"Inner painting" is painting inside a transparent object, the painter must use the "mirror principle","reverse direction" painting, and need to use a special pen, this pen body is generally made of small bamboo, and the tip is similar to "fish hook", so that it is convenient to put the pen into the transparent container "reverse direction" painting. This kind of production process is very difficult, because it is painted inside the glass container, once there is a mistake, it is impossible to correct the smear.