Two-dimensional (2D) hand-drawn animation is still being produced in Japan, and the market is growing each year against a backdrop of worldwide popularity. Inbetweening, the process of creating interpolated frames between key frames, is an important but laborious task in 2D hand-drawn animation. With this process, we aim to avoid the need for a special interface or vectorization. We therefore propose an optical-flow based line drawing frame interpolation method using a distance transform. In general, an optical flow is not applicable to line drawings owing to a lack of intensity in the gradients and colors. Therefore, we use a distance transform to add intensity gradients to line drawings, and it is possible to estimate an optical flow between them. We evaluated our method quantitatively using commercial hand-drawn animation inbetweens. Our method significantly outperforms a baseline method that does not use a distance transform. In addition, we also compared our method to an existing image-based method qualitatively and showed that our method generates better results.
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