line of sight idiom
视线（某人看某物的方向）
the direction in which a person must look in order to see a particular object
The cannon can be linked to the line of sight of the gunner so that it points in the same direction that he is looking in. 大炮可与炮手的视线连成一线，因此它所瞄准的方向和炮手观察的方向一致。