Content for  TR 26.928  Word version:  18.0.0

Polygon meshes may be represented in a variety of ways, using different methods to store the vertex, edge and face data such as face-vertex, winged, half or quad-edge meshes, corner-table or vertex-vertex meshes. Different formats also store other per vertex and materials related data in different ways. Each of the representations have particular advantages and drawbacks. The choice of the data structure is governed by the application, the required performance, the size of the data, and the operations to be performed. For example, it is easier to deal with triangles than general polygons. For certain operations it is necessary to have a fast access to topological information such as edges or neighboring faces; this requires more complex structures such as the winged-edge representation. For hardware rendering, compact, simple structures are needed and are as such commonly incorporated into low-level rendering APIs such as DirectX and OpenGL. Many different formats for storage and data formats exist for storing polygon mesh data, as an example PLY-format is introduced below, because it provides 3D data in a human readable format. In practice PLY-format is rarely used in real-time rendering. Typically the data format is configured to the need of the 3D engine and as such the landscape is littered with proprietary 3D formats. 3D formats may be roughly divided in two categories, run time friendly formats such as glTF and formats which enable transferring 3D assets between systems like PLY.

The PoLYgon (PLY) format (see http://paulbourke.net/dataformats/ply/) is used to describe a 3D object as a list of vertices, faces and other elements, along with associated attributes. A single PLY file describes exactly one 3D object. The 3D object may be generated synthetically or captured from a real scene. Attributes of the 3D object elements that might be stored with the object include: colour, surface normals, texture coordinates, transparency, etc. The format permits one object to have different properties for the front and back of a polygon. The PLY does not intend to act as a Scene Graph, so it does not include transformation matrices, multiple 3D objects, modeling hierarchies, or object sub-parts. A typical PLY object definition is a list of (x,y,z,r,g,b) triples for vertices and their colour attributes (r,g,b), so they represent a point cloud. It may also include a list of faces that are described by indices into the list of the vertices. Vertices and faces are primary elements of the 3D object representation. PLY allows applications to create new attributes that are attached to the elements of an object. New attributes are appended to the list of attributes of an element, in a way to maintain backwards compatibility. Attributes that are not understood by a parser are simply skipped. Furthermore, PLY allows for extensions to create new element types and their associated attributes. Examples of such elements could be materials (ambient, diffuse and specular colours and coefficients). New elements can also be discarded by programs that do not understand them. A PLY file is structured as follows:

  Header
  Vertex List
  Face List
  (lists of other elements)

The header is a human-readable textual description of the PLY file. It contains a description of each element type, including the element's name (e.g. "vertex"), how many of such elements are in the object, and a list of the various attributes associated with the element. The header also indicates whether the file is in binary or ASCII format. A list of elements for each element type follows the header in the order described in the header. The following is an example PLY in binary format with 19928 vertices and 39421 faces: This example demonstrates the different components of a PLY file header. Each part of the header is a carriage-return terminated ASCII string that begins with a keyword. In case of binary representation, the file will be a mix of an ASCII header and binary representation of the elements, in little or big endian, depending on the architecture on which the PLY file has been generated. The PLY file must start with the characters "ply". The vertex attributes listed in this example are the (x,y,z) floating point coordinates, the (nx,ny,nz) representation of the normal vectors, a 32 bit flag mask, (r,g,b) 8-bit representations of the colour of each vertex, an 8-bit representation of the transparency alpha channel. Faces are represented as a list of vertex indices with a flags attribute associated with each face.