Currently you can only scale between large and small. I suggest changing code like this in order to scale smoothly in interaction.js. “_pmouseWheelDeltaY” may be removed.
const scaleFactor = 50;
// ZOOM if there is a change in mouseWheelDelta
if (this._mouseWheelDeltaY !=0) {
// zoom according to direction of mouseWheelDeltaY rather than value
if (this._mouseWheelDeltaY > 0) {
this._renderer._curCamera._orbit(0, 0, sensitivityZ * scaleFactor);
} else {
this._renderer._curCamera._orbit(0, 0, -sensitivityZ * scaleFactor);
}
this._mouseWheelDeltaY =0;
}