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Surface Simplification & Edgebreaker Compression for 2D Cel Animations. Vivek Kwatra & Jarek Rossignac GVU Center, College of Computing Georgia Institute of Technology. Cel Animations: Example. Foreground Cel. Background Cel. Animation. Motivation. Need to compress

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## Surface Simplification & Edgebreaker Compression for 2D Cel Animations

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**Surface Simplification & Edgebreaker Compression for 2D Cel**Animations Vivek Kwatra & Jarek Rossignac GVU Center, College of Computing Georgia Institute of Technology**Cel Animations: Example**Foreground Cel Background Cel Animation**Motivation**• Need to compress • To speed up transmission • Want to compress boundary of region • Image/video compression too expensive • Compress pixels instead of regions • MPEG, RLE, Octree, Gzip • Applications • Entertainment: Animations • Web-based instruction: manuals, tutorials, explanations • Visualization: weather, environmental, medical**Cel Animations: Observations**• Collection of regions • Uniform color per region • Only few colors/regions • Shape of region changes slowly with time • Want to encode shape instead of pixels**Iso-surface Extraction**Voxelization Playback Simplification Clipping Overview Compression Decompression Video**Prior Art (building blocks)**• Iso-surface extraction to build 3D model • Marching Cubes [Lorensen & Cline’87] • Tetrahedral Decomposition [Gueziec’95] • Surface simplification for lossy compression • [Hoppe’96, Lindstrom & Turk’98, Rossignac & Borrel’93, Ronfard & Rossignac’96…] • Geometric Compression • [King & Rossignac’99, King et al’00, Rossignac’99: Edgebreaker, Rossignac & Szymczak’99, Rossignac et al’01, Taubin & Rossignac’98, Touma & Gotsman’98…]**Iso-surface**Voxelization Simplification Clipping Playback Overview Compression Video Decompression**Voxelization**• Assume animation is segmented into regions • Pixels P(x,y,t) sample 3D image-time space • We could extract the black/white bounding faces • But it would be too jaggy • Given region R, we define a scalar field: • As t changes, R sweeps a volume V:**Iso-surface Extraction**• Compute S (bounding surface of V): • Interpolate F between pixel centers • piecewise constant • bilinear interpolation • linear interpolation(over triangular elements) • Use Marching tetrahedra [Gueziec’95]: simpler and more robust than marching cubes**Marching Tetrahedra**• Each cube is divided into five tetrahedra • The iso-surface of a tetrahedron is either a triangle or a quad • Consistent tetrahedral decomposition of cubes ensures a watertight mesh**Marching Tetrahedra (contd…)**• Tetrahedralization may produce sharp corners; depends upon the particular decomposition • Move corner away from the midpoint to smooth it out • Result is then independent of tetrahedralization**Iso-surface**Voxelization Simplification Clipping Playback Overview Compression Video Decompression**Odd parity**Odd parity Odd parity Simplification • Triangle count reduced by 95-99% for geometric error of less than 0.2% • Metro [Cignoni et al.’98] • Simplify using edge-collapse operations • [Lindstrom & Turk’98] • Degenerate cases • Surface folds onto itself • Manifold connectivity preserved • Surface remains watertight • Parity preserved**R**R R b L C L E a S R E R C Compression • Edgebreaker for connectivity compression • Compresses connectivity to 1 bit per triangle • Vertex quantization • Span the range represented by the animation • Predictive encoding for vertices • Parallelogram prediction rule • Compresses geometry to 13 bits per vertex ...CRSRLECRRRLE (2T bit code: C=0, L=110, R=101, S=100, E=111)**Iso-surface**Voxelization Playback Simplification Clipping Overview Compression Video Decompression**Animation Playback**• Clip the surface with the plane t = frame # • Paint the front-facing triangles (shown in green here) in color of background (black) • Paint the back-facing triangles (shown in gray) with the region color • Degenerate triangles can causing popping artifacts**Animation Playback (contd…)**• Parity checking logic improved to handle overlapping triangles • No need for z-buffer • Odd parity: point on clipping plane is inside the solid • Even parity: Front facing is visible • Hardware supported OpenGL stencil buffers to expedite process Parity: Even Odd Odd Even Clipping Plane View Plane**Summary**• Input: series of images (uniform regions) • Represent each region as pixels • Represent its evolution as volume of voxels • Compute 3D boundary as iso-surface • Simplify while preserving parity test • Compress using Edgebreaker • Use clipping and stencils to display slices**Conclusion & Future Work**• Conclusion • Improve cel animation compression • Ratios of 200 - 300 • Leverage existing techniques • Iso-surface extraction for binary voxels • Simplification • Compression • Future work • Compressing multiple regions together • Extensions to other applications: • medical imaging, weather visualization etc.

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