Inverse toon shading: interactive normal field modeling with isophotes

We introduce an interactive modeling tool for designing a smooth 3D normal field from the isophotes of a discretely shaded 2D image. Block or cartoon shading is a visual style in which artists depict a smoothly shaded 3D object using a small number of discrete brightness values, manifested as regions or bands of constant color. In our approach, artists trace isophotes, or curves of constant brightness, along the boundaries between constant color bands. Our algorithm first estimates light directions and computes 3D normals along the object silhouette and at intersections between isophotes from different light sources. We then propagate these 3D normals smoothly along isophotes, and subsequently throughout the interior of the shape. We describe our user interface for editing isophotes and correcting unintended normals produced by our algorithm. We validate our approach with a perceptual experiment and comparisons to ground truth data. Finally, we present a set of 3D renderings created using our interface.