Minor updates and documentation, see #123

js/buoyancy/model/ApplicationsMass.ts

@@ -26,7 +26,8 @@ export default abstract class ApplicationsMass extends Mass {
 
   protected massLabelOffsetVector3: Vector3;
 
-  public readonly intersectionGroup: THREE.Group;
+  // For ray casting/hit testing
+  public readonly intersectionGroup = new THREE.Group();
 
   protected constructor( engine: PhysicsEngine, displacementVolumeProperty: NumberProperty, options: ApplicationsMassOptions ) {
 
@@ -41,8 +42,6 @@ export default abstract class ApplicationsMass extends Mass {
 
     this.massLabelOffsetOrientationProperty.value = new Vector2( 1, -1 / 2 );
     this.massLabelOffsetProperty.value = this.massLabelOffsetVector3;
-
-    this.intersectionGroup = new THREE.Group();
   }
 
   /**
@@ -55,7 +54,7 @@ export default abstract class ApplicationsMass extends Mass {
 
   /**
    * If there is an intersection with the ray and this mass, the t-value (distance the ray would need to travel to
-   * reach the intersection, e.g. ray.position + ray.distance * t === intersectionPoint) will be returned. Otherwise
+   * reach the intersection, e.g. ray.position + ray.distance * t === intersectionPoint) will be returned. Otherwise,
    * if there is no intersection, null will be returned.
    */
   public override intersect( ray: Ray3, isTouch: boolean, scale = 1 ): number | null {
@@ -77,11 +76,14 @@ export default abstract class ApplicationsMass extends Mass {
    * Returns the displayed area of this object at a given y level
    *
    * Assumes step information was updated.
+   *
+   * TODO: Why is this different than getDisplacedVolume? Should they share implementation? See https://github.com/phetsims/density-buoyancy-common/issues/123
    */
   public getDisplacedArea( liquidLevel: number ): number {
     const bottom = this.stepBottom;
     const top = this.stepTop;
 
+    // TODO: https://github.com/phetsims/density-buoyancy-common/issues/123 if the liquid level is beyond the top, it probably shouldn't be 0, right?
     if ( liquidLevel < bottom || liquidLevel > top ) {
       return 0;
     }

js/common/model/Mass.ts

@@ -263,8 +263,6 @@ export default abstract class Mass extends PhetioObject {
     }, providedOptions );
 
     assert && assert( options.body, 'options.body required' );
-    assert && assert( options.shape instanceof Shape, 'options.shape required as a Shape' );
-    assert && assert( options.material instanceof Material, 'options.material required as a Material' );
     assert && assert( options.volume > 0, 'non-zero options.volume required' );
 
     super( options );
@@ -629,11 +627,11 @@ export default abstract class Mass extends PhetioObject {
   public abstract setRatios( widthRatio: number, heightRatio: number ): void;
 
   /**
-   * Called after a engine-physics-model step once before doing other operations (like computing buoyant forces,
+   * Called after the engine-physics-model step once before doing other operations (like computing buoyant forces,
    * displacement, etc.) so that it can set high-performance flags used for this purpose.
    *
    * Type-specific values are likely to be set, but this should set at least stepX/stepBottom/stepTop (as those are
-   * used for determining basin volumes and cross sections)
+   * used for determining basin volumes and cross-sections)
    */
   public updateStepInformation(): void {
     this.engine.bodyGetStepMatrixTransform( this.body, this.stepMatrix );