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简述正投影法的基本特性。

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(1) 真实性 .当直线、曲线成平面平行于投影面时,直线或曲线的投影反映实长,平面的投影反映真实形状

(2) 积聚性 。当直线、曲线或平 面垂直 于投影面时,直线的投影枳聚成一点,平面或曲线的投影积聚成直线成曲线

(3) 类似性。当直线、曲线或平面倾斜于投影面时,直线或曲线 的投影仍然为直线或曲线、但小于实长。平面的投影小 于实际形状的大小,是实际形状的类似形

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