在画法几何投影变换中，不论用换面法或旋转法，一般位置直线均需两次变换才能变成投影面垂直线。
In the transformation of descriptive geometry, no matter the method of change plane or revolve, the oblique line can be changed to a vertical line in two successive transformations.
投影面可为基本投影面也可为新设的垂直面或一般位置平面。
The projection plane may be the basic projection plane, newly-established vertical plane or common place plane, and the direction of the projection may slope.
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