To make use of these coordinates, we must first re-define them in terms of the visible area of the viewing frustum.

了利用好这些坐标，我们首先需根据视图锥截体的可视区重新定义它们。

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The viewing frustum has an origin (0, 0) at the center of the screen, while screen coordinates have an origin at the upper left of the screen.

视图锥截体的原点(，0)屏幕中心，而屏幕坐标的则在屏幕的左上角。

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