EaseMarks: Using Secondary Sketch Marks To Author and Communicate Motion Interpolation

Abstract

**Please note that the full text is embargoed until 08/01/2024** Motion interpolation is a process where an animator transforms jerky frame transitions into rich motions that communicate anticipation, urgency, hysteresis, and even calmness. Animators leverage mathematical functions known as easing curves to modify the rate at which in-betweens are added to keyframes. While effective, easing curves are tedious to tune since they fundamentally lack the ability to encode spatial information. Inspired by timing charts and other standards from traditional cel animation, we introduce a motion animation technique where secondary marks, which we term EaseMark (e.g., hatches, loops), are used to denote motion interpolation decisions. We synthesize an EaseMark Sketching Language and evaluate it through a crowdsourced study. This sketching language is then put in practice in a tool that allows animators to author motion interpolation by sketching or selecting different EaseMarks that affect an object's visual and spatial expression over time. We discuss how secondary marks can be used to expand the expressiveness and utility of sketching languages in other complex design practices.