Fix hit testing threshold (#969)

* Fix hit testing threshold

- The bounding box was not correctly extended to take into account the threshold. It was only for y axis but not x.
- The bezier threshold was using 20 instead of 10 and not taking into account zoom level.

Both those issues are fixed and now the behavior looks good on all the shapes I can test.

* fix_tests

src/element/collision.ts

@@ -165,7 +165,12 @@ export function hitTest(
     const shape = getShapeForElement(element) as Drawable[];
 
     const [x1, y1, x2, y2] = getLinearElementAbsoluteBounds(element);
-    if (x < x1 || y < y1 - 10 || x > x2 || y > y2 + 10) {
+    if (
+      x < x1 - lineThreshold ||
+      y < y1 - lineThreshold ||
+      x > x2 + lineThreshold ||
+      y > y2 + lineThreshold
+    ) {
       return false;
     }
 
@@ -173,7 +178,9 @@ export function hitTest(
     const relY = y - element.y;
 
     // hit thest all "subshapes" of the linear element
-    return shape.some(subshape => hitTestRoughShape(subshape.sets, relX, relY));
+    return shape.some(subshape =>
+      hitTestRoughShape(subshape.sets, relX, relY, lineThreshold),
+    );
   } else if (element.type === "text") {
     const [x1, y1, x2, y2] = getElementAbsoluteCoords(element);
 
@@ -191,6 +198,7 @@ const pointInBezierEquation = (
   p2: Point,
   p3: Point,
   [mx, my]: Point,
+  lineThreshold: number,
 ) => {
   // B(t) = p0 * (1-t)^3 + 3p1 * t * (1-t)^2 + 3p2 * t^2 * (1-t) + p3 * t^3
   const equation = (t: number, idx: number) =>
@@ -199,7 +207,6 @@ const pointInBezierEquation = (
     3 * Math.pow(t, 2) * (1 - t) * p1[idx] +
     p0[idx] * Math.pow(t, 3);
 
-  const epsilon = 20;
   // go through t in increments of 0.01
   let t = 0;
   while (t <= 1.0) {
@@ -208,7 +215,7 @@ const pointInBezierEquation = (
 
     const diff = Math.sqrt(Math.pow(tx - mx, 2) + Math.pow(ty - my, 2));
 
-    if (diff < epsilon) {
+    if (diff < lineThreshold) {
       return true;
     }
 
@@ -218,7 +225,12 @@ const pointInBezierEquation = (
   return false;
 };
 
-const hitTestRoughShape = (opSet: OpSet[], x: number, y: number) => {
+const hitTestRoughShape = (
+  opSet: OpSet[],
+  x: number,
+  y: number,
+  lineThreshold: number,
+) => {
   // read operations from first opSet
   const ops = opSet[0].ops;
 
@@ -248,7 +260,14 @@ const hitTestRoughShape = (opSet: OpSet[], x: number, y: number) => {
       // check if points are on the curve
       // cubic bezier curves require four parameters
       // the first parameter is the last stored position (p0)
-      const retVal = pointInBezierEquation(p0, p1, p2, p3, [x, y]);
+      const retVal = pointInBezierEquation(
+        p0,
+        p1,
+        p2,
+        p3,
+        [x, y],
+        lineThreshold,
+      );
 
       // set end point of bezier curve as the new starting point for
       // upcoming operations as each operation is based on the last drawn

src/tests/selection.test.tsx

@@ -163,10 +163,23 @@ describe("select single element on the scene", () => {
       fireEvent.keyDown(document, { key: KEYS.ESCAPE });
     }
 
+    /*
+        1 2 3 4 5 6 7 8 9
+      1
+      2     x
+      3
+      4       .
+      5
+      6
+      7           x
+      8
+      9
+    */
+
     const tool = getByToolName("selection");
     fireEvent.click(tool);
-    // click on a line on the rectangle
-    fireEvent.pointerDown(canvas, { clientX: 45, clientY: 20 });
+    // click on a line on the arrow
+    fireEvent.pointerDown(canvas, { clientX: 40, clientY: 40 });
     fireEvent.pointerUp(canvas);
 
     expect(renderScene).toHaveBeenCalledTimes(7);
@@ -175,7 +188,7 @@ describe("select single element on the scene", () => {
     expect(h.appState.selectedElementIds[h.elements[0].id]).toBeTruthy();
   });
 
-  it("arrow", () => {
+  it("arrow escape", () => {
     const { getByToolName, container } = render(<App />);
     const canvas = container.querySelector("canvas")!;
     {
@@ -188,10 +201,23 @@ describe("select single element on the scene", () => {
       fireEvent.keyDown(document, { key: KEYS.ESCAPE });
     }
 
+    /*
+        1 2 3 4 5 6 7 8 9
+      1
+      2     x
+      3
+      4       .
+      5
+      6
+      7           x
+      8
+      9
+    */
+
     const tool = getByToolName("selection");
     fireEvent.click(tool);
-    // click on a line on the rectangle
-    fireEvent.pointerDown(canvas, { clientX: 45, clientY: 20 });
+    // click on a line on the arrow
+    fireEvent.pointerDown(canvas, { clientX: 40, clientY: 40 });
     fireEvent.pointerUp(canvas);
 
     expect(renderScene).toHaveBeenCalledTimes(7);