Compression of Cell Animation

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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
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Cel Animations Example
Foreground Cel
Background Cel
Animation
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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

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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

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Overview
Compression
Decompression
Video
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Prior Art (building blocks)

• Iso-surface extraction to build 3D model
• Marching Cubes Lorensen Cline87
• Tetrahedral Decomposition Gueziec95
• Surface simplification for lossy compression
• Hoppe96, Lindstrom Turk98, Rossignac
  Borrel93, Ronfard Rossignac96
• Geometric Compression
• King Rossignac99, King et al00,
  Rossignac99 Edgebreaker, Rossignac
  Szymczak99, Rossignac et al01, Taubin
  Rossignac98, Touma Gotsman98

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Overview
Compression
Video
Decompression
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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

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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 Gueziec95 simpler and
  more robust than marching cubes

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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

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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

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Overview
Compression
Video
Decompression
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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 Turk98
• Degenerate cases
• Surface folds onto itself
• Manifold connectivity preserved
• Surface remains watertight
• Parity preserved

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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 C0, L110, R101,
S100, E111)
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Overview
Compression
Video
Decompression
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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

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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
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Results
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Results (contd)
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Show Animation
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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

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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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• Thank You