Bob Sumner (Disney Research Zurich)
Antoine Milliez (Disney Research Zurich)
Stelian Coros (Disney Research Zurich)
Ilya Baran (Disney Research Zurich)
Johannes Schmid (Disney Research Zurich, ETH)
Katie Bassett (Disney Research Zurich)
We present a technique to generalize the 2D painting metaphor to 3D that allows the artist to treat the full 3D space as a canvas. Strokes painted in the 2D viewport window must be embedded in 3D space in a way that gives creative freedom to the artist while maintaining an acceptable level of controllability. We address this challenge by proposing a canvas concept defined implicitly by a 3D scalar field. The artist shapes the implicit canvas by creating approximate 3D proxy geometry. An optimization procedure is then used to embed painted strokes in space by satisfying different objective criteria defined on the scalar field. This functionality allows us to implement tools for painting along level set surfaces or across different level sets. Our method gives the power of fine-tuning the implicit canvas to the artist using a unified painting/sculpting metaphor. In OverCoat, scenes are rendered by projecting each brush stroke onto the current view plane and rasterizing it as one or more fragments for every pixel that the stroke overlaps. This rendering method exposes a technical dilemma about the order in which the fragments should be composited. In the 2D painting metaphor, when the artist places a new paint stroke, it obscures all previous paint strokes that it overlaps. Such behavior is achieved by compositing in stroke order. From a 3D point of view, however, strokes that are closer to the viewer should obscure those that are farther away, which amounts to compositing in depth order. Compositing purely in stroke order negates much of the benefit of 3D painting, as the sense of tangible objects is lost when the view is changed. Compositing purely in depth order, on the other hand, leads to Z-fighting, precluding the artist from painting over existing strokes, and thus ignores an important part of the 2D painting metaphor. In this work, we formalize this idea for the first time and design a new mixed-order rendering algorithm that addresses these challenges.