fix: Freedraw hit test precision

Signed-off-by: Mark Tolmacs <mark@lazycat.hu>

packages/element/src/distance.ts

@@ -2,6 +2,7 @@ import {
   curvePointDistance,
   distanceToLineSegment,
   pointRotateRads,
+  polygonIncludesPointNonZero,
 } from "@excalidraw/math";
 
 import { ellipse, ellipseDistanceFromPoint } from "@excalidraw/math/ellipse";
@@ -47,8 +48,9 @@ export const distanceToElement = (
       return distanceToEllipseElement(element, elementsMap, p);
     case "line":
     case "arrow":
-    case "freedraw":
       return distanceToLinearOrFreeDraElement(element, elementsMap, p);
+    case "freedraw":
+      return distanceToFreeDrawElement(element, elementsMap, p);
   }
 };
 
@@ -145,3 +147,30 @@ const distanceToLinearOrFreeDraElement = (
     ...curves.map((a) => curvePointDistance(a, p)),
   );
 };
+
+/**
+ * Returns the distance of a point to a freedraw element.
+ *
+ * @param element The freedraw element
+ * @param p The point to consider
+ * @returns 0 if the point is within the inked area, otherwise the euclidean
+ * distance to the stroke outline
+ */
+const distanceToFreeDrawElement = (
+  element: ExcalidrawFreeDrawElement,
+  elementsMap: ElementsMap,
+  p: GlobalPoint,
+) => {
+  const [lines] = deconstructLinearOrFreeDrawElement(element, elementsMap);
+
+  if (lines.length === 0) {
+    return Infinity;
+  }
+
+  const polygon = lines.map((line) => line[0]);
+  if (polygonIncludesPointNonZero(p, polygon)) {
+    return 0;
+  }
+
+  return Math.min(...lines.map((s) => distanceToLineSegment(p, s)));
+};

packages/element/src/shape.ts

@@ -677,67 +677,43 @@ export const generateLinearCollisionShape = (
         });
     }
     case "freedraw": {
-      if (element.points.length < 2) {
+      const outlinePoints = getFreedrawOutlinePoints(element);
+
+      if (outlinePoints.length < 2) {
         return [];
       }
 
-      const simplifiedPoints = simplify(
-        element.points as Mutable<LocalPoint[]>,
-        0.75,
-      );
+      const collisionOutline =
+        element.strokeOptions?.variability === "constant" &&
+        CONSTANT_WIDTH_FREEDRAW.STREAMLINE > 0
+          ? (simplify(
+              outlinePoints as Mutable<LocalPoint[]>,
+              CONSTANT_WIDTH_FREEDRAW.STREAMLINE,
+            ) as [number, number][])
+          : outlinePoints;
 
-      return generator
-        .curve(simplifiedPoints as [number, number][], options)
-        .sets[0].ops.slice(0, element.points.length)
-        .map((op, i) => {
-          if (i === 0) {
-            const p = pointRotateRads<GlobalPoint>(
-              pointFrom<GlobalPoint>(
-                element.x + op.data[0],
-                element.y + op.data[1],
-              ),
-              center,
-              element.angle,
-            );
-
-            return {
-              op: "move",
-              data: pointFrom<LocalPoint>(p[0] - element.x, p[1] - element.y),
-            };
+      if (collisionOutline.length < 2) {
+        return [];
       }
 
+      // Close the outline polygon so its boundary never has a gap at the seam.
+      const [firstX, firstY] = collisionOutline[0];
+      const [lastX, lastY] = collisionOutline[collisionOutline.length - 1];
+      const closedOutline =
+        firstX === lastX && firstY === lastY
+          ? collisionOutline
+          : [...collisionOutline, collisionOutline[0]];
+
+      return closedOutline.map((point, idx) => {
+        const p = pointRotateRads<GlobalPoint>(
+          pointFrom<GlobalPoint>(element.x + point[0], element.y + point[1]),
+          center,
+          element.angle,
+        );
+
         return {
-            op: "bcurveTo",
-            data: [
-              pointRotateRads(
-                pointFrom<GlobalPoint>(
-                  element.x + op.data[0],
-                  element.y + op.data[1],
-                ),
-                center,
-                element.angle,
-              ),
-              pointRotateRads(
-                pointFrom<GlobalPoint>(
-                  element.x + op.data[2],
-                  element.y + op.data[3],
-                ),
-                center,
-                element.angle,
-              ),
-              pointRotateRads(
-                pointFrom<GlobalPoint>(
-                  element.x + op.data[4],
-                  element.y + op.data[5],
-                ),
-                center,
-                element.angle,
-              ),
-            ]
-              .map((p) =>
-                pointFrom<LocalPoint>(p[0] - element.x, p[1] - element.y),
-              )
-              .flat(),
+          op: idx === 0 ? "move" : "lineTo",
+          data: pointFrom<LocalPoint>(p[0] - element.x, p[1] - element.y),
         };
       });
     }

packages/element/tests/collision.test.tsx

@@ -218,3 +218,54 @@ describe("hitElementItself cache", () => {
     ).toBe(true);
   });
 });
+
+describe("freedraw collision matches the rendered stroke width", () => {
+  // A straight, horizontal stroke centered on y === 0.
+  const points = Array.from({ length: 21 }, (_, i) =>
+    pointFrom<LocalPoint>(i * 5, 0),
+  );
+
+  const createFreeDraw = (variability: "variable" | "constant") =>
+    API.createElement({
+      type: "freedraw",
+      x: 0,
+      y: 0,
+      strokeWidth: 10,
+      points,
+      strokeOptions: { variability, streamline: 0.5 },
+    });
+
+  const distanceAt = (
+    element: ReturnType<typeof createFreeDraw>,
+    x: number,
+    y: number,
+  ) =>
+    distance.distanceToElement(
+      element,
+      arrayToMap([element]),
+      pointFrom<GlobalPoint>(x, y),
+    );
+
+  it("treats a point on the centerline as a direct hit for both modes", () => {
+    expect(distanceAt(createFreeDraw("variable"), 50, 0)).toBe(0);
+    expect(distanceAt(createFreeDraw("constant"), 50, 0)).toBe(0);
+  });
+
+  it("hits across the body of a thick stroke, not just near the centerline", () => {
+    expect(distanceAt(createFreeDraw("variable"), 50, 20)).toBe(0);
+  });
+
+  it("uses a wider hit area for variable-width than for constant-width strokes", () => {
+    const offset = 20;
+    const variableDistance = distanceAt(createFreeDraw("variable"), 50, offset);
+    const constantDistance = distanceAt(createFreeDraw("constant"), 50, offset);
+
+    expect(variableDistance).toBe(0);
+    expect(constantDistance).toBeGreaterThan(0);
+    expect(variableDistance).toBeLessThan(constantDistance);
+  });
+
+  it("does not hit points clearly outside even the widest stroke", () => {
+    expect(distanceAt(createFreeDraw("variable"), 50, 45)).toBeGreaterThan(0);
+  });
+});