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Geometry Compression

Geometry Compression. Michael Deering, Sun Microsystems SIGGRAPH (1995) Presented by: Michael Chung. Geometry Compression What is it?. Lossy technique for reducing the size of geometry representation. Motivation. Save bandwidth and transmission time in graphics accelerators and networks.

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Geometry Compression

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  1. Geometry Compression Michael Deering, Sun Microsystems SIGGRAPH (1995) Presented by: Michael Chung

  2. Geometry CompressionWhat is it? • Lossy technique for reducing the size of geometry representation.

  3. Motivation • Save bandwidth and transmission time in graphics accelerators and networks. • Save storage space in main memory and on disk.

  4. Proposed Contributions • Technique for lossy compression ratios of between 6 and 10 to 1 • Claims only slight losses in object quality • Depends on original representation format and final quality level desired

  5. Geometry CompressionWhat is it? • Trade-off between quality (subjective) and amount of compression. • Compression steps can be reversed for decompression

  6. Geometry CompressionWhat is it? • Goal: represent geometry with geometry compression instructions

  7. Insights • Reduce size of geometry representation in several ways. • Reuse vertices in triangle strip via reference • Bit shaving • Geometry is local, encode deltas • Normals as indices

  8. Compression Steps • Convert triangle data to generalized triangle mesh • Quantization of positions, colors, normals • Delta encoding of quantized values • Huffman tag-based variable-length encoding of deltas • Output binary output stream with Huffman table initializations and geometry compression instructions

  9. Compression Steps • Convert triangle data to generalized triangle mesh • Quantization of positions, colors, normals • Delta encoding of quantized values • Huffman tag-based variable-length encoding of deltas • Output binary output stream with Huffman table initializations and geometry compression instructions

  10. Step 1: Conversion to Generalized Triangle Mesh generalized triangle strip  generalized triangle mesh • Generalized triangle strip • Specifies vertices with four vertex replacement codes (2 bits): • Replace oldest • Replace middle • Restart clockwise • Restart counterclockwise

  11. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle strip (example)

  12. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle strip (example)

  13. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle strip (example)

  14. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle strip (example)

  15. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle strip (example)

  16. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle strip (example)

  17. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle strip (example)

  18. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle strip (example)

  19. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle strip (example)

  20. Geometry Compression Instruction Set

  21. Geometry Compression Instruction Set

  22. Geometry Compression Instruction Set

  23. Step 1: Conversion to Generalized Triangle Mesh generalized triangle strip  generalized triangle mesh • Generalized triangle mesh • Generalized triangle strip • Mesh buffer • 16 slot queue • 4 bit index • Explicitly push vertices onto mesh buffer for reuse. • We save because only 4 bits are required to reference old vertex.

  24. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle mesh (example)

  25. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle mesh (example)

  26. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle mesh (example)

  27. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle mesh (example)

  28. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle mesh (example)

  29. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle mesh (example)

  30. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle mesh (example)

  31. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle mesh (example)

  32. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle mesh (example)

  33. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle mesh (example)

  34. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle mesh (example)

  35. Step 1: Conversion to Generalized Triangle Mesh • Generalized triangle mesh (example)

  36. Step 1: Conversion to Generalized Triangle Mesh

  37. Step 1: Conversion to Generalized Triangle Mesh

  38. Geometry Compression Instruction Set

  39. Compression Steps • Convert triangle data to generalized triangle mesh • Quantization of positions, colors, normals • Delta encoding of quantized values • Huffman tag-based variable-length encoding of deltas • Output binary output stream with Huffman table initializations and geometry compression instructions

  40. Step 2: Quantization

  41. Step 2: Quantization

  42. Step 2: Quantization • Some parts of the geometry may require more or less precision than others.

  43. Step 2: Quantization • Some parts of the geometry may require more or less precision than others. • So, the amount of quantization we perform per position, normal, and color is variable.

  44. Step 2: Quantization • Position • 32-bit floating-point coordinates are wasteful. • 8-bit exponent allows an unnecessary range of values. • 24-bit fixed-point mantissa offers unnecessary precision.

  45. Step 2: Quantization • Position • Based on empirical visual tests, allow at most 16 bits per component (X, Y, Z)

  46. Step 2: Quantization • Color • Linear reflectivity values R, G, B, (optional) A • Range from 0.0 to 1.0 per component • cap state bit sets alpha ON and OFF • At most 12 unsigned fraction bits per component

  47. Geometry Compression Instruction Set

  48. Step 2: Quantization • Normal • 96 bits can represent up to 296 different normals • We don’t need so many • Angular density of 0.01 radians between normals visually indistinguishable • This is about 100,000 normals distributed over a unit sphere • 48 bits to represent a normal (16 bits per X, Y, Z) • We can do better than 48 bits per normal • Use clever indexing to represent ~100,000 normals with 18 bits…

  49. Step 2: Quantization • Normal • 96 bits can represent up to 296 different normals • We don’t need so many • Angular density of 0.01 radians between normals visually indistinguishable • This is about 100,000 normals distributed over a unit sphere • 48 bits to represent a normal (16 bits per X, Y, Z) • We can do better than 48 bits per normal • Use clever indexing to represent ~100,000 normals with 18 bits…

  50. Step 2: Quantization • Normal • 96 bits can represent up to 296 different normals • We don’t need so many • Angular density of 0.01 radians between normals visually indistinguishable • This is about 100,000 normals distributed over a unit sphere • 48 bits to represent a normal (16 bits per X, Y, Z) • We can do better than 48 bits per normal • Use clever indexing to represent ~100,000 normals with 18 bits…

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