Replay fixes for Quaternion.computeAxis

CHANGES.md

@@ -8,6 +8,7 @@
 
 - Changes the default `RequestScheduler.maximumRequestsPerServer` from 6 to 18. This should improve performance on HTTP/2 servers and above [#11627](https://github.com/CesiumGS/cesium/issues/11627)
 - Corrected JSDoc and Typescript definitions that marked optional arguments as required in `ImageryProvider` constructor [#11625](https://github.com/CesiumGS/cesium/issues/11625)
+- The `Quaternion.computeAxis` function created an axis that was `(0,0,0)` for the unit quaternion, and an axis that was `(NaN,NaN,NaN)` for the quaternion `(0,0,0,-1)` (which describes a rotation about 360 degrees). Now, it returns the x-axis `(1,0,0)` in both of these cases.
 
 ### 1.112 - 2023-12-01
 

packages/engine/Source/Core/Quaternion.js

@@ -677,8 +677,12 @@ Quaternion.computeAxis = function (quaternion, result) {
   //>>includeEnd('debug');
 
   const w = quaternion.w;
-  if (Math.abs(w - 1.0) < CesiumMath.EPSILON6) {
-    result.x = result.y = result.z = 0;
+  if (
+    Math.abs(w - 1.0) < CesiumMath.EPSILON6 ||
+    Math.abs(w + 1.0) < CesiumMath.EPSILON6
+  ) {
+    result.x = 1;
+    result.y = result.z = 0;
     return result;
   }
 

packages/engine/Specs/Core/QuaternionSpec.js

@@ -449,15 +449,24 @@ describe("Core/Quaternion", function () {
     expect(result).toBe(returnedResult);
   });
 
-  it("axis returns Cartesian3 0 when w equals 1.0", function () {
-    const expected = new Cartesian3(0.0, 0.0, 0.0);
+  it("axis returns Cartesian3 (1,0,0) when w equals 1.0", function () {
+    const expected = new Cartesian3(1.0, 0.0, 0.0);
     const quaternion = new Quaternion(4.0, 2.0, 3.0, 1.0);
     const result = new Cartesian3(1, 2, 3);
     const returnedResult = Quaternion.computeAxis(quaternion, result);
     expect(returnedResult).toEqual(expected);
     expect(result).toBe(returnedResult);
   });
 
+  it("axis returns Cartesian3 (1,0,0) when w equals -1.0", function () {
+    const expected = new Cartesian3(1.0, 0.0, 0.0);
+    const quaternion = new Quaternion(4.0, 2.0, 3.0, -1.0);
+    const result = new Cartesian3(1, 2, 3);
+    const returnedResult = Quaternion.computeAxis(quaternion, result);
+    expect(returnedResult).toEqual(expected);
+    expect(result).toBe(returnedResult);
+  });
+
   it("angle works", function () {
     // 60 degrees is used here to ensure that the sine and cosine of the half angle are not equal.
     const angle = Math.PI / 3.0;