Being of good quality to ask the front or top surface , stop up head , jade quality bad to ask the back jade have 玉质地好一面叫阳面或顶面堵头，玉质地差的一面叫阴面。
2.
In order to get the shade of the object , we must decide which is the open surface and which is the hidden surface , decide the line between the open surface and the hidden surface , and then draw the shade one by one 求作平面立体的阴影，首先要判别立体表面的阳面和阴面，从而确定阴线，也就是阳面和阴面的分界线，然后逐段作出阴线在投影面以及自身阳面上的落影。
