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Mesh, 3D

objects coding object meshes

Definition: Mesh is a 3D object representation consisting of a collection of vertices and polygons.

The 3D object representation can be a polygon mesh, which consists of a collection of vertices and polygons that define the shape of an object in 3D (Figure 1). To benefit from current 3D graphic cards hardware acceleration polygon meshes usually contain only triangles because the hardware is optimized for triangle mesh computation.

Polygon meshes, as a 3D surface representation with 3D points and connectivity, have initially been considered for compression in a static way. The contributions were focused on compression of vertex positions and connectivity. Static mesh coding, exploiting spatial dependencies of adjacent polygons, is also currently part of MPEG-4. The MPEG-4 computer graphics part AFX (Animation Framework extension) offers a tool called AFX Interpolator Compression which allows the compression of such a linear keyframe animation.

Animated sequences of 3D objects stem from a number of different applications, starting with pure virtual Computer Graphics objects that deform over time. One of the newer application fields includes mixed-reality object reconstruction, where natural images are captured to reconstruct 3D objects with natural appearance for virtual environments. Those objects are referred to as “3D video objects” or 3DVOs, a term that describes a new representation format for visual media. It allows free navigation around real world dynamic objects by choosing arbitrary viewpoints and viewing directions similar to navigation in virtual-reality environments. 3DVOs are an extension of classical computer graphic objects towards representing motion and appearance of real world moving objects. Similar to animated synthetic objects, there is a 3D geometry, which changes over time. Animation however refers to a 3D geometry that applies a deformation while representing the same object with constant topology. 3DVOs require a more general approach of change over time, as there is not only geometry deformation, but also topology changes. Even new scene content, analogue to scene cuts in 2D films may appear. A coding scheme must therefore consider these general requirements. This can be achieved by coding dynamic meshes as separated groups of meshes (GOMs) with constant connectivity over a certain period of time within each GOM. In this case conventional static mesh coding (e.g. MPEG-4 3DMC) can be used for the Intra-Mesh (I-Mesh) coding and a predictive coding scheme can be applied for the positions of the vertices for all following P-Meshes of each GOM.

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