Vision

You are given two points $P$ and $Q$ and an opaque sphere in a three-dimensional space. The point $P$ is not moving, while $Q$ is moving in a straight line with constant velocity. You are also given a direction vector $d$ with the following meaning: the position of $Q$ at time $t$ is $Q(t) = Q(0) + d \cdot t$, where $Q(0)$ is the initial position of $Q$. It is guaranteed that $Q$ is not visible f