Applying a depth effect or a 3D-like look to hand-drawn animations can be just as visually pleasing as the rendering of a full 3D animation character. In this work we present a method for computer-assisted shading of 2D animation sequences where the main character undergoes non-rigid deformations between the frames.

The system receives as input a sequence of vectorized 2D animation frames containing an outline sketch of the character and applies a 3D shading effect directly to these 2D frames. The system allows the animator to specify shading constraints on a specific frame and addresses the challenge of propagating this information to the consequent frames with minimal user intervention. Eventually, this produces a coherently shaded animation sequence, and significantly reduces the effort spent by the animator on the shading process.

In order to achieve visually appealing results and keep the shading coherent between the frames, we describe a graphical user interface and registration method that is able to propagate shading constraints through the animation sequence despite non-rigid character deformations. Finally, we describe a method for generating the 3D-like shading to animation frames based on the constraints.