More progress on intersecting

polygon.cpp

@@ -12,64 +12,56 @@ struct point {
 
 typedef std::pair<point, point> segment;
 
-// https://stackoverflow.com/questions/9043805/test-if-two-lines-intersect-javascript-function/16725715#16725715
-// this does not seem to be correct
-bool intersects(double x1, double y1, double x2, double y2, double x3, double y3, double x4, double y4) {
-	double det = (x2 - x1) * (y4 - y3) - (x4 - x3) * (y2 - y1);
-	if (det == 0) {
-		return false;
-	} else {
-		double lambda = ((y4 - y3) * (x4 - x1) + (x3 - x4) * (y4 - y1)) / det;
-		double gamma = ((y1 - y2) * (x4 - x1) + (x2 - x1) * (y4 - y1)) / det;
-
-		if (lambda > 0 && lambda < 1 && gamma > 0 && gamma < 1) {
-			printf("%f,%f to %f,%f and %f,%f to %f,%f: %f and %f\n",
-			       x1, y1, x2, y2, x3, y3, x4, y4, lambda, gamma);
-			printf("%f,%f or %f,%f\n",
-			       x1 + (x2 - x1) * lambda, y1 + (y2 - y1) * lambda,
-			       x3 + (x4 - x3) * gamma, y3 + (y4 - y3) * gamma);
-		}
-		return (0 < lambda && lambda < 1) && (0 < gamma && gamma < 1);
-	}
-}
-
 // https://stackoverflow.com/questions/563198/how-do-you-detect-where-two-line-segments-intersect
-// this seems to produce the same point along both segments
-bool get_line_intersection(double p0_x, double p0_y, double p1_x, double p1_y,
-			   double p2_x, double p2_y, double p3_x, double p3_y) {
-	double s1_x, s1_y, s2_x, s2_y;
-	s1_x = p1_x - p0_x;
-	s1_y = p1_y - p0_y;
-	s2_x = p3_x - p2_x;
-	s2_y = p3_y - p2_y;
+//
+// beware of
+// https://stackoverflow.com/questions/9043805/test-if-two-lines-intersect-javascript-function/16725715#16725715
+// which does not seem to produce correct results.
+std::pair<double, double> get_line_intersection(double p0_x, double p0_y, double p1_x, double p1_y,
+						double p2_x, double p2_y, double p3_x, double p3_y) {
+	double d01_x, d01_y, d23_x, d23_y;
+	d01_x = p1_x - p0_x;
+	d01_y = p1_y - p0_y;
+	d23_x = p3_x - p2_x;
+	d23_y = p3_y - p2_y;
 
-	float det = (-s2_x * s1_y + s1_x * s2_y);
+	float det = (-d23_x * d01_y + d01_x * d23_y);
 
 	if (det != 0) {
-		double s, t;
-		s = (-s1_y * (p0_x - p2_x) + s1_x * (p0_y - p2_y)) / det;
-		t = (s2_x * (p0_y - p2_y) - s2_y * (p0_x - p2_x)) / det;
+		double t, s;
+		t = (d23_x * (p0_y - p2_y) - d23_y * (p0_x - p2_x)) / det;
+		s = (-d01_y * (p0_x - p2_x) + d01_x * (p0_y - p2_y)) / det;
 
 		if (s >= 0 && s <= 1 && t >= 0 && t <= 1) {
+			return std::make_pair(t, s);
+
+#if 0
 			printf("%f,%f to %f,%f and %f,%f to %f,%f: %f and %f\n",
 			       p0_x, p0_y, p1_x, p1_y, p2_x, p2_y, p3_x, p3_y, t, s);
 			printf("%f,%f or %f,%f\n",
-			       p0_x + t * s1_x, p0_y + t * s1_y,
-			       p2_x + s * s2_x, p2_y + s * s2_y);
-
-			return 1;
+			       p0_x + t * d01_x, p0_y + t * d01_y,
+			       p2_x + s * d23_x, p2_y + s * d23_y);
+#endif
 		}
 	}
 
-	return 0;  // No collision
+	return std::make_pair(-1, -1);
 }
 
 bool intersect(std::vector<segment> &segs, size_t s1, size_t s2) {
-	get_line_intersection(std::round(segs[s1].first.x), std::round(segs[s1].first.y),
-			      std::round(segs[s1].second.x), std::round(segs[s1].second.y),
-			      std::round(segs[s2].first.x), std::round(segs[s2].first.y),
-			      std::round(segs[s2].second.x), std::round(segs[s2].second.y));
-	return false;
+	auto intersections = get_line_intersection(std::round(segs[s1].first.x), std::round(segs[s1].first.y),
+						   std::round(segs[s1].second.x), std::round(segs[s1].second.y),
+						   std::round(segs[s2].first.x), std::round(segs[s2].first.y),
+						   std::round(segs[s2].second.x), std::round(segs[s2].second.y));
+
+	bool changed = false;
+	if (intersections.first >= 0) {
+		// XXX introduce a new node
+	} else {
+		// XXX handle collinear
+	}
+
+	return changed;
 }
 
 struct scan_transition {
@@ -120,8 +112,8 @@ std::vector<segment> snap_round(std::vector<segment> segs) {
 
 		// do the scan
 
-		std::set<std::pair<size_t, size_t>> already;
 		std::set<size_t> active;
+		std::set<std::pair<size_t, size_t>> already;
 		size_t bottom = 0;
 
 		for (size_t i = 0; i < tops.size(); i++) {
@@ -145,6 +137,7 @@ std::vector<segment> snap_round(std::vector<segment> segs) {
 								// may have caused new intersections
 								again = true;
 							}
+
 							already.insert(std::make_pair(s1, s2));
 						}
 					}
@@ -161,6 +154,18 @@ std::vector<segment> snap_round(std::vector<segment> segs) {
 				}
 			}
 		}
+
+		if (again) {
+			// let the intersections settle down before we start trying
+			// to make additional changes
+			continue;
+		}
+
+		// find identical opposite-winding segments and adjust for them
+		//
+		// this is in the same loop because we may introduce new self-intersections
+		// in the course of trying to keep spindles alive, and will then need to
+		// resolve those.
 	}
 
 	return segs;
@@ -199,8 +204,6 @@ drawvec clean_polygon(drawvec const &geom, int z, int detail) {
 
 	segments = snap_round(segments);
 
-	// remove duplicate segments with opposite windings
-
 	// reassemble segments into rings
 
 	// remove collinear points?