More intersection detection functions

packages/math/ellipse.test.ts

@@ -3,7 +3,9 @@ import {
   ellipseSegmentInterceptPoints,
   ellipseIncludesPoint,
   ellipseTouchesPoint,
+  ellipseIntersectsLine,
 } from "./ellipse";
+import { line } from "./line";
 import { point } from "./point";
 import { segment } from "./segment";
 import type { Ellipse, GlobalPoint } from "./types";
@@ -47,7 +49,7 @@ describe("point and ellipse", () => {
   });
 });
 
-describe("line and ellipse", () => {
+describe("segment and ellipse", () => {
   it("detects outside segment", () => {
     const e = ellipse(point(0, 0), 2, 2);
 
@@ -77,3 +79,32 @@ describe("line and ellipse", () => {
     ).toEqual([]);
   });
 });
+
+describe("line and ellipse", () => {
+  const e = ellipse(point(0, 0), 2, 2);
+
+  it("detects outside line", () => {
+    expect(
+      ellipseIntersectsLine(
+        e,
+        line<GlobalPoint>(point(-10, -10), point(10, -10)),
+      ),
+    ).toEqual([]);
+  });
+  it("detects line intersecting ellipse", () => {
+    expect(
+      ellipseIntersectsLine(e, line<GlobalPoint>(point(0, -1), point(0, 1))),
+    ).toEqual([point(0, 2), point(0, -2)]);
+    expect(
+      ellipseIntersectsLine(
+        e,
+        line<GlobalPoint>(point(-100, 0), point(-10, 0)),
+      ).map(([x, y]) => point(Math.round(x), Math.round(y))),
+    ).toEqual([point(2, 0), point(-2, 0)]);
+  });
+  it("detects line touching ellipse", () => {
+    expect(
+      ellipseIntersectsLine(e, line<GlobalPoint>(point(-2, -2), point(2, -2))),
+    ).toEqual([point(0, -2)]);
+  });
+});

packages/math/ellipse.ts

@@ -1,5 +1,5 @@
-import { point, pointDistance, pointFromVector } from "./point";
-import type { Ellipse, GenericPoint, Segment } from "./types";
+import { point, pointDistance, pointFromVector, pointsEqual } from "./point";
+import type { Ellipse, GenericPoint, Line, Segment } from "./types";
 import { PRECISION } from "./utils";
 import {
   vector,
@@ -180,3 +180,37 @@ export function ellipseSegmentInterceptPoints<Point extends GenericPoint>(
 
   return intersections;
 }
+
+export function ellipseIntersectsLine<Point extends GenericPoint>(
+  { center, halfWidth, halfHeight }: Ellipse<Point>,
+  [g, h]: Line<Point>,
+): Point[] {
+  const [cx, cy] = center;
+  const x1 = g[0] - cx;
+  const y1 = g[1] - cy;
+  const x2 = h[0] - cx;
+  const y2 = h[1] - cy;
+  const a =
+    Math.pow(x2 - x1, 2) / Math.pow(halfWidth, 2) +
+    Math.pow(y2 - y1, 2) / Math.pow(halfHeight, 2);
+  const b =
+    2 *
+    ((x1 * (x2 - x1)) / Math.pow(halfWidth, 2) +
+      (y1 * (y2 - y1)) / Math.pow(halfHeight, 2));
+  const c =
+    Math.pow(x1, 2) / Math.pow(halfWidth, 2) +
+    Math.pow(y1, 2) / Math.pow(halfHeight, 2) -
+    1;
+  const t1 = (-b + Math.sqrt(Math.pow(b, 2) - 4 * a * c)) / (2 * a);
+  const t2 = (-b - Math.sqrt(Math.pow(b, 2) - 4 * a * c)) / (2 * a);
+  const candidates = [
+    point<Point>(x1 + t1 * (x2 - x1) + cx, y1 + t1 * (y2 - y1) + cy),
+    point<Point>(x1 + t2 * (x2 - x1) + cx, y1 + t2 * (y2 - y1) + cy),
+  ].filter((p) => !isNaN(p[0]) && !isNaN(p[1]));
+
+  if (candidates.length === 2 && pointsEqual(candidates[0], candidates[1])) {
+    return [candidates[0]];
+  }
+
+  return candidates;
+}

packages/math/line.ts

@@ -1,3 +1,4 @@
+import { ellipseIntersectsLine } from "./ellipse";
 import { point, pointCenter, pointRotateRads } from "./point";
 import { segmentIncludesPoint } from "./segment";
 import type { GenericPoint, Line, Radians, Segment } from "./types";
@@ -93,3 +94,5 @@ export function lineIntersectsSegment<Point extends GenericPoint>(
 
   return candidate;
 }
+
+export const lineInterceptsEllipse = ellipseIntersectsLine;

packages/math/segment.ts

@@ -6,7 +6,7 @@ import {
   pointRotateRads,
   pointsEqual,
 } from "./point";
-import type { GenericPoint, Segment, Radians, Line } from "./types";
+import type { GenericPoint, Segment, Radians } from "./types";
 import { PRECISION } from "./utils";
 import {
   vectorAdd,
@@ -180,15 +180,10 @@ export function segmentDistanceToPoint<Point extends GenericPoint>(
 }
 
 /**
- * Returns the intersection point between a segment and a line, if any
+ * Returns the intersection point of a segment and a line
  *
- * @param s
  * @param l
+ * @param s
  * @returns
  */
-export function segmentIntersectsLine<Point extends GenericPoint>(
-  s: Segment<Point>,
-  l: Line<Point>,
-): Point | null {
-  return lineIntersectsSegment(l, s);
-}
+export const segmentIntersectsLine = lineIntersectsSegment;