Using isophotes and shadows to interactively model normal and height fields

• Published in: Computers & graphics 2016-10, Vol.59, p.130-142
• Main Authors: Xu, Qiuying, Liu, Songrun, Gingold, Yotam, Singh, Karan
• Format: Article
• Language: English
• Subjects: Isophotes; Non-photorealistic rendering; Sketching
• ISSN: 0097-8493; 1873-7684
• DOI: 10.1016/j.cag.2016.02.004

We introduce an interactive modeling tool for designing (a) a smooth 3D normal field from the isophotes of a discretely shaded 2D image and (b) lifting the normal field into a smooth height field given a cast shadow. 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 also describe a technique for lifting the generated normal field into a height field given the boundary of the shadow cast by the object. 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. [Display omitted] •We present an interactive modeling solution for inverse toon (discrete tone) shading.•We describe an interactive tool for drawing and shading silhouettes and isophotes.•We describe an algorithm to compute a smooth 3D normal field from toon shaded input.•We describe an algorithm to lift a normal field with a shadow into a height field.•We describe studies to assess humans' ability to perceive and draw isophotes.