1, y, i, j, xl, xr;
int *ind; /* list of vertex indices, sorted by pt[ind[j]].y */

n = nvert;
pt = point;
if (n<=0) return;
ALLOC(ind, int, n);
ALLOC(active, Edge, n);

/* create y-sorted array of indices ind[k] into vertex list */
for (k=0; k ind[k] = k;
qsort(ind, n, sizeof ind[0], compare_ind); /* sort ind by pt[ind[k]].y */

nact = 0; /* start with empty active list */
k = 0; /* ind[k] is next vertex to process */
y0 = MAX(win->y0, ceil(pt[ind[0]].y-.5)); /* ymin of polygon */
y1 = MIN(win->y1, floor(pt[ind[n-1]].y-.5)); /* ymax of polygon */

for (y=y0; y<=y1; y++) { /* step through scanlines */
/* scanline y is at y+.5 in continuous coordinates */

/* check vertices between previous scanline and current one, if any */
for (; k /* to simplify, if pt.y=y+.5, pretend it's above */
/* invariant: y-.5 < pt[i].y <= y+.5 */
i = ind[k];
/*
* insert or delete edges before and after vertex i (i-1 to i,
* and i to i+1) from active list if they cross scanline y
*/
j = i>0 ? i-1 : n-1; /* vertex previous to i */
if (pt[j].y <= y-.5) /* old edge, remove from active list */
delete(j);
else if (pt[j].y > y+.5) /* new edge, add to active list */
insert(j, y);
j = i if (pt[j].y <= y-.5) /* old edge, remove from active list */
delete(i);
else if (pt[j].y > y+.5) /* new edge, add to active list */
insert(i, y);
}

/* sort active edge list by active[j].x */
qsort(active, nact, sizeof active[0], compare_active);

/* draw horizontal segments for scanline y */
for (j=0; j /* span 'tween j & j+1 is inside, span tween j+1 & j+2 is outside */
xl = ceil(active[j].x-.5); /* left end of span */
if (xlx0) xl = win->x0;
xr = floor(active[j+1].x-.5); /* right end of span */
if (xr>win->x1) xr = win->x1;
if (xl<=xr)
(*spanproc)(y, xl, xr); /* draw pixels in span */
active[j].x += active[j].dx; /* increment edge coords */
active[j+1].x += active[j+1].dx;
}
}
}

static delete(i) /* remove edge i from active list */
int i;
{
int j;

for (j=0; j if (j>=nact) return; /* edge not in active list; happens at win->y0*/
nact--;
bcopy(&active[j+1], &active[j], (nact-j)*sizeof active[0]);
}

static insert(i, y) /* append edge i to end of active list */
int i, y;
{
int j;
double dx;
Point2 *p, *q;

j = i if (pt[i].y < pt[j].y) {p = &pt[i]; q = &pt[j];}
else {p = &pt[j]; q = &pt[i];}
/* initialize x position at intersection of edge with scanline y */
active[nact].dx = dx = (q->x-p->x)/(q->y-p->y);
active[nact].x = dx*(y+.5-p->y)+p->x;
active[nact].i = i;
nact++;
}

/* comparison routines for qsort */
compare_ind(u, v) int *u, *v; {return pt[*u].y <= pt[*v].y ? -1 : 1;}
compare_active(u, v) Edge *u, *v; {return u->x <= v->x ? -1 : 1;}