Since we defined the frustum, we know the angle at which the sides of the frustum meet - this is the field of view we used to create the projection matrix in the first place.
Since we defined the frustum, we know the angle at which the sides of the frustum meet - this is the field of view we used to create the projection matrix in the first place.
确定好锥截体，我们就知道锥截体顶角的大小—这是我们在第一个地方用来创建投影矩阵的视场。