在mfc画图时，画笔宽度设为0，为什么还可以画出东西来。

nWidth
Specifies the width of the pen.
For the first version of the constructor, if this value is zero, the width in device units is always 1 pixel, regardless of the mapping mode.
For the second version of the constructor, if nPenStyle is PS_GEOMETRIC, the width is given in logical units. If nPenStyle is PS_COSMETIC, the width must be set to 1.

0代表默认宽度