INTERNATIONAL ORGANIZATION FOR STANDARDIZATION
ORGANISATION INTERNATIONALE DE NORMALISATION
ISO/IEC JTC 1/SC 29/WG 11
CODING OF MOVING PICTURES AND AUDIO

ISO/IEC JTC 1/SC 29/WG 11N7630

October 2005, Nice

Introduction

Recently, 3D mesh models are used in various multimedia applications such as computer game, animation, and simulation applications. To maintain a convincing level of realism, many applications require highly detailed complex models. However, such models demand broad bandwidth and much storage capacity to transmit and store. To address these problems, many 3-D mesh compression algorithms have been proposed by increasing the coding efficiency for 3D models.

 As one of the well-known conventional algorithms, three-dimensional mesh coding (3DMC) was introduced in MPEG-4 Visual, Version 2. 3DMC provides a representation and compression tool for IndexedFaceSet node of 3D objects onto which images and video may be mapped. 3D mesh coding is to compress static mesh models. The animation of 3D mesh models is possible by using key-frame animation (with Interpolators). 3DMC provides additional functionalities—such as high compression, incremental rendering, and error resilience—that are useful to many applications.

Technology overview

The MPEG-4 3D mesh object is a compressed bitstream of the IndexedFaceSet VRML/BIFS node. Major components in IndexedFaceSet are these:

• Connectivity: A 3D mesh consists of connected polygons. As for 2D meshes, the way to form a polygon from the given vertices is called connectivity information. Using a wireframe, it is easy to see how polygons form a 3D mesh.
• Geometry: As for 2D meshes, the 3D coordinates of the nodes or vertices are called geometry. The coordinates are represented in the Cartesian coordinate system. A basic 3D model can be generated with only connectivity and geometry information. Therefore, these two information are the most important components in 3D mesh coding.
•   Other properties: In designing the appearance of the 3D mesh object, one can add colors, normals, and texture coordinates on the top of the model representation.

3DMC scheme

3DMC comprises three major coding blocks: topological surgery (data transformation); differential quantization of connectivity, geometry, and other properties (quantization); and entropy coding. In figure 1, the 3DMC decoder architecture is presented.

Figure 1. 3DMC decoder architecture

Functionalities

3DMC provides 30:1 to 40:1 compression ratio without noticeable visual degradation. However, compression is not the only advantage to using 3DMC. The following functionalities are supported by 3DMC:

n        Compression: Near-lossless to lossy compression of 3D models is supported. Usually 30:1 to 40:1 compression ratio over a VRML ASCII file can be achieved without visual degradation.

n        Incremental rendering: With 3DMC, there is no need to wait until the complete bitstream is received to start rendering it. With the incremental rendering capability, the decoder can begin building the model with just a fraction of the entire bitstream. This functionality is important when the latency is a critical issue, such as for home shopping.

n        Support for nonmanifold models: Because of the compression characteristic using 3DMC topological surgery, only orientable[1] and manifold[2] models are supported. For nonorientable or nonmanifold[3] models, a dedicated operation called stitching is performed to support these models.

n        Support for error resilience: With a built-in error-resilience capability, 3DMC can suffer less from network errors, as the decoder can build a model from the partitions that are not corrupted by the errors.

n        Support for progressive transmission: 3D mesh models can be quite complex, with millions of polygons. Depending on the viewing distance, the user may not need million-triangle accuracy, but may be satisfied with hundreds of triangles. A scalable bitstream similar to LOD (level of detail) representation allows building 3D models with different resolutions to serve such a case.

Summary

3D mesh compression is one of the very first attempts to address compression of 3D objects. 3DMC can be efficiently used and applied with AFX tools, where 3DMC can be utilized to represent static models as well as some animated 3D models.

References

[1]       MPEG SNHC Homepage, http://www.sait.samsung.co.kr/snhc.

[2]       ISO/IEC 14772-1, “The Virtual Reality Modeling Language,” 1997, www.vrml.org/Specifications/VRML97.

[3]       Taubin, G., W.P. Horn, F. Lazarus, and J. Rossignac, “Geometric Coding and VRML.” Proceedings of the IEEE, July 1998.

[4]       Taubin G., J. Rossignac, “Course on 3D Geometry Compression.” SIGGRAPH’99, Los Angeles 1999.

[5]       Jang, Euee S., “3D Animation Coding-its History and Framework.” Proceedings of International Conference of Multimedia and Expo 2000, New York. July 2000.

[6]       Fernando Pereira and Touradj Ebrahimi, The MPEG-4 Book, Prentice Hall, 2002.

[7]       Aaron E. Walsh and Mikael Bourges-Sevenier, MPEG-4 Jump-Start, Prentice Hall, 2002.

[8]       Craig Gotsman, Stefan Gumhold, and Leif Kobbelt, “Simplification and Compression of 3D Meshes.” Tutorials on Multiresolution in Geometric Modelling, A. Iske, E. Quak, M.S. Floater (Eds.), Springer-Verlag, Hidelberg, 2002.