intersections

docs/chapters/intersections/line-line.js

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+let Point = Vector, points = [];
+
+setup() {
+    points.push(new Point(50,50));
+    points.push(new Point(150,110));
+    points.push(new Point(50,250));
+    points.push(new Point(170,170));
+    setMovable(points);
+}
+
+draw() {
+  clear();
+  const [p1, p2, p3, p4] = points;
+
+  setStroke(`black`);
+  this.drawLine(p1,p2);
+  this.drawLine(p3,p4);
+
+  const p = this.lli(
+    p1.x, p1.y,
+    p2.x, p2.y,
+    p3.x, p3.y,
+    p4.x, p4.y
+  );
+
+  if (p) {
+    if (this.onLine(p, p1, p2) && this.onLine(p, p3, p4)) {
+      setColor(`lime`);
+      circle(p.x, p.y, 3);
+      setFill(`black`);
+      text(`Intersection at (${p.x|0}, ${p.y|0})`, this.width/2, 15, CENTER);
+    } else {
+      setColor(`red`);
+      circle(p.x, p.y, 3);
+    }
+  }
+}
+
+drawLine(p1, p2) {
+  setColor(`black`);
+  line(p1.x, p1.y, p2.x, p2.y);
+  setStroke( randomColor() );
+  circle(p1.x, p1.y, 2);
+  circle(p2.x, p2.y, 2);
+}
+
+lli(x1, y1, x2, y2, x3, y3, x4, y4) {
+  const nx = (x1 * y2 - y1 * x2) * (x3 - x4) - (x1 - x2) * (x3 * y4 - y3 * x4),
+        ny = (x1 * y2 - y1 * x2) * (y3 - y4) - (y1 - y2) * (x3 * y4 - y3 * x4),
+        d = (x1 - x2) * (y3 - y4) - (y1 - y2) * (x3 - x4);
+  if (d == 0) return;
+  return { x: nx / d, y: ny / d };
+}
+
+onLine(p, p1, p2) {
+  const mx = min(p1.x, p2.x),
+        my = min(p1.y, p2.y),
+        Mx = max(p1.x, p2.x),
+        My = max(p1.y, p2.y);
+  return (mx <= p.x && my <= p.y && Mx >= p.x && My >= p.y);
+}