Initial import of libart 2.3.21

1 files changed, 1803 insertions, 0 deletions

diff --git a/art_svp_intersect.c b/art_svp_intersect.c
new file mode 100644
index 0000000..4ece5f4
--- /dev/null
+++ b/art_svp_intersect.c
@@ -0,0 +1,1803 @@
+/* Libart_LGPL - library of basic graphic primitives
+ * Copyright (C) 2001 Raph Levien
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+/* This file contains a testbed implementation of the new intersection
+   code.
+*/
+
+#include "config.h"
+#include "art_svp_intersect.h"
+
+#include <math.h> /* for sqrt */
+
+/* Sanitychecking verifies the main invariant on every priority queue
+   point. Do not use in production, as it slows things down way too
+   much. */
+#define noSANITYCHECK
+
+/* This can be used in production, to prevent hangs. Eventually, it
+   should not be necessary. */
+#define CHEAP_SANITYCHECK
+
+#define noVERBOSE
+
+#include "art_misc.h"
+
+/* A priority queue - perhaps move to a separate file if it becomes
+   needed somewhere else */
+
+#define ART_PRIQ_USE_HEAP
+
+typedef struct _ArtPriQ ArtPriQ;
+typedef struct _ArtPriPoint ArtPriPoint;
+
+struct _ArtPriQ {
+  int n_items;
+  int n_items_max;
+  ArtPriPoint **items;
+};
+
+struct _ArtPriPoint {
+  double x;
+  double y;
+  void *user_data;
+};
+
+static ArtPriQ *
+art_pri_new (void)
+{
+  ArtPriQ *result = art_new (ArtPriQ, 1);
+
+  result->n_items = 0;
+  result->n_items_max = 16;
+  result->items = art_new (ArtPriPoint *, result->n_items_max);
+  return result;
+}
+
+static void
+art_pri_free (ArtPriQ *pq)
+{
+  art_free (pq->items);
+  art_free (pq);
+}
+
+static art_boolean
+art_pri_empty (ArtPriQ *pq)
+{
+  return pq->n_items == 0;
+}
+
+#ifdef ART_PRIQ_USE_HEAP
+
+/* This heap implementation is based on Vasek Chvatal's course notes:
+   http://www.cs.rutgers.edu/~chvatal/notes/pq.html#heap */
+
+static void
+art_pri_bubble_up (ArtPriQ *pq, int vacant, ArtPriPoint *missing)
+{
+  ArtPriPoint **items = pq->items;
+  int parent;
+
+  parent = (vacant - 1) >> 1;
+  while (vacant > 0 && (missing->y < items[parent]->y ||
+			(missing->y == items[parent]->y &&
+			 missing->x < items[parent]->x)))
+    {
+      items[vacant] = items[parent];
+      vacant = parent;
+      parent = (vacant - 1) >> 1;
+    }
+
+  items[vacant] = missing;
+}
+
+static void
+art_pri_insert (ArtPriQ *pq, ArtPriPoint *point)
+{
+  if (pq->n_items == pq->n_items_max)
+    art_expand (pq->items, ArtPriPoint *, pq->n_items_max);
+
+  art_pri_bubble_up (pq, pq->n_items++, point);
+}
+
+static void
+art_pri_sift_down_from_root (ArtPriQ *pq, ArtPriPoint *missing)
+{
+  ArtPriPoint **items = pq->items;
+  int vacant = 0, child = 2;
+  int n = pq->n_items;
+
+  while (child < n)
+    {
+      if (items[child - 1]->y < items[child]->y ||
+	  (items[child - 1]->y == items[child]->y &&
+	   items[child - 1]->x < items[child]->x))
+	child--;
+      items[vacant] = items[child];
+      vacant = child;
+      child = (vacant + 1) << 1;
+    }
+  if (child == n)
+    {
+      items[vacant] = items[n - 1];
+      vacant = n - 1;
+    }
+
+  art_pri_bubble_up (pq, vacant, missing);
+}
+
+static ArtPriPoint *
+art_pri_choose (ArtPriQ *pq)
+{
+  ArtPriPoint *result = pq->items[0];
+
+  art_pri_sift_down_from_root (pq, pq->items[--pq->n_items]);
+  return result;
+}
+
+#else
+
+/* Choose least point in queue */
+static ArtPriPoint *
+art_pri_choose (ArtPriQ *pq)
+{
+  int i;
+  int best = 0;
+  double best_x, best_y;
+  double y;
+  ArtPriPoint *result;
+
+  if (pq->n_items == 0)
+    return NULL;
+
+  best_x = pq->items[best]->x;
+  best_y = pq->items[best]->y;
+
+  for (i = 1; i < pq->n_items; i++)
+    {
+      y = pq->items[i]->y;
+      if (y < best_y || (y == best_y && pq->items[i]->x < best_x))
+	{
+	  best = i;
+	  best_x = pq->items[best]->x;
+	  best_y = y;
+	}
+    }
+  result = pq->items[best];
+  pq->items[best] = pq->items[--pq->n_items];
+  return result;
+}
+
+static void
+art_pri_insert (ArtPriQ *pq, ArtPriPoint *point)
+{
+  if (pq->n_items == pq->n_items_max)
+    art_expand (pq->items, ArtPriPoint *, pq->n_items_max);
+
+  pq->items[pq->n_items++] = point;
+}
+
+#endif
+
+#ifdef TEST_PRIQ
+
+#include <stdlib.h> /* for rand() */
+#include <stdio.h>
+
+static double
+double_rand (double lo, double hi, int quant)
+{
+  int tmp = rand () / (RAND_MAX * (1.0 / quant)) + 0.5;
+  return lo + tmp * ((hi - lo) / quant);
+}
+
+/*
+ * This custom allocator for priority queue points is here so I can
+ * test speed. It doesn't look like it will be that significant, but
+ * if I want a small improvement later, it's something.
+ */
+
+typedef ArtPriPoint *ArtPriPtPool;
+
+static ArtPriPtPool *
+art_pri_pt_pool_new (void)
+{
+  ArtPriPtPool *result = art_new (ArtPriPtPool, 1);
+  *result = NULL;
+  return result;
+}
+
+static ArtPriPoint *
+art_pri_pt_alloc (ArtPriPtPool *pool)
+{
+  ArtPriPoint *result = *pool;
+  if (result == NULL)
+    return art_new (ArtPriPoint, 1);
+  else
+    {
+      *pool = result->user_data;
+      return result;
+    }
+}
+
+static void
+art_pri_pt_free (ArtPriPtPool *pool, ArtPriPoint *pt)
+{
+  pt->user_data = *pool;
+  *pool = pt;
+}
+
+static void
+art_pri_pt_pool_free (ArtPriPtPool *pool)
+{
+  ArtPriPoint *pt = *pool;
+  while (pt != NULL)
+    {
+      ArtPriPoint *next = pt->user_data;
+      art_free (pt);
+      pt = next;
+    }
+  art_free (pool);
+}
+
+int
+main (int argc, char **argv)
+{
+  ArtPriPtPool *pool = art_pri_pt_pool_new ();
+  ArtPriQ *pq;
+  int i, j;
+  const int n_iter = 1;
+  const int pq_size = 100;
+
+  for (j = 0; j < n_iter; j++)
+    {
+      pq = art_pri_new ();
+
+      for (i = 0; i < pq_size; i++)
+	{
+	  ArtPriPoint *pt = art_pri_pt_alloc (pool);
+	  pt->x = double_rand (0, 1, 100);
+	  pt->y = double_rand (0, 1, 100);
+	  pt->user_data = (void *)i;
+	  art_pri_insert (pq, pt);
+	}
+
+      while (!art_pri_empty (pq))
+	{
+	  ArtPriPoint *pt = art_pri_choose (pq);
+	  if (n_iter == 1)
+	    printf ("(%g, %g), %d\n", pt->x, pt->y, (int)pt->user_data);
+	  art_pri_pt_free (pool, pt);
+	}
+
+      art_pri_free (pq);
+    }
+  art_pri_pt_pool_free (pool);
+  return 0;
+}
+
+#else /* TEST_PRIQ */
+
+/* A virtual class for an "svp writer". A client of this object creates an
+   SVP by repeatedly calling "add segment" and "add point" methods on it.
+*/
+
+typedef struct _ArtSvpWriterRewind ArtSvpWriterRewind;
+
+/* An implementation of the svp writer virtual class that applies the
+   winding rule. */
+
+struct _ArtSvpWriterRewind {
+  ArtSvpWriter super;
+  ArtWindRule rule;
+  ArtSVP *svp;
+  int n_segs_max;
+  int *n_points_max;
+};
+
+static int
+art_svp_writer_rewind_add_segment (ArtSvpWriter *self, int wind_left,
+				   int delta_wind, double x, double y)
+{
+  ArtSvpWriterRewind *swr = (ArtSvpWriterRewind *)self;
+  ArtSVP *svp;
+  ArtSVPSeg *seg;
+  art_boolean left_filled, right_filled;
+  int wind_right = wind_left + delta_wind;
+  int seg_num;
+  const int init_n_points_max = 4;
+
+  switch (swr->rule)
+    {
+    case ART_WIND_RULE_NONZERO:
+      left_filled = (wind_left != 0);
+      right_filled = (wind_right != 0);
+      break;
+    case ART_WIND_RULE_INTERSECT:
+      left_filled = (wind_left > 1);
+      right_filled = (wind_right > 1);
+      break;
+    case ART_WIND_RULE_ODDEVEN:
+      left_filled = (wind_left & 1);
+      right_filled = (wind_right & 1);
+      break;
+    case ART_WIND_RULE_POSITIVE:
+      left_filled = (wind_left > 0);
+      right_filled = (wind_right > 0);
+      break;
+    default:
+      art_die ("Unknown wind rule %d\n", swr->rule);
+    }
+  if (left_filled == right_filled)
+    {
+      /* discard segment now */
+#ifdef VERBOSE
+      art_dprint ("swr add_segment: %d += %d (%g, %g) --> -1\n",
+	      wind_left, delta_wind, x, y);
+#endif
+      return -1;
+   }
+
+  svp = swr->svp;
+  seg_num = svp->n_segs++;
+  if (swr->n_segs_max == seg_num)
+    {
+      swr->n_segs_max <<= 1;
+      svp = (ArtSVP *)art_realloc (svp, sizeof(ArtSVP) +
+				   (swr->n_segs_max - 1) *
+				   sizeof(ArtSVPSeg));
+      swr->svp = svp;
+      swr->n_points_max = art_renew (swr->n_points_max, int,
+				     swr->n_segs_max);
+    }
+  seg = &svp->segs[seg_num];
+  seg->n_points = 1;
+  seg->dir = right_filled;
+  swr->n_points_max[seg_num] = init_n_points_max;
+  seg->bbox.x0 = x;
+  seg->bbox.y0 = y;
+  seg->bbox.x1 = x;
+  seg->bbox.y1 = y;
+  seg->points = art_new (ArtPoint, init_n_points_max);
+  seg->points[0].x = x;
+  seg->points[0].y = y;
+#ifdef VERBOSE
+    art_dprint ("swr add_segment: %d += %d (%g, %g) --> %d(%s)\n",
+	    wind_left, delta_wind, x, y, seg_num,
+	    seg->dir ? "v" : "^");
+#endif
+  return seg_num;
+}
+
+static void
+art_svp_writer_rewind_add_point (ArtSvpWriter *self, int seg_id,
+				 double x, double y)
+{
+  ArtSvpWriterRewind *swr = (ArtSvpWriterRewind *)self;
+  ArtSVPSeg *seg;
+  int n_points;
+
+#ifdef VERBOSE
+  art_dprint ("swr add_point: %d (%g, %g)\n", seg_id, x, y);
+#endif
+  if (seg_id < 0)
+    /* omitted segment */
+    return;
+
+  seg = &swr->svp->segs[seg_id];
+  n_points = seg->n_points++;
+  if (swr->n_points_max[seg_id] == n_points)
+    art_expand (seg->points, ArtPoint, swr->n_points_max[seg_id]);
+  seg->points[n_points].x = x;
+  seg->points[n_points].y = y;
+  if (x < seg->bbox.x0)
+    seg->bbox.x0 = x;
+  if (x > seg->bbox.x1)
+    seg->bbox.x1 = x;
+  seg->bbox.y1 = y;
+}
+
+static void
+art_svp_writer_rewind_close_segment (ArtSvpWriter *self, int seg_id)
+{
+  /* Not needed for this simple implementation. A potential future
+     optimization is to merge segments that can be merged safely. */
+#ifdef SANITYCHECK
+  ArtSvpWriterRewind *swr = (ArtSvpWriterRewind *)self;
+  ArtSVPSeg *seg;
+
+  if (seg_id >= 0)
+    {
+      seg = &swr->svp->segs[seg_id];
+      if (seg->n_points < 2)
+	art_warn ("*** closing segment %d with only %d point%s\n",
+		  seg_id, seg->n_points, seg->n_points == 1 ? "" : "s");
+    }
+#endif
+
+#ifdef VERBOSE
+  art_dprint ("swr close_segment: %d\n", seg_id);
+#endif
+}
+
+ArtSVP *
+art_svp_writer_rewind_reap (ArtSvpWriter *self)
+{
+  ArtSvpWriterRewind *swr = (ArtSvpWriterRewind *)self;
+  ArtSVP *result = swr->svp;
+
+  art_free (swr->n_points_max);
+  art_free (swr);
+  return result;
+}
+
+ArtSvpWriter *
+art_svp_writer_rewind_new (ArtWindRule rule)
+{
+  ArtSvpWriterRewind *result = art_new (ArtSvpWriterRewind, 1);
+
+  result->super.add_segment = art_svp_writer_rewind_add_segment;
+  result->super.add_point = art_svp_writer_rewind_add_point;
+  result->super.close_segment = art_svp_writer_rewind_close_segment;
+
+  result->rule = rule;
+  result->n_segs_max = 16;
+  result->svp = art_alloc (sizeof(ArtSVP) + 
+			   (result->n_segs_max - 1) * sizeof(ArtSVPSeg));
+  result->svp->n_segs = 0;
+  result->n_points_max = art_new (int, result->n_segs_max);
+
+  return &result->super;
+}
+
+/* Now, data structures for the active list */
+
+typedef struct _ArtActiveSeg ArtActiveSeg;
+
+/* Note: BNEG is 1 for \ lines, and 0 for /. Thus,
+   x[(flags & BNEG) ^ 1] <= x[flags & BNEG] */
+#define ART_ACTIVE_FLAGS_BNEG 1
+
+/* This flag is set if the segment has been inserted into the active
+   list. */
+#define ART_ACTIVE_FLAGS_IN_ACTIVE 2
+
+/* This flag is set when the segment is to be deleted in the
+   horiz commit process. */
+#define ART_ACTIVE_FLAGS_DEL 4
+
+/* This flag is set if the seg_id is a valid output segment. */
+#define ART_ACTIVE_FLAGS_OUT 8
+
+/* This flag is set if the segment is in the horiz list. */
+#define ART_ACTIVE_FLAGS_IN_HORIZ 16
+
+struct _ArtActiveSeg {
+  int flags;
+  int wind_left, delta_wind;
+  ArtActiveSeg *left, *right; /* doubly linked list structure */
+
+  const ArtSVPSeg *in_seg;
+  int in_curs;
+
+  double x[2];
+  double y0, y1;
+  double a, b, c; /* line equation; ax+by+c = 0 for the line, a^2 + b^2 = 1,
+		     and a>0 */
+
+  /* bottom point and intersection point stack */
+  int n_stack;
+  int n_stack_max;
+  ArtPoint *stack;
+
+  /* horiz commit list */
+  ArtActiveSeg *horiz_left, *horiz_right;
+  double horiz_x;
+  int horiz_delta_wind;
+  int seg_id;
+};
+
+typedef struct _ArtIntersectCtx ArtIntersectCtx;
+
+struct _ArtIntersectCtx {
+  const ArtSVP *in;
+  ArtSvpWriter *out;
+
+  ArtPriQ *pq;
+
+  ArtActiveSeg *active_head;
+
+  double y;
+  ArtActiveSeg *horiz_first;
+  ArtActiveSeg *horiz_last;
+
+  /* segment index of next input segment to be added to pri q */
+  int in_curs;
+};
+
+#define EPSILON_A 1e-5 /* Threshold for breaking lines at point insertions */
+
+/**
+ * art_svp_intersect_setup_seg: Set up an active segment from input segment.
+ * @seg: Active segment.
+ * @pri_pt: Priority queue point to initialize.
+ *
+ * Sets the x[], a, b, c, flags, and stack fields according to the
+ * line from the current cursor value. Sets the priority queue point
+ * to the bottom point of this line. Also advances the input segment
+ * cursor.
+ **/
+static void
+art_svp_intersect_setup_seg (ArtActiveSeg *seg, ArtPriPoint *pri_pt)
+{
+  const ArtSVPSeg *in_seg = seg->in_seg;
+  int in_curs = seg->in_curs++;
+  double x0, y0, x1, y1;
+  double dx, dy, s;
+  double a, b, r2;
+
+  x0 = in_seg->points[in_curs].x;
+  y0 = in_seg->points[in_curs].y;
+  x1 = in_seg->points[in_curs + 1].x;
+  y1 = in_seg->points[in_curs + 1].y;
+  pri_pt->x = x1;
+  pri_pt->y = y1;
+  dx = x1 - x0;
+  dy = y1 - y0;
+  r2 = dx * dx + dy * dy;
+  s = r2 == 0 ? 1 : 1 / sqrt (r2);
+  seg->a = a = dy * s;
+  seg->b = b = -dx * s;
+  seg->c = -(a * x0 + b * y0);
+  seg->flags = (seg->flags & ~ART_ACTIVE_FLAGS_BNEG) | (dx > 0);
+  seg->x[0] = x0;
+  seg->x[1] = x1;
+  seg->y0 = y0;
+  seg->y1 = y1;
+  seg->n_stack = 1;
+  seg->stack[0].x = x1;
+  seg->stack[0].y = y1;
+}
+
+/**
+ * art_svp_intersect_add_horiz: Add point to horizontal list.
+ * @ctx: Intersector context.
+ * @seg: Segment with point to insert into horizontal list.
+ *
+ * Inserts @seg into horizontal list, keeping it in ascending horiz_x
+ * order.
+ *
+ * Note: the horiz_commit routine processes "clusters" of segs in the
+ * horiz list, all sharing the same horiz_x value. The cluster is
+ * processed in active list order, rather than horiz list order. Thus,
+ * the order of segs in the horiz list sharing the same horiz_x
+ * _should_ be irrelevant. Even so, we use b as a secondary sorting key,
+ * as a "belt and suspenders" defensive coding tactic.
+ **/
+static void
+art_svp_intersect_add_horiz (ArtIntersectCtx *ctx, ArtActiveSeg *seg)
+{
+  ArtActiveSeg **pp = &ctx->horiz_last;
+  ArtActiveSeg *place;
+  ArtActiveSeg *place_right = NULL;
+
+
+#ifdef CHEAP_SANITYCHECK
+  if (seg->flags & ART_ACTIVE_FLAGS_IN_HORIZ)
+    {
+      art_warn ("*** attempt to put segment in horiz list twice\n");
+      return;
+    }
+  seg->flags |= ART_ACTIVE_FLAGS_IN_HORIZ;
+#endif
+
+#ifdef VERBOSE
+  art_dprint ("add_horiz %lx, x = %g\n", (unsigned long) seg, seg->horiz_x);
+#endif
+  for (place = *pp; place != NULL && (place->horiz_x > seg->horiz_x ||
+				      (place->horiz_x == seg->horiz_x &&
+				       place->b < seg->b));
+       place = *pp)
+    {
+      place_right = place;
+      pp = &place->horiz_left;
+    }
+  *pp = seg;
+  seg->horiz_left = place;
+  seg->horiz_right = place_right;
+  if (place == NULL)
+    ctx->horiz_first = seg;
+  else
+    place->horiz_right = seg;
+}
+
+static void
+art_svp_intersect_push_pt (ArtIntersectCtx *ctx, ArtActiveSeg *seg,
+			   double x, double y)
+{
+  ArtPriPoint *pri_pt;
+  int n_stack = seg->n_stack;
+
+  if (n_stack == seg->n_stack_max)
+    art_expand (seg->stack, ArtPoint, seg->n_stack_max);
+  seg->stack[n_stack].x = x;
+  seg->stack[n_stack].y = y;
+  seg->n_stack++;
+
+  seg->x[1] = x;
+  seg->y1 = y;
+
+  pri_pt = art_new (ArtPriPoint, 1);
+  pri_pt->x = x;
+  pri_pt->y = y;
+  pri_pt->user_data = seg;
+  art_pri_insert (ctx->pq, pri_pt);
+}
+
+typedef enum {
+  ART_BREAK_LEFT = 1,
+  ART_BREAK_RIGHT = 2
+} ArtBreakFlags;
+
+/**
+ * art_svp_intersect_break: Break an active segment.
+ *
+ * Note: y must be greater than the top point's y, and less than
+ * the bottom's.
+ *
+ * Return value: x coordinate of break point.
+ */
+static double
+art_svp_intersect_break (ArtIntersectCtx *ctx, ArtActiveSeg *seg,
+			 double x_ref, double y, ArtBreakFlags break_flags)
+{
+  double x0, y0, x1, y1;
+  const ArtSVPSeg *in_seg = seg->in_seg;
+  int in_curs = seg->in_curs;
+  double x;
+
+  x0 = in_seg->points[in_curs - 1].x;
+  y0 = in_seg->points[in_curs - 1].y;
+  x1 = in_seg->points[in_curs].x;
+  y1 = in_seg->points[in_curs].y;
+  x = x0 + (x1 - x0) * ((y - y0) / (y1 - y0));
+  if ((break_flags == ART_BREAK_LEFT && x > x_ref) ||
+      (break_flags == ART_BREAK_RIGHT && x < x_ref))
+    {
+#ifdef VERBOSE
+      art_dprint ("art_svp_intersect_break: limiting x to %f, was %f, %s\n",
+		  x_ref, x, break_flags == ART_BREAK_LEFT ? "left" : "right");
+      x = x_ref;
+#endif
+    }
+
+  /* I think we can count on min(x0, x1) <= x <= max(x0, x1) with sane
+     arithmetic, but it might be worthwhile to check just in case. */
+
+  if (y > ctx->y)
+    art_svp_intersect_push_pt (ctx, seg, x, y);
+  else
+    {
+      seg->x[0] = x;
+      seg->y0 = y;
+      seg->horiz_x = x;
+      art_svp_intersect_add_horiz (ctx, seg);
+    }
+
+  return x;
+}
+
+/**
+ * art_svp_intersect_add_point: Add a point, breaking nearby neighbors.
+ * @ctx: Intersector context.
+ * @x: X coordinate of point to add.
+ * @y: Y coordinate of point to add.
+ * @seg: "nearby" segment, or NULL if leftmost.
+ *
+ * Return value: Segment immediately to the left of the new point, or
+ * NULL if the new point is leftmost.
+ **/
+static ArtActiveSeg *
+art_svp_intersect_add_point (ArtIntersectCtx *ctx, double x, double y,
+			     ArtActiveSeg *seg, ArtBreakFlags break_flags)
+{
+  ArtActiveSeg *left, *right;
+  double x_min = x, x_max = x;
+  art_boolean left_live, right_live;
+  double d;
+  double new_x;
+  ArtActiveSeg *test, *result = NULL;
+  double x_test;
+
+  left = seg;
+  if (left == NULL)
+    right = ctx->active_head;
+  else
+    right = left->right; 
+  left_live = (break_flags & ART_BREAK_LEFT) && (left != NULL);
+  right_live = (break_flags & ART_BREAK_RIGHT) && (right != NULL);
+  while (left_live || right_live)
+    {
+      if (left_live)
+	{
+	  if (x <= left->x[left->flags & ART_ACTIVE_FLAGS_BNEG] &&
+	      /* It may be that one of these conjuncts turns out to be always
+		 true. We test both anyway, to be defensive. */
+	      y != left->y0 && y < left->y1)
+	    {
+	      d = x_min * left->a + y * left->b + left->c;
+	      if (d < EPSILON_A)
+		{
+		  new_x = art_svp_intersect_break (ctx, left, x_min, y,
+						   ART_BREAK_LEFT);
+		  if (new_x > x_max)
+		    {
+		      x_max = new_x;
+		      right_live = (right != NULL);
+		    }
+		  else if (new_x < x_min)
+		    x_min = new_x;
+		  left = left->left;
+		  left_live = (left != NULL);
+		}
+	      else
+		left_live = ART_FALSE;
+	    }
+	  else
+	    left_live = ART_FALSE;
+	}
+      else if (right_live)
+	{
+	  if (x >= right->x[(right->flags & ART_ACTIVE_FLAGS_BNEG) ^ 1] &&
+	      /* It may be that one of these conjuncts turns out to be always
+		 true. We test both anyway, to be defensive. */
+	      y != right->y0 && y < right->y1)
+	    {
+	      d = x_max * right->a + y * right->b + right->c;
+	      if (d > -EPSILON_A)
+		{
+		  new_x = art_svp_intersect_break (ctx, right, x_max, y,
+						   ART_BREAK_RIGHT);
+		  if (new_x < x_min)
+		    {
+		      x_min = new_x;
+		      left_live = (left != NULL);
+		    }
+		  else if (new_x >= x_max)
+		    x_max = new_x;
+		  right = right->right;
+		  right_live = (right != NULL);
+		}
+	      else
+		right_live = ART_FALSE;
+	    }
+	  else
+	    right_live = ART_FALSE;
+	}
+    }
+
+  /* Ascending order is guaranteed by break_flags. Thus, we don't need
+     to actually fix up non-ascending pairs. */
+
+  /* Now, (left, right) defines an interval of segments broken. Sort
+     into ascending x order. */
+  test = left == NULL ? ctx->active_head : left->right;
+  result = left;
+  if (test != NULL && test != right)
+    {
+      if (y == test->y0)
+	x_test = test->x[0];
+      else /* assert y == test->y1, I think */
+	x_test = test->x[1];
+      for (;;)
+	{
+	  if (x_test <= x)
+	    result = test;
+	  test = test->right;
+	  if (test == right)
+	    break;
+	  new_x = x_test;
+	  if (new_x < x_test)
+	    {
+	      art_warn ("art_svp_intersect_add_point: non-ascending x\n");
+	    }
+	  x_test = new_x;
+	}
+    }
+  return result;
+}
+
+static void
+art_svp_intersect_swap_active (ArtIntersectCtx *ctx,
+			       ArtActiveSeg *left_seg, ArtActiveSeg *right_seg)
+{
+  right_seg->left = left_seg->left;
+  if (right_seg->left != NULL)
+    right_seg->left->right = right_seg;
+  else
+    ctx->active_head = right_seg;
+  left_seg->right = right_seg->right;
+  if (left_seg->right != NULL)
+    left_seg->right->left = left_seg;
+  left_seg->left = right_seg;
+  right_seg->right = left_seg;
+}
+
+/**
+ * art_svp_intersect_test_cross: Test crossing of a pair of active segments.
+ * @ctx: Intersector context.
+ * @left_seg: Left segment of the pair.
+ * @right_seg: Right segment of the pair.
+ * @break_flags: Flags indicating whether to break neighbors.
+ *
+ * Tests crossing of @left_seg and @right_seg. If there is a crossing,
+ * inserts the intersection point into both segments.
+ *
+ * Return value: True if the intersection took place at the current
+ * scan line, indicating further iteration is needed.
+ **/
+static art_boolean
+art_svp_intersect_test_cross (ArtIntersectCtx *ctx,
+			      ArtActiveSeg *left_seg, ArtActiveSeg *right_seg,
+			      ArtBreakFlags break_flags)
+{
+  double left_x0, left_y0, left_x1;
+  double left_y1 = left_seg->y1;
+  double right_y1 = right_seg->y1;
+  double d;
+
+  const ArtSVPSeg *in_seg;
+  int in_curs;
+  double d0, d1, t;
+  double x, y; /* intersection point */
+
+#ifdef VERBOSE 
+  static int count = 0;
+
+  art_dprint ("art_svp_intersect_test_cross %lx <-> %lx: count=%d\n",
+	  (unsigned long)left_seg, (unsigned long)right_seg, count++);
+#endif
+
+  if (left_seg->y0 == right_seg->y0 && left_seg->x[0] == right_seg->x[0])
+    {
+      /* Top points of left and right segments coincide. This case
+	 feels like a bit of duplication - we may want to merge it
+	 with the cases below. However, this way, we're sure that this
+	 logic makes only localized changes. */
+
+      if (left_y1 < right_y1)
+	{
+	  /* Test left (x1, y1) against right segment */
+	  double left_x1 = left_seg->x[1];
+
+	  if (left_x1 <
+	      right_seg->x[(right_seg->flags & ART_ACTIVE_FLAGS_BNEG) ^ 1] ||
+	      left_y1 == right_seg->y0)
+	    return ART_FALSE;
+	  d = left_x1 * right_seg->a + left_y1 * right_seg->b + right_seg->c;
+	  if (d < -EPSILON_A)
+	    return ART_FALSE;
+	  else if (d < EPSILON_A)
+	    {
+	      /* I'm unsure about the break flags here. */
+	      double right_x1 = art_svp_intersect_break (ctx, right_seg,
+							 left_x1, left_y1,
+							 ART_BREAK_RIGHT);
+	      if (left_x1 <= right_x1)
+		return ART_FALSE;
+	    }
+	}
+      else if (left_y1 > right_y1)
+	{
+	  /* Test right (x1, y1) against left segment */
+	  double right_x1 = right_seg->x[1];
+
+	  if (right_x1 > left_seg->x[left_seg->flags & ART_ACTIVE_FLAGS_BNEG] ||
+	      right_y1 == left_seg->y0)
+	    return ART_FALSE;
+	  d = right_x1 * left_seg->a + right_y1 * left_seg->b + left_seg->c;
+	  if (d > EPSILON_A)
+	    return ART_FALSE;
+	  else if (d > -EPSILON_A)
+	    {
+	      /* See above regarding break flags. */
+	      double left_x1 = art_svp_intersect_break (ctx, left_seg,
+							right_x1, right_y1,
+							ART_BREAK_LEFT);
+	      if (left_x1 <= right_x1)
+		return ART_FALSE;
+	    }
+	}
+      else /* left_y1 == right_y1 */
+	{
+	  double left_x1 = left_seg->x[1];
+	  double right_x1 = right_seg->x[1];
+
+	  if (left_x1 <= right_x1)
+	    return ART_FALSE;
+	}
+      art_svp_intersect_swap_active (ctx, left_seg, right_seg);
+      return ART_TRUE;
+    }
+
+  if (left_y1 < right_y1)
+    {
+      /* Test left (x1, y1) against right segment */
+      double left_x1 = left_seg->x[1];
+
+      if (left_x1 <
+	  right_seg->x[(right_seg->flags & ART_ACTIVE_FLAGS_BNEG) ^ 1] ||
+	  left_y1 == right_seg->y0)
+	return ART_FALSE;
+      d = left_x1 * right_seg->a + left_y1 * right_seg->b + right_seg->c;
+      if (d < -EPSILON_A)
+	return ART_FALSE;
+      else if (d < EPSILON_A)
+	{
+	  double right_x1 = art_svp_intersect_break (ctx, right_seg,
+						     left_x1, left_y1,
+						     ART_BREAK_RIGHT);
+	  if (left_x1 <= right_x1)
+	    return ART_FALSE;
+	}
+    }
+  else if (left_y1 > right_y1)
+    {
+      /* Test right (x1, y1) against left segment */
+      double right_x1 = right_seg->x[1];
+
+      if (right_x1 > left_seg->x[left_seg->flags & ART_ACTIVE_FLAGS_BNEG] ||
+	  right_y1 == left_seg->y0)
+	return ART_FALSE;
+      d = right_x1 * left_seg->a + right_y1 * left_seg->b + left_seg->c;
+      if (d > EPSILON_A)
+	return ART_FALSE;
+      else if (d > -EPSILON_A)
+	{
+	  double left_x1 = art_svp_intersect_break (ctx, left_seg,
+						    right_x1, right_y1,
+						    ART_BREAK_LEFT);
+	  if (left_x1 <= right_x1)
+	    return ART_FALSE;
+	}
+    }
+  else /* left_y1 == right_y1 */
+    { 
+      double left_x1 = left_seg->x[1];
+      double right_x1 = right_seg->x[1];
+
+      if (left_x1 <= right_x1)
+	return ART_FALSE;
+    }
+
+  /* The segments cross. Find the intersection point. */
+
+  in_seg = left_seg->in_seg;
+  in_curs = left_seg->in_curs;
+  left_x0 = in_seg->points[in_curs - 1].x;
+  left_y0 = in_seg->points[in_curs - 1].y;
+  left_x1 = in_seg->points[in_curs].x;
+  left_y1 = in_seg->points[in_curs].y;
+  d0 = left_x0 * right_seg->a + left_y0 * right_seg->b + right_seg->c;
+  d1 = left_x1 * right_seg->a + left_y1 * right_seg->b + right_seg->c;
+  if (d0 == d1)
+    {
+      x = left_x0;
+      y = left_y0;
+    }
+  else
+    {
+      /* Is this division always safe? It could possibly overflow. */
+      t = d0 / (d0 - d1);
+      if (t <= 0)
+	{
+	  x = left_x0;
+	  y = left_y0;
+	}
+      else if (t >= 1)
+	{
+	  x = left_x1;
+	  y = left_y1;
+	}
+      else
+	{
+	  x = left_x0 + t * (left_x1 - left_x0);
+	  y = left_y0 + t * (left_y1 - left_y0);
+	}
+    }
+
+  /* Make sure intersection point is within bounds of right seg. */
+  if (y < right_seg->y0)
+    {
+      x = right_seg->x[0];
+      y = right_seg->y0;
+    }
+  else if (y > right_seg->y1)
+    {
+      x = right_seg->x[1];
+      y = right_seg->y1;
+    }
+  else if (x < right_seg->x[(right_seg->flags & ART_ACTIVE_FLAGS_BNEG) ^ 1])
+    x = right_seg->x[(right_seg->flags & ART_ACTIVE_FLAGS_BNEG) ^ 1];
+  else if (x > right_seg->x[right_seg->flags & ART_ACTIVE_FLAGS_BNEG])
+    x = right_seg->x[right_seg->flags & ART_ACTIVE_FLAGS_BNEG];
+
+  if (y == left_seg->y0)
+    {
+      if (y != right_seg->y0)
+	{
+#ifdef VERBOSE
+	  art_dprint ("art_svp_intersect_test_cross: intersection (%g, %g) matches former y0 of %lx, %lx\n",
+		    x, y, (unsigned long)left_seg, (unsigned long)right_seg);
+#endif
+	  art_svp_intersect_push_pt (ctx, right_seg, x, y);
+	  if ((break_flags & ART_BREAK_RIGHT) && right_seg->right != NULL)
+	    art_svp_intersect_add_point (ctx, x, y, right_seg->right,
+					 break_flags);
+	}
+      else
+	{
+	  /* Intersection takes place at current scan line; process
+	     immediately rather than queueing intersection point into
+	     priq. */
+	  ArtActiveSeg *winner, *loser;
+
+	  /* Choose "most vertical" segement */
+	  if (left_seg->a > right_seg->a)
+	    {
+	      winner = left_seg;
+	      loser = right_seg;
+	    }
+	  else
+	    {
+	      winner = right_seg;
+	      loser = left_seg;
+	    }
+
+	  loser->x[0] = winner->x[0];
+	  loser->horiz_x = loser->x[0];
+	  loser->horiz_delta_wind += loser->delta_wind;
+	  winner->horiz_delta_wind -= loser->delta_wind;
+
+	  art_svp_intersect_swap_active (ctx, left_seg, right_seg);
+	  return ART_TRUE;
+	}
+    }
+  else if (y == right_seg->y0)
+    {
+#ifdef VERBOSE
+      art_dprint ("*** art_svp_intersect_test_cross: intersection (%g, %g) matches latter y0 of %lx, %lx\n",
+	      x, y, (unsigned long)left_seg, (unsigned long)right_seg);
+#endif
+      art_svp_intersect_push_pt (ctx, left_seg, x, y);
+      if ((break_flags & ART_BREAK_LEFT) && left_seg->left != NULL)
+	art_svp_intersect_add_point (ctx, x, y, left_seg->left,
+				     break_flags);
+    }
+  else
+    {
+#ifdef VERBOSE
+      art_dprint ("Inserting (%g, %g) into %lx, %lx\n",
+	      x, y, (unsigned long)left_seg, (unsigned long)right_seg);
+#endif
+      /* Insert the intersection point into both segments. */
+      art_svp_intersect_push_pt (ctx, left_seg, x, y);
+      art_svp_intersect_push_pt (ctx, right_seg, x, y);
+      if ((break_flags & ART_BREAK_LEFT) && left_seg->left != NULL)
+	art_svp_intersect_add_point (ctx, x, y, left_seg->left, break_flags);
+      if ((break_flags & ART_BREAK_RIGHT) && right_seg->right != NULL)
+	art_svp_intersect_add_point (ctx, x, y, right_seg->right, break_flags);
+    }
+  return ART_FALSE;
+}
+
+/**
+ * art_svp_intersect_active_delete: Delete segment from active list.
+ * @ctx: Intersection context.
+ * @seg: Segment to delete.
+ *
+ * Deletes @seg from the active list.
+ **/
+static /* todo inline */ void
+art_svp_intersect_active_delete (ArtIntersectCtx *ctx, ArtActiveSeg *seg)
+{
+  ArtActiveSeg *left = seg->left, *right = seg->right;
+
+  if (left != NULL)
+    left->right = right;
+  else
+    ctx->active_head = right;
+  if (right != NULL)
+    right->left = left;
+}
+
+/**
+ * art_svp_intersect_active_free: Free an active segment.
+ * @seg: Segment to delete.
+ *
+ * Frees @seg.
+ **/
+static /* todo inline */ void
+art_svp_intersect_active_free (ArtActiveSeg *seg)
+{
+  art_free (seg->stack);
+#ifdef VERBOSE
+  art_dprint ("Freeing %lx\n", (unsigned long) seg);
+#endif
+  art_free (seg);
+}
+
+/**
+ * art_svp_intersect_insert_cross: Test crossings of newly inserted line.
+ *
+ * Tests @seg against its left and right neighbors for intersections.
+ * Precondition: the line in @seg is not purely horizontal.
+ **/
+static void
+art_svp_intersect_insert_cross (ArtIntersectCtx *ctx,
+				ArtActiveSeg *seg)
+{
+  ArtActiveSeg *left = seg, *right = seg;
+
+  for (;;)
+    {
+      if (left != NULL)
+	{
+	  ArtActiveSeg *leftc;
+
+	  for (leftc = left->left; leftc != NULL; leftc = leftc->left)
+	    if (!(leftc->flags & ART_ACTIVE_FLAGS_DEL))
+	      break;
+	  if (leftc != NULL &&
+	      art_svp_intersect_test_cross (ctx, leftc, left,
+					    ART_BREAK_LEFT))
+	    {
+	      if (left == right || right == NULL)
+		right = left->right;
+	    }
+	  else
+	    {
+	      left = NULL;
+	    }
+	}
+      else if (right != NULL && right->right != NULL)
+	{
+	  ArtActiveSeg *rightc;
+
+	  for (rightc = right->right; rightc != NULL; rightc = rightc->right)
+	    if (!(rightc->flags & ART_ACTIVE_FLAGS_DEL))
+	      break;
+	  if (rightc != NULL &&
+	      art_svp_intersect_test_cross (ctx, right, rightc,
+					    ART_BREAK_RIGHT))
+	    {
+	      if (left == right || left == NULL)
+		left = right->left;
+	    }
+	  else
+	    {
+	      right = NULL;
+	    }
+	}
+      else
+	break;
+    }
+}
+
+/**
+ * art_svp_intersect_horiz: Add horizontal line segment.
+ * @ctx: Intersector context.
+ * @seg: Segment on which to add horizontal line.
+ * @x0: Old x position.
+ * @x1: New x position.
+ *
+ * Adds a horizontal line from @x0 to @x1, and updates the current
+ * location of @seg to @x1.
+ **/
+static void
+art_svp_intersect_horiz (ArtIntersectCtx *ctx, ArtActiveSeg *seg,
+			 double x0, double x1)
+{
+  ArtActiveSeg *hs;
+
+  if (x0 == x1)
+    return;
+
+  hs = art_new (ArtActiveSeg, 1);
+
+  hs->flags = ART_ACTIVE_FLAGS_DEL | (seg->flags & ART_ACTIVE_FLAGS_OUT);
+  if (seg->flags & ART_ACTIVE_FLAGS_OUT)
+    {
+      ArtSvpWriter *swr = ctx->out;
+
+      swr->add_point (swr, seg->seg_id, x0, ctx->y);
+    }
+  hs->seg_id = seg->seg_id;
+  hs->horiz_x = x0;
+  hs->horiz_delta_wind = seg->delta_wind;
+  hs->stack = NULL;
+
+  /* Ideally, the (a, b, c) values will never be read. However, there
+     are probably some tests remaining that don't check for _DEL
+     before evaluating the line equation. For those, these
+     initializations will at least prevent a UMR of the values, which
+     can crash on some platforms. */
+  hs->a = 0.0;
+  hs->b = 0.0;
+  hs->c = 0.0;
+
+  seg->horiz_delta_wind -= seg->delta_wind;
+
+  art_svp_intersect_add_horiz (ctx, hs);
+
+  if (x0 > x1)
+    {
+      ArtActiveSeg *left;
+      art_boolean first = ART_TRUE;
+
+      for (left = seg->left; left != NULL; left = seg->left)
+	{
+	  int left_bneg = left->flags & ART_ACTIVE_FLAGS_BNEG;
+
+	  if (left->x[left_bneg] <= x1)
+	    break;
+	  if (left->x[left_bneg ^ 1] <= x1 &&
+	      x1 * left->a + ctx->y * left->b + left->c >= 0)
+	    break;
+	  if (left->y0 != ctx->y && left->y1 != ctx->y)
+	    {
+	      art_svp_intersect_break (ctx, left, x1, ctx->y, ART_BREAK_LEFT);
+	    }
+#ifdef VERBOSE
+	  art_dprint ("x0=%g > x1=%g, swapping %lx, %lx\n",
+		  x0, x1, (unsigned long)left, (unsigned long)seg);
+#endif
+	  art_svp_intersect_swap_active (ctx, left, seg);
+	  if (first && left->right != NULL)
+	    {
+	      art_svp_intersect_test_cross (ctx, left, left->right,
+					    ART_BREAK_RIGHT);
+	      first = ART_FALSE;
+	    }
+	}
+    }
+  else
+    {
+      ArtActiveSeg *right;
+      art_boolean first = ART_TRUE;
+
+      for (right = seg->right; right != NULL; right = seg->right)
+	{
+	  int right_bneg = right->flags & ART_ACTIVE_FLAGS_BNEG;
+
+	  if (right->x[right_bneg ^ 1] >= x1)
+	    break;
+	  if (right->x[right_bneg] >= x1 &&
+	      x1 * right->a + ctx->y * right->b + right->c <= 0)
+	    break;
+	  if (right->y0 != ctx->y && right->y1 != ctx->y)
+	    {
+	      art_svp_intersect_break (ctx, right, x1, ctx->y,
+				       ART_BREAK_LEFT);
+	    }
+#ifdef VERBOSE
+	  art_dprint ("[right]x0=%g < x1=%g, swapping %lx, %lx\n",
+		  x0, x1, (unsigned long)seg, (unsigned long)right);
+#endif
+	  art_svp_intersect_swap_active (ctx, seg, right);
+	  if (first && right->left != NULL)
+	    {
+	      art_svp_intersect_test_cross (ctx, right->left, right,
+					    ART_BREAK_RIGHT);
+	      first = ART_FALSE;
+	    }
+	}
+    }
+
+  seg->x[0] = x1;
+  seg->x[1] = x1;
+  seg->horiz_x = x1;
+  seg->flags &= ~ART_ACTIVE_FLAGS_OUT;
+}
+
+/**
+ * art_svp_intersect_insert_line: Insert a line into the active list.
+ * @ctx: Intersector context.
+ * @seg: Segment containing line to insert.
+ *
+ * Inserts the line into the intersector context, taking care of any
+ * intersections, and adding the appropriate horizontal points to the
+ * active list.
+ **/
+static void
+art_svp_intersect_insert_line (ArtIntersectCtx *ctx, ArtActiveSeg *seg)
+{
+  if (seg->y1 == seg->y0)
+    {
+#ifdef VERBOSE
+      art_dprint ("art_svp_intersect_insert_line: %lx is horizontal\n",
+	      (unsigned long)seg);
+#endif
+      art_svp_intersect_horiz (ctx, seg, seg->x[0], seg->x[1]);
+    }
+  else
+    {
+      art_svp_intersect_insert_cross (ctx, seg);
+      art_svp_intersect_add_horiz (ctx, seg);
+    }
+}
+
+static void
+art_svp_intersect_process_intersection (ArtIntersectCtx *ctx,
+					ArtActiveSeg *seg)
+{
+  int n_stack = --seg->n_stack;
+  seg->x[1] = seg->stack[n_stack - 1].x;
+  seg->y1 = seg->stack[n_stack - 1].y;
+  seg->x[0] = seg->stack[n_stack].x;
+  seg->y0 = seg->stack[n_stack].y;
+  seg->horiz_x = seg->x[0];
+  art_svp_intersect_insert_line (ctx, seg);
+}
+
+static void
+art_svp_intersect_advance_cursor (ArtIntersectCtx *ctx, ArtActiveSeg *seg,
+				  ArtPriPoint *pri_pt)
+{
+  const ArtSVPSeg *in_seg = seg->in_seg;
+  int in_curs = seg->in_curs;
+  ArtSvpWriter *swr = seg->flags & ART_ACTIVE_FLAGS_OUT ? ctx->out : NULL;
+
+  if (swr != NULL)
+    swr->add_point (swr, seg->seg_id, seg->x[1], seg->y1);
+  if (in_curs + 1 == in_seg->n_points)
+    {
+      ArtActiveSeg *left = seg->left, *right = seg->right;
+
+#if 0
+      if (swr != NULL)
+	swr->close_segment (swr, seg->seg_id);
+      seg->flags &= ~ART_ACTIVE_FLAGS_OUT;
+#endif
+      seg->flags |= ART_ACTIVE_FLAGS_DEL;
+      art_svp_intersect_add_horiz (ctx, seg);
+      art_svp_intersect_active_delete (ctx, seg);
+      if (left != NULL && right != NULL)
+	art_svp_intersect_test_cross (ctx, left, right,
+				      ART_BREAK_LEFT | ART_BREAK_RIGHT);
+      art_free (pri_pt);
+    }
+  else
+    {
+      seg->horiz_x = seg->x[1];
+
+      art_svp_intersect_setup_seg (seg, pri_pt);
+      art_pri_insert (ctx->pq, pri_pt);
+      art_svp_intersect_insert_line (ctx, seg);
+    }
+}
+
+static void
+art_svp_intersect_add_seg (ArtIntersectCtx *ctx, const ArtSVPSeg *in_seg)
+{
+  ArtActiveSeg *seg = art_new (ArtActiveSeg, 1);
+  ArtActiveSeg *test;
+  double x0, y0;
+  ArtActiveSeg *beg_range;
+  ArtActiveSeg *last = NULL;
+  ArtActiveSeg *left, *right;
+  ArtPriPoint *pri_pt = art_new (ArtPriPoint, 1);
+
+  seg->flags = 0;
+  seg->in_seg = in_seg;
+  seg->in_curs = 0;
+
+  seg->n_stack_max = 4;
+  seg->stack = art_new (ArtPoint, seg->n_stack_max);
+
+  seg->horiz_delta_wind = 0;
+  
+  seg->wind_left = 0;
+
+  pri_pt->user_data = seg;
+  art_svp_intersect_setup_seg (seg, pri_pt);
+  art_pri_insert (ctx->pq, pri_pt);
+
+  /* Find insertion place for new segment */
+  /* This is currently a left-to-right scan, but should be replaced
+     with a binary search as soon as it's validated. */
+
+  x0 = in_seg->points[0].x;
+  y0 = in_seg->points[0].y;
+  beg_range = NULL;
+  for (test = ctx->active_head; test != NULL; test = test->right)
+    {
+      double d;
+      int test_bneg = test->flags & ART_ACTIVE_FLAGS_BNEG;
+
+      if (x0 < test->x[test_bneg])
+	{
+	  if (x0 < test->x[test_bneg ^ 1])
+	    break;
+	  d = x0 * test->a + y0 * test->b + test->c;
+	  if (d < 0)
+	    break;
+	}
+      last = test;
+    }
+
+  left = art_svp_intersect_add_point (ctx, x0, y0, last, ART_BREAK_LEFT | ART_BREAK_RIGHT);
+  seg->left = left;
+  if (left == NULL)
+    {
+      right = ctx->active_head;
+      ctx->active_head = seg;
+    }
+  else
+    {
+      right = left->right;
+      left->right = seg;
+    }
+  seg->right = right;
+  if (right != NULL)
+    right->left = seg;
+
+  seg->delta_wind = in_seg->dir ? 1 : -1;
+  seg->horiz_x = x0;
+
+  art_svp_intersect_insert_line (ctx, seg);
+}
+
+#ifdef SANITYCHECK
+static void
+art_svp_intersect_sanitycheck_winding (ArtIntersectCtx *ctx)
+{
+#if 0
+  /* At this point, we seem to be getting false positives, so it's
+     turned off for now. */
+
+  ArtActiveSeg *seg;
+  int winding_number = 0;
+
+  for (seg = ctx->active_head; seg != NULL; seg = seg->right)
+    {
+      /* Check winding number consistency. */
+      if (seg->flags & ART_ACTIVE_FLAGS_OUT)
+	{
+	  if (winding_number != seg->wind_left)
+	    art_warn ("*** art_svp_intersect_sanitycheck_winding: seg %lx has wind_left of %d, expected %d\n",
+		  (unsigned long) seg, seg->wind_left, winding_number);
+	  winding_number = seg->wind_left + seg->delta_wind;
+	}
+    }
+  if (winding_number != 0)
+    art_warn ("*** art_svp_intersect_sanitycheck_winding: non-balanced winding number %d\n",
+	      winding_number);
+#endif
+}
+#endif
+
+/**
+ * art_svp_intersect_horiz_commit: Commit points in horiz list to output.
+ * @ctx: Intersection context.
+ *
+ * The main function of the horizontal commit is to output new
+ * points to the output writer.
+ *
+ * This "commit" pass is also where winding numbers are assigned,
+ * because doing it here provides much greater tolerance for inputs
+ * which are not in strict SVP order.
+ *
+ * Each cluster in the horiz_list contains both segments that are in
+ * the active list (ART_ACTIVE_FLAGS_DEL is false) and that are not,
+ * and are scheduled to be deleted (ART_ACTIVE_FLAGS_DEL is true). We
+ * need to deal with both.
+ **/
+static void
+art_svp_intersect_horiz_commit (ArtIntersectCtx *ctx)
+{
+  ArtActiveSeg *seg;
+  int winding_number = 0; /* initialization just to avoid warning */
+  int horiz_wind = 0;
+  double last_x = 0; /* initialization just to avoid warning */
+
+#ifdef VERBOSE
+  art_dprint ("art_svp_intersect_horiz_commit: y=%g\n", ctx->y);
+  for (seg = ctx->horiz_first; seg != NULL; seg = seg->horiz_right)
+    art_dprint (" %lx: %g %+d\n",
+	    (unsigned long)seg, seg->horiz_x, seg->horiz_delta_wind);
+#endif
+
+  /* Output points to svp writer. */
+  for (seg = ctx->horiz_first; seg != NULL;)
+    {
+      /* Find a cluster with common horiz_x, */
+      ArtActiveSeg *curs;
+      double x = seg->horiz_x;
+
+      /* Generate any horizontal segments. */
+      if (horiz_wind != 0)
+	{
+	  ArtSvpWriter *swr = ctx->out;
+	  int seg_id;
+
+	  seg_id = swr->add_segment (swr, winding_number, horiz_wind,
+				     last_x, ctx->y);
+	  swr->add_point (swr, seg_id, x, ctx->y);
+	  swr->close_segment (swr, seg_id);
+	}
+
+      /* Find first active segment in cluster. */
+
+      for (curs = seg; curs != NULL && curs->horiz_x == x;
+	   curs = curs->horiz_right)
+	if (!(curs->flags & ART_ACTIVE_FLAGS_DEL))
+	  break;
+
+      if (curs != NULL && curs->horiz_x == x)
+	{
+	  /* There exists at least one active segment in this cluster. */
+
+	  /* Find beginning of cluster. */
+	  for (; curs->left != NULL; curs = curs->left)
+	    if (curs->left->horiz_x != x)
+	      break;
+
+	  if (curs->left != NULL)
+	    winding_number = curs->left->wind_left + curs->left->delta_wind;
+	  else
+	    winding_number = 0;
+
+	  do
+	    {
+#ifdef VERBOSE
+	      art_dprint (" curs %lx: winding_number = %d += %d\n",
+		      (unsigned long)curs, winding_number, curs->delta_wind);
+#endif
+	      if (!(curs->flags & ART_ACTIVE_FLAGS_OUT) ||
+		  curs->wind_left != winding_number)
+		{
+		  ArtSvpWriter *swr = ctx->out;
+
+		  if (curs->flags & ART_ACTIVE_FLAGS_OUT)
+		    {
+		      swr->add_point (swr, curs->seg_id,
+				      curs->horiz_x, ctx->y);
+		      swr->close_segment (swr, curs->seg_id);
+		    }
+
+		  curs->seg_id = swr->add_segment (swr, winding_number,
+						   curs->delta_wind,
+						   x, ctx->y);
+		  curs->flags |= ART_ACTIVE_FLAGS_OUT;
+		}
+	      curs->wind_left = winding_number;
+	      winding_number += curs->delta_wind;
+	      curs = curs->right;
+	    }
+	  while (curs != NULL && curs->horiz_x == x);
+	}
+
+      /* Skip past cluster. */
+      do
+	{
+	  ArtActiveSeg *next = seg->horiz_right;
+
+	  seg->flags &= ~ART_ACTIVE_FLAGS_IN_HORIZ;
+	  horiz_wind += seg->horiz_delta_wind;
+	  seg->horiz_delta_wind = 0;
+	  if (seg->flags & ART_ACTIVE_FLAGS_DEL)
+	    {
+	      if (seg->flags & ART_ACTIVE_FLAGS_OUT)
+		{
+		  ArtSvpWriter *swr = ctx->out;
+		  swr->close_segment (swr, seg->seg_id);
+		}
+	      art_svp_intersect_active_free (seg);
+	    }
+	  seg = next;
+	}
+      while (seg != NULL && seg->horiz_x == x);
+
+      last_x = x;
+    }
+  ctx->horiz_first = NULL;
+  ctx->horiz_last = NULL;
+#ifdef SANITYCHECK
+  art_svp_intersect_sanitycheck_winding (ctx);
+#endif
+}
+
+#ifdef VERBOSE
+static void
+art_svp_intersect_print_active (ArtIntersectCtx *ctx)
+{
+  ArtActiveSeg *seg;
+
+  art_dprint ("Active list (y = %g):\n", ctx->y);
+  for (seg = ctx->active_head; seg != NULL; seg = seg->right)
+    {
+      art_dprint (" %lx: (%g, %g)-(%g, %g), (a, b, c) = (%g, %g, %g)\n",
+	      (unsigned long)seg,
+	      seg->x[0], seg->y0, seg->x[1], seg->y1,
+	      seg->a, seg->b, seg->c);
+    }
+}
+#endif
+
+#ifdef SANITYCHECK
+static void
+art_svp_intersect_sanitycheck (ArtIntersectCtx *ctx)
+{
+  ArtActiveSeg *seg;
+  ArtActiveSeg *last = NULL;
+  double d;
+
+  for (seg = ctx->active_head; seg != NULL; seg = seg->right)
+    {
+      if (seg->left != last)
+	{
+	  art_warn ("*** art_svp_intersect_sanitycheck: last=%lx, seg->left=%lx\n",
+		    (unsigned long)last, (unsigned long)seg->left);
+	}
+      if (last != NULL)
+	{
+	  /* pairwise compare with previous seg */
+
+	  /* First the top. */
+	  if (last->y0 < seg->y0)
+	    {
+	    }
+	  else
+	    {
+	    }
+
+	  /* Then the bottom. */
+	  if (last->y1 < seg->y1)
+	    {
+	      if (!((last->x[1] <
+		     seg->x[(seg->flags & ART_ACTIVE_FLAGS_BNEG) ^ 1]) ||
+		    last->y1 == seg->y0))
+		{
+		  d = last->x[1] * seg->a + last->y1 * seg->b + seg->c;
+		  if (d >= -EPSILON_A)
+		    art_warn ("*** bottom (%g, %g) of %lx is not clear of %lx to right (d = %g)\n",
+			      last->x[1], last->y1, (unsigned long) last,
+			      (unsigned long) seg, d);
+		}
+	    }
+	  else if (last->y1 > seg->y1)
+
+	    {
+	      if (!((seg->x[1] >
+		     last->x[last->flags & ART_ACTIVE_FLAGS_BNEG]) ||
+		    seg->y1 == last->y0))
+	      {
+		d = seg->x[1] * last->a + seg->y1 * last->b + last->c;
+		if (d <= EPSILON_A)
+		  art_warn ("*** bottom (%g, %g) of %lx is not clear of %lx to left (d = %g)\n",
+			      seg->x[1], seg->y1, (unsigned long) seg,
+			      (unsigned long) last, d);
+	      }
+	    }
+	  else
+	    {
+	      if (last->x[1] > seg->x[1])
+		art_warn ("*** bottoms (%g, %g) of %lx and (%g, %g) of %lx out of order\n",
+			  last->x[1], last->y1, (unsigned long)last,
+			  seg->x[1], seg->y1, (unsigned long)seg);
+	    }
+	}
+      last = seg;
+    }
+}
+#endif
+
+void
+art_svp_intersector (const ArtSVP *in, ArtSvpWriter *out)
+{
+  ArtIntersectCtx *ctx;
+  ArtPriQ *pq;
+  ArtPriPoint *first_point;
+#ifdef VERBOSE
+  int count = 0;
+#endif
+
+  if (in->n_segs == 0)
+    return;
+
+  ctx = art_new (ArtIntersectCtx, 1);
+  ctx->in = in;
+  ctx->out = out;
+  pq = art_pri_new ();
+  ctx->pq = pq;
+
+  ctx->active_head = NULL;
+
+  ctx->horiz_first = NULL;
+  ctx->horiz_last = NULL;
+
+  ctx->in_curs = 0;
+  first_point = art_new (ArtPriPoint, 1);
+  first_point->x = in->segs[0].points[0].x;
+  first_point->y = in->segs[0].points[0].y;
+  first_point->user_data = NULL;
+  ctx->y = first_point->y;
+  art_pri_insert (pq, first_point);
+
+  while (!art_pri_empty (pq))
+    {
+      ArtPriPoint *pri_point = art_pri_choose (pq);
+      ArtActiveSeg *seg = (ArtActiveSeg *)pri_point->user_data;
+
+#ifdef VERBOSE
+      art_dprint ("\nIntersector step %d\n", count++);
+      art_svp_intersect_print_active (ctx);
+      art_dprint ("priq choose (%g, %g) %lx\n", pri_point->x, pri_point->y,
+	      (unsigned long)pri_point->user_data);
+#endif
+#ifdef SANITYCHECK
+      art_svp_intersect_sanitycheck(ctx);
+#endif
+
+      if (ctx->y != pri_point->y)
+	{
+	  art_svp_intersect_horiz_commit (ctx);
+	  ctx->y = pri_point->y;
+	}
+
+      if (seg == NULL)
+	{
+	  /* Insert new segment from input */
+	  const ArtSVPSeg *in_seg = &in->segs[ctx->in_curs++];
+	  art_svp_intersect_add_seg (ctx, in_seg);
+	  if (ctx->in_curs < in->n_segs)
+	    {
+	      const ArtSVPSeg *next_seg = &in->segs[ctx->in_curs];
+	      pri_point->x = next_seg->points[0].x;
+	      pri_point->y = next_seg->points[0].y;
+	      /* user_data is already NULL */
+	      art_pri_insert (pq, pri_point);
+	    }
+	  else
+	    art_free (pri_point);
+	}
+      else
+	{
+	  int n_stack = seg->n_stack;
+
+	  if (n_stack > 1)
+	    {
+	      art_svp_intersect_process_intersection (ctx, seg);
+	      art_free (pri_point);
+	    }
+	  else
+	    {
+	      art_svp_intersect_advance_cursor (ctx, seg, pri_point);
+	    }
+	}
+    }
+
+  art_svp_intersect_horiz_commit (ctx);
+
+  art_pri_free (pq);
+  art_free (ctx);
+}
+
+#endif /* not TEST_PRIQ */