Martin Guay, Rémi Ronfard, Michael Gleicher, Marie-Paule Cani
ACM Transactions on Graphics (Proceedings of SIGGRAPH), 2015
Left image: character model and space-time curve (STC). Right image: we dynamically warp (red window) within the parametric space of the STC to produce a dynamic line of action. Squash and stretch is controlled directly by drawing faster and slower.
Abstract
We present a space-time abstraction for the sketch-based design of character animation. It allows animators to draft a full coordinated motion using a single stroke called the space-time curve (STC). From the STC we compute a dynamic line of action (DLOA) that drives the motion of a 3D character through projective constraints. Our dynamic models for the line’s motion are entirely geometric, require no pre-existing data, and allow full artistic control. The resulting DLOA can be refined by over-sketching strokes along the space-time curve, or by composing another DLOA on top leading to control over complex motions with few strokes. Additionally , the resulting dynamic line of action can be applied to arbitrary body parts or characters. To match a 3D character to the 2D line over time, we introduce a robust matching algorithm based on closed-form solutions, yielding a tight match while allowing squash and stretch of the character’s skeleton. Our experiments show that space-time sketching has the potential of bringing animation design within the reach of beginners while saving time for skilled artists.
