Calculate rotation matrix for non Y up upvector only when up vector set

Instead of each time _getViewMatrix is called.

src/Cameras/arcRotateCamera.ts

@@ -74,6 +74,41 @@ export class ArcRotateCamera extends TargetCamera {
         this.setPosition(newPosition);
     }
 
+    @serializeAsVector3("upVector")
+    protected _upVector = Vector3.Up();
+
+    protected _upToYMatrix: Matrix;
+    protected _YToUpMatrix: Matrix;
+
+    /**The vector the camera should consider as up. (default is Vector3(0, 1, 0) aka Vector3.Up())
+     * Setting this will copy the given vec to the camera's upVector, and set rotation matrices to and from the Y up.  
+     * DO NOT set the up vector using copyFrom or copyFromFloats, as this bypasses setting the above matrices.
+     */
+    set upVector(vec: Vector3) {
+        if (!this._upToYMatrix) {
+            this._YToUpMatrix = new Matrix();
+            this._upToYMatrix = new Matrix();
+
+            this._upVector = Vector3.Zero();
+        }
+
+        vec.normalize();
+        this._upVector.copyFrom(vec);
+        this.setMatUp(); // still more efficient to calculate this once
+    }
+
+    get upVector() {
+        return this._upVector;
+    }
+
+    setMatUp() {
+        // from y-up to custom-up (used in _getViewMatrix)
+        Matrix.RotationAlignToRef(Vector3.UpReadOnly, this._upVector, this._YToUpMatrix);
+
+        // from custom-up to y-up (used in rebuildAnglesAndRadians)
+        Matrix.RotationAlignToRef(this._upVector, Vector3.UpReadOnly, this._upToYMatrix);
+    }
+
     /**
      * Current inertia value on the longitudinal axis.
      * The bigger this number the longer it will take for the camera to stop.
@@ -574,8 +609,6 @@ export class ArcRotateCamera extends TargetCamera {
     protected _targetBoundingCenter: Nullable<Vector3>;
 
     private _computationVector: Vector3 = Vector3.Zero();
-    private _tempAxisVector: Vector3;
-    private _tempAxisRotationMatrix: Matrix;
 
     /**
      * Instantiates a new ArcRotateCamera in a given scene
@@ -942,22 +975,8 @@ export class ArcRotateCamera extends TargetCamera {
         this._computationVector.copyFromFloats(this.radius * cosa * sinb, this.radius * cosb, this.radius * sina * sinb);
 
         // Rotate according to up vector
-        if (this.upVector.x !== 0 || this.upVector.y !== 1.0 || this.upVector.z !== 0) {
-
-            if (!this._tempAxisVector) {
-                this._tempAxisVector = new Vector3();
-                this._tempAxisRotationMatrix = new Matrix();
-            }
-
-            Vector3.CrossToRef(Vector3.Up(), this.upVector, this._tempAxisVector);
-            this._tempAxisVector.normalize();
-
-            let angle = Math.acos(Vector3.Dot(Vector3.UpReadOnly, this.upVector));
-
-            Matrix.RotationAxisToRef(this._tempAxisVector, angle, this._tempAxisRotationMatrix);
-
-            this._tempAxisVector.copyFrom(this._computationVector);
-            Vector3.TransformCoordinatesToRef(this._tempAxisVector, this._tempAxisRotationMatrix, this._computationVector);
+        if (this._upVector.x !== 0 || this._upVector.y !== 1.0 || this._upVector.z !== 0) {
+            Vector3.TransformCoordinatesToRef(this._computationVector, this._YToUpMatrix, this._computationVector);
         }
 
         target.addToRef(this._computationVector, this._newPosition);