一个手绘normal的方法

https://polycount.com/discussion/98983/how-to-paint-flow-anisotropic-comb-maps-in-photoshop

flow map

comb map---for anistropic spec hair

normal

总之这就是个画方向的

色盘 

根据brush方向在色盘上取颜色 起点在圆心的vector 2d