3D objects in the virtual world are represented by triangle meshes mapped with textures. In order to achieve fast interactivity and transmission, online games generally apply synthetic texture, which is designed to be small in size and often composed of simple duplicable patterns. However, synthetic texture or color per vertex is not sufficient to meet the requirements of other applications, such as Tele-health and virutal museum, which require real life high resolution texture. High resolution texture can exceed millions of pixels, and is significantly bigger than mesh data.

In general, a dense mesh with high resolution texture is more realistic than a sparse mesh with low resolution texture. However, due to limited resources, a compromise has to be made between quality and bandwidth. The goal is to achieve best-effort quality in a specified time period, taking limited bandwidth into account. The challenge is how to select an optimal or best-effort combination of mesh and texture data, so that the time limit given will not be exceeded or under-utilized.

Many Level-of-detail (LOD) or multiscale techniques have been discussed extensively in the literature. The basic idea is to display a coarse version initially, and a refined version gradually as the viewing distance decreases, or when a zoomed-in version of the 3D object is requested by the viewer. If 180 triangles can provide satisfactory quality, rendering 1800 triangles is a waste of resources. The application should be able to determine which version to display at a given viewing distance or zoom level.

Example:
(left) 180 triangles mesh half mapped with texture (right) 1800 triangles mesh half mapped with texture

Example: A head model displayed from fine to coarse (left to right)

Number of vertices are: 1872, 1187, 1128, 1048, 887 and 676.

Note that face features diminish gradually from fine to coarse.

Generation of LOD involves the reduction of number of faces on a mesh, associated with a reduction of texture quality. The question is:

• When should a simplified version be used?
  
• What to determine the priority of vertices to be removed?
  
• How to ensure there is no excessive simplification degrading visual fidelity?
  
• Given a time limit and fluctuating bandwidth, how to combine mesh and texture simplification in order to achieve the best result?"
  

Geometric metrics have traditionally been used to compare the performance of different simplification techniques. However, the perceptual metric has gained popularity in recent research. In fact, using the perceptual metric is more appropriate because visual fidelity of 3D objects is ultimately determined by the HVS. Human observers or judges are therefore invited in perceptual evaluation experiments in order to verify the results of simplification algorithms.

Perceptual evaluation experiments are designed to explore how online 3D objects are visualized by the human visual system.