The PLY Polygon File Format --------------------------- Introduction ------------ This document presents the PLY polygon file format, a format for storing graphical objects that are described as a collection of polygons. Our goal is to provide a format that is simple and easy to implement but that is general enough to be useful for a wide range of models. The file format has two sub-formats: an ASCII representation for easily getting started, and a binary version for compact storage and for rapid saving and loading. We hope that this format will promote the exchange of graphical object between programs and also between groups of people. Overview -------- Anyone who has worked in the field of computer graphics for even a short time knows about the bewildering array of storage formats for graphical objects. It seems as though every programmer creates a new file format for nearly every new programming project. The way out of this morass of formats is to create a single file format that is both flexible enough to anticipate future needs and that is simple enough so as not to drive away potential users. Once such a format is defined, a suite of utilities (both procedures and entire programs) can be written that are centered around this format. Each new utility that is added to the suite can leverage off the power of the others. The PLY format describes an object as a collection of vertices, faces and other elements, along with properties such as color and normal direction that can be attached to these elements. A PLY file contains the description of exactly one object. Sources of such objects include: hand-digitized objects, polygon objects from modeling programs, range data, triangles from marching cubes (isosurfaces from volume data), terrain data, radiosity models. Properties that might be stored with the object include: color, surface normals, texture coordinates, transparency, range data confidence, and different properties for the front and back of a polygon. The PLY format is NOT intended to be a general scene description language, a shading language or a catch-all modeling format. This means that it includes no transformation matrices, object instantiation, modeling hierarchies, or object sub-parts. A typical PLY object definition is simply a list of (x,y,z) triples for vertices and a list of faces that are described by indices into the list of vertices. Most PLY files include this core information. Vertices and faces are two examples of "elements", and the bulk of a PLY file is its list of elements. Each element in a given file has a fixed number of "properties" that are specified for each element. The typical information in a PLY file contains just two elements, the (x,y,z) triples for vertices and the vertex indices for each face. Applications can create new properties that are attached to elements of an object. For example, the properties red, green and blue are commonly associated with vertex elements. New properties are added in such a way that old programs do not break when these new properties are encountered. Properties that are not understood by a program can either be carried along uninterpreted or can be discarded. In addition, one can create a new element type and define the properties associated with this element. Examples of new elements are edges, cells (lists of pointers to faces) and materials (ambient, diffuse and specular colors and coefficients). New elements can also be carried along or discarded by programs that do not understand them. File Structure -------------- This is the structure of a typical PLY file: Header Vertex List Face List (lists of other elements) The header is a series of carraige-return terminated lines of text that describe the remainder of the file. The header includes a description of each element type, including the element's name (e.g. "edge"), how many such elements are in the object, and a list of the various properties associated with the element. The header also tells whether the file is binary or ASCII. Following the header is one list of elements for each element type, presented in the order described in the header. Below is the complete ASCII description for a cube. The header of a binary version of the same object would differ only in substituting the word "binary_little_endian" or "binary_big_endian" for the word "ascii". The comments in brackets are NOT part of the file, they are annotations to this example. Comments in files are ordinary keyword-identified lines that begin with the word "comment". ply format ascii 1.0 { ascii/binary, format version number } comment made by anonymous { comments keyword specified, like all lines } comment this file is a cube element vertex 8 { define "vertex" element, 8 of them in file } property float32 x { vertex contains float "x" coordinate } property float32 y { y coordinate is also a vertex property } property float32 z { z coordinate, too } element face 6 { there are 6 "face" elements in the file } property list uint8 int32 vertex_index { "vertex_indices" is a list of ints } end_header { delimits the end of the header } 0 0 0 { start of vertex list } 0 0 1 0 1 1 0 1 0 1 0 0 1 0 1 1 1 1 1 1 0 4 0 1 2 3 { start of face list } 4 7 6 5 4 4 0 4 5 1 4 1 5 6 2 4 2 6 7 3 4 3 7 4 0 This example demonstrates the basic components of the header. Each part of the header is a carraige-return terminated ASCII string that begins with a keyword. Even the start and end of the header ("ply" and "end_header") are in this form. The characters "ply" must be the first four characters of the file, since they serve as the fileÕs magic number. Following the start of the header is the keyword "format" and a specification of either ASCII or binary format, followed by a version number. Next is the description of each of the elements in the polygon file, and within each element description is the specification of the properties. Then generic element description has this form: element property property property ... The properties listed after an "element" line define both the data type of the property and also the order in which the property appears for each element. There are three kinds of data types a property may have: scalar, string and list. Here is a list of the scalar data types a property may have: name type number of bytes --------------------------------------- int8 character 1 uint8 unsigned character 1 int16 short integer 2 uint16 unsigned short integer 2 int32 integer 4 uint32 unsigned integer 4 float32 single-precision float 4 float64 double-precision float 8 These byte counts are important and must not vary across implementations in order for these files to be portable. There is a special form of property definitions that uses the list data type: property list An example of this is from the cube file above: property list uint8 int32 vertex_index This means that the property "vertex_index" contains first an unsigned char telling how many indices the property contains, followed by a list containing that many integers. Each integer in this variable-length list is an index to a vertex. Another Example --------------- Here is another cube definition: ply format ascii 1.0 comment author: anonymous comment object: another cube element vertex 8 property float32 x property float32 y property float32 z property red uint8 { start of vertex color } property green uint8 property blue uint8 element face 7 property list uint8 int32 vertex_index { number of vertices for each face } element edge 5 { five edges in object } property int32 vertex1 { index to first vertex of edge } property int32 vertex2 { index to second vertex } property uint8 red { start of edge color } property uint8 green property uint8 blue end_header 0 0 0 255 0 0 { start of vertex list } 0 0 1 255 0 0 0 1 1 255 0 0 0 1 0 255 0 0 1 0 0 0 0 255 1 0 1 0 0 255 1 1 1 0 0 255 1 1 0 0 0 255 3 0 1 2 { start of face list, begin with a triangle } 3 0 2 3 { another triangle } 4 7 6 5 4 { now some quadrilaterals } 4 0 4 5 1 4 1 5 6 2 4 2 6 7 3 4 3 7 4 0 0 1 255 255 255 { start of edge list, begin with white edge } 1 2 255 255 255 2 3 255 255 255 3 0 255 255 255 2 0 0 0 0 { end with a single black line } This file specifies a red, green and blue value for each vertex. To illustrate the variable-length nature of vertex_index, the first two faces of the object are triangles instead of a single square. This means that the number of faces in the object is 7. This object also contains a list of edges. Each edge contains two pointers to the vertices that delinate the edge. Each edge also has a color. The five edges defined above were specified so as to highlight the two triangles in the file. The first four edges are white, and they surround the two triangles. The final edge is black, and it is the edge that separates the triangles. User-Defined Elements --------------------- The examples above showed the use of three elements: vertices, faces and edges. The PLY format allows users to define their own elements as well. The format for defining a new element is exactly the same as for vertices, faces and edges. Here is the section of a header that defines a material property: element material 6 property ambient_red uint8 { ambient color } property ambient_green uint8 property ambient_blue uint8 property ambient_coeff float32 property diffuse_red uint8 { diffuse color } property diffuse_green uint8 property diffuse_blue uint8 property diffuse_coeff float32 property specular_red uint8 { specular color } property specular_green uint8 property specular_blue uint8 property specular_coeff float32 property specular_power float32 { Phong power } These lines would appear in the header directly after the specification of vertices, faces and edges. If we want each vertex to have a material specification, we might add this line to the end of the properties for a vertex: property material_index int32 This integer is now an index into the list of materials contained in the file. It may be tempting for the author of a new application to invent several new elements to be stored in PLY files. This practice should be kept to a minimum. Much better is to try adapting common elements (vertices, faces, edges, materials) to new uses, so that other programs that understand these elements might be useful in manipulating these adapted elements. Take, for example, an application that describes molecules as collections of spheres and cylinders. It would be tempting define sphere and cylinder elements for the PLY files containing the molecules. If, however, we use the vertex and edge elements for this purpose (adding the radius property to each), we can make use of programs that manipulate and display vertices and edges. Clearly one should not create special elements for triangles and quadrilaterals, but instead use the face element. (c) 1998 Greg Turk