Space-Time Surface Simplification and Edgebreaker Compression for 2D Cel Animations

Digitized cel animations are typically composed of frames that contain a small number of regions; each region contains pixels of the same color and exhibits a significant level of shape coherence through time. To exploit this coherence, we treat the stack of frames as a 3D volume and represent the evolution of each region by the bounding surface of the 3D sub-volume V that it sweeps out. To reduce transmission costs, we triangulate and simplify the bounding surface and then encode it using the Edgebreaker compression scheme. To restore a close approximation of the original animation, the client player decompresses the surface and produces the successive frames by intersecting V with constant-time planes. The intersection is generated in real-time with standard graphics hardware through an improved capping (i.e. solid clipping) technique, which correctly handles overlapping facets. We have tested this approach on real and synthetic black&white animations and report compression ratios that improve upon those produced using the MPEG, MRLE, and GZIP compression standards for an equivalent quality result.