public void setFaceNormal(Vector3f faceNormal) {
        if (faceNormal == null || !Vector3f.isValidVector(faceNormal)) {
            this.faceNormal.set(Vector3f.NAN);
        } else {
            this.faceNormal = faceNormal;
        }
    }
Sets the normal which particles are facing.

<p>By default, particles
will face the camera, but for some effects (e.g. shockwave) it may
be necessary to face a specific direction instead. To restore
normal functionality, provide <code>null</code> as the argument for
<code>faceNormal</code>.

@param faceNormal The normals particles should face, or <code>null</code>
if particles should face the camera.
jme3-core/src/main/java/com/jme3/effect/ParticleEmitter.java