code: protected float intersectionZ (IRay3 ray, float z) {
          IVector3 origin = ray.origin(), dir = ray.direction();
          float t = (z - origin.z()) / dir.z();
          if (t < 0f) {
              return Float.MAX_VALUE;
          }
          float ix = origin.x() + t*dir.x(), iy = origin.y() + t*dir.y();
          return (ix >= _minExtent.x && ix <= _maxExtent.x &&
              iy >= _minExtent.y && iy <= _maxExtent.y) ? 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 z equals the value specified, or returns
  {@link Float#MAX_VALUE} if there is no such intersection.