<p>I think the change is valid. Unfortunately it is not possible to make balls even tighter since each ball of <code>HollowBallBound</code> always intersects a point. So, the volume of each bound depends only on the vantage point. I think the number of base cases is huge since outer balls cover a lot of empty space.</p>

<p>I think it is possible to make <code>HollowBallBound</code> tighter. I suggest to consider an intersection of <code>HollowBallBound</code> with the minimum bounding ball. I guess it is easy to compute <code>MinDistance</code> and <code>MaxDistance</code>. And I think that should reduce the volume of the bound significantly. How do you think?</p>

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