Match-free Inbetweening Assistant (MIBA): A Practical Animation Tool without User Stroke Correspondence

Shuhong Chen, Matthias Zwicker; Proceedings of the Asian Conference on Computer Vision (ACCV), 2024, pp. 179-193

In traditional 2D frame-by-frame animation, inbetweening (interpolating line drawings, abbr. "IB") is still a manual and labor-intensive task. Despite the abundance of literature and software offering automation and claiming speedups, animators and the industry as a whole have been hesitant to adopt these new tools. Upon inspection, we find prior work often unreasonably expects adoption of novel stroke-matching workflows, naively assumes access to adequate center-line vectorization, and lacks rigorous evaluation with professional users on real production data. Facing these challenges, we leverage optical flow estimation and differentiable vector graphics to design a "Match-free Inbetweening Assistant" (MIBA). Unrestricted by the need for user stroke correspondence, MIBA integrates into the existing IB workflow without introducing additional requirements, and makes the raster input case feasible thanks to its robustness to vectorization quality. MIBA's simplicity and effectiveness is demonstrated in our comprehensive user study, where users with professional IB experience achieved 4.2x average speedup and better chamfer distance scores on real-world production data, given only a 5-minute tutorial of new functionality.