One of the primary reasons for the high cost of traditional animation is the inbetweening process, where artists manually draw each intermediate frame necessary for smooth motion. Making this process more efficient has been at the core of computer graphics research for years, yet the industry has adopted very few solutions. Most existing solutions either require vector input or resort to tight inbetweening; often, they attempt to fully automate the process. In industry, however, keyframes are often spaced far apart, drawn in raster format, and contain occlusions. Moreover, inbetweening is fundamentally an artistic process, so the artist should maintain high-level control over it. We address these issues by proposing a novel inbetweening system for bitmap character drawings, supporting both tight and far inbetweening. In our setup, the artist can control motion by animating a skeleton between the keyframe poses. Our system then performs skeleton-based deformation of the bitmap drawings into the same pose and employs discrete optimization and deep learning to blend the deformed images. Besides the skeleton and the two drawn bitmap keyframes, we require very little annotation. However, deforming drawings with occlusions is complex, as it requires a piecewise smooth deformation field. To address this, we observe that this deformation field is smooth when the drawing is lifted into 3D. Our system therefore optimizes topology of a 2.5D partially layered template that we use to lift the drawing into 3D and get the final piecewise-smooth deformaton, effectively resolving occlusions. We validate our system through a series of animations, qualitative and quantitative comparisons, and user studies, demonstrating that our approach consistently outperforms the state of the art and our results are consistent with the viewers' perception. Code and data for our paper are available at http://www-labs.iro.umontreal.ca/~bmpix/inbetweening/.

Kirill Brodt

Mikhail Bessmeltsev

ACM Transactions on Graphics

ACM TRANSACTIONS ON GRAPHICS (TOG) 是一份经过同行评审的科学期刊，旨在传播计算机图形学领域的最新发现。它自 1982 年以来由计算机协会出版。从 2003 年开始，所有在 SIGGRAPH 年度会议上被接受发表的论文都印在该期刊的夏季特刊上。

ACM Transactions on Graphics (TOG) is a peer-reviewed scientific journal aimed at disseminating the latest research findings in the field of computer graphics. It has been published by the Association for Computing Machinery since 1982. Since 2003, all papers accepted for presentation at the SIGGRAPH annual conference are included in the summer issue of this journal.

《ACM Transactions on Graphics》（TOG）是一本经过同行评审的科学期刊，旨在传播计算机图形学领域的最新研究成果。该期刊自1982年由计算机协会出版。从2003年起，所有在SIGGRAPH年度会议上接受发表的论文均会收录在该期刊的夏季特刊中。

Skeleton-Driven Inbetweening of Bitmap Character Drawings

A novel inbetweening system for bitmap character drawings, supporting both tight and far inbetweening, and validated through a series of animations, qualitative and quantitative comparisons, and user studies, demonstrating that this approach consistently outperforms the state of the art.

Category	Quartile
计算机科学	1
计算机科学, 计算机软件工程	1

Skeleton-Driven Inbetweening of Bitmap Character Drawings

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Category	Quartile
COMPUTER SCIENCE, SOFTWARE ENGINEERING	1