Abstract: Thangka is a unique form of artistic paintings in Buddhism,depicting a variety of objects and including intricate details.Black and white Thangka is a special branch of Thangka,in which objects are depicted by gray gradient.Manually painting such gray gradient is highly tedious and time consuming.This paper presents a computer assisted system for rendering black and white Thangka.With different objects labelled by using ID colors in the input Thangka line drawings,our system assigns black/white feature points as boundary constraints semi-automatically for regions with gray values increasing/decreasing gradually from one side of the region to another,by taking occlusions and internal geometric features into account.It also takes grayscale images of distance field of object boundaries,or grayscale images drawn manually inside a few regions on human body,as the given brightness distribution for object regions with gray values increasing gradually from the boundaries to the interiors.Black/white feature points and grayscale images are used as constraints to solve Poisson equation to obtain gray gradient corresponding to different objects,as demonstrated by examples in the paper.