code: protected float intersectionY (IRay3 ray, float y) {
          IVector3 origin = ray.origin(), dir = ray.direction();
          float t = (y - origin.y()) / dir.y();
          if (t < 0f) {
              return Float.MAX_VALUE;
          }
          float ix = origin.x() + t*dir.x(), iz = origin.z() + t*dir.z();
          return (ix >= _minExtent.x && ix <= _maxExtent.x &&
              iz >= _minExtent.z && iz <= _maxExtent.z) ? t : Float.MAX_VALUE;
      }
docs: Helper method for {@link #intersection}. Finds the <code>t</code> value where the ray
  intersects the box at the plane where y equals the value specified, or returns
  {@link Float#MAX_VALUE} if there is no such intersection.