Polygon File Format

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Polygon File Format
File extension : .ply
MIME type : text / plain
Magic number : PLY
Developed by: Greg Turk , Stanford University
Initial release: 1994
Current version: 1.0
Type: ASCII format / binary file
Container for: 3D models


3D models

The Polygon File Format ( PLY for short or also known as Stanford Triangle Format ) is a file format for storing three-dimensional data that was originally designed for use with 3D scanners .

The format is characterized by a simple description of individual objects as lists of polygons . Different attributes can be defined for the front and back of a polygon, such as: color , transparency , surface normals , texture coordinates or confidence values ​​of measured data.

A PLY file can be saved in ASCII format or as a binary file.

Stanford University's Digital Michelangelo Project used the PLY format for an extremely high-resolution 3D scan of Michelangelo's " David " sculpture.

File structure

The files begin with a header that specifies the elements of the mesh and their types. The list following the header contains elements such as vertices , triangles (faces) and edges.

In both the ASCII and the binary version of the file, the header always consists of ASCII characters, only the numerical part of the file is saved differently. The first line of the header consists only of the magic number : 

ply

The second line specifies the format of the file and should be one of the following: 

format ascii 1.0
format binary_little_endian 1.0
format binary_big_endian 1.0

The marking 1.0refers to the version number of the standard used. 1.0however, it is the only version currently in use.

The word commentat the beginning of a line allows you to insert comments, everything that follows on the line is ignored: 

comment Dies ist ein Kommentar!

The elementdescription of the elements used begins with the keyword, followed by their number.

For example, the following is in a file that contains 12 vertices, each of which is defined as a number triplet from (X, Y, Z) floats . 

element vertex 12
property float x
property float y
property float z

Further propertylines of an element could also describe other data such as color values. It looks like this, for example: 

property uchar red
property uchar green
property uchar blue

Please note that colors typically have the data type uchar(unsigned char, i.e. integers with a value range from 0 to 255). If colors are mentioned in the header, each line contains not only the number triple (X, Y, Z) for the vertex position, but also a number triple for the colors (R, G, B).

There are two different notations for data types, which are specified as scalar variables or as a list , as above .

char, uchar, short, ushort, int, uint, float, double are data types that can be used for scalar notation.

For the list notation you can choose from the purely numerical types int8, uint8, int16, uint16, int32, uint32, float32, float64.

An object with ten polygon faces can be described with the list notation as follows: 

element face 10
property list uchar int vertex_indices

The word listsignals the beginning of a list, the first entry in the list (in this case of the type uchar) contains the number of elements in the list. All of the following entries are of the type int.

The following line closes the header and is followed by the actual content of the file: 

end_header

example

The following describes a simple cube in a PLY file in ASCII format and comes from the original file structure definition by Greg Turk . 

ply
format ascii 1.0						{ ascii/binary, format version number }
comment made by Greg Turk				{ comments keyword specified, like all lines }
comment this file is a cube
element vertex 8						{ define "vertex" element, 8 of them in file }
property float x						{ vertex contains float "x" coordinate }
property float y						{ y coordinate is also a vertex property }
property float z						{ z coordinate, too }
element face 6							{ there are 6 "face" elements in the file }
property list uchar int vertex_indices	{ "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

ASCII or binary format

In the ASCII version of the format, vertices and faces are each described in one line, numerical values ​​are separated by spaces. In binary format, the numerical values ​​are simply summarized more compactly; the values ​​can continue to be interpreted individually thanks to the endian specified in the header and the given data types of the propertyattributes. The property listnotation for polygons provides for both versions that the first number indicates the vertex number of the polygon and the following list then lined up the vertex indices.

Emergence

The PLY format was developed by Greg Turk and others under the direction of Marc Levoy at Stanford University in the mid-1990s . The basic structure was inspired by the Wavefront OBJ format, which, however, did not offer the desired flexibility to formulate various additional information or groupings. As a result, the propertyand elementkeywords were introduced to generalize the description of vertices, polygons, associated data, and other groupings.

Related file formats

Scientific software

for unstructured triangular grids in PLY format:

Program libraries and source texts

Data collections

Individual evidence

  1. ^ A b Greg Turk: The PLY Polygon File Format. (No longer available online.) Archived from the original on December 4, 2016 ; accessed on February 5, 2017 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.dcs.ed.ac.uk
  2. ^ The Digital Michelangelo Project
  3. ↑ Hubert Mara and Bartosz Bogacz: Breaking the Code on Broken Tablets: The Learning Challenge for Annotated Cuneiform Script in Normalized 2D and 3D Datasets . In: Proceedings of the 15th International Conference on Document Analysis and Recognition ( ICDAR ) . Sydney, Australia 2019.