Формат файлов геометрии OBJ: Описание и особенности

Geometry definition file format

OBJ (or OBJ) is a geometry definition file format first developed by Wavefront Technologies for its Advanced Visualizer animation package. The file format is open and has been adopted by other 3D graphics application vendors. The OBJ file format is a simple data format that represents 3D geometry alone — namely, the position of each vertex, the UV position of each texture coordinate vertex, vertex normals, and the faces that make each polygon defined as a list of vertices, and texture vertices. Vertices are stored in a counter clockwise order by default, making explicit declaration of face normals unnecessary. OBJ coordinates have no units, but OBJ files can contain scale information in a human readable comment line.

A vertex is specified via a line starting with the letter v. That is followed by (x,y,z[,w]) coordinates. W is optional and defaults to 1.0. A right hand coordinate system is used to specify the coordinate locations. Some applications support vertex colors, by putting red, green and blue values after x y and z (this precludes specifying w). The color values range from 0 to 1.

A free form geometry statement can be specified in a line starting with the string vp. Define points in parameter space of a curve or surface. u only is required for curve points, u and v for surface points and control points of non rational trimming curves, and u, v and w (weight) for control points of rational trimming curves.

Faces are defined using lists of vertex, texture and normal indices in the format vertex index/texture index/normal index for which each index starts at 1 and increases corresponding to the order in which the referenced element was defined. Polygons such as quadrilaterals can be defined by using more than three indices. OBJ files also support free form geometry which use curves and surfaces to define objects, such as NURBS surfaces.

A valid vertex index matches the corresponding vertex elements of a previously defined vertex list. If an index is positive then it refers to the offset in that vertex list, starting at 1. If an index is negative then it relatively refers to the end of the vertex list, 1 referring to the last element. Each face can contain three or more vertices. f v1 v2 v3

Records starting with the letter "l" (lowercase L) specify the order of the vertices which build a polyline. l v1 v2 v3 v4 v5 v6

Obj files support higher order surfaces using several different kinds of interpolation, such as Taylor and B splines, although support for those features in third party file readers is far from universal. Obj files also do not support mesh hierarchies or any kind of animation or deformation, such as vertex skinning or mesh morphing.

OBJ files, due to their list structure, are able to reference vertices, normals, etc. either by their absolute position (1 represents the first defined vertex, N representing the Nth defined vertex), or by their relative position ( 1 represents the latest defined vertex). However, not all software supports the latter approach, and conversely some software inherently writes only the latter form (due to the convenience of appending elements without needing to recalculate vertex offsets, etc. ), leading to occasional incompatibilities.

The Material Template Library format (MTL) or File Format is a companion file format to OBJ, also defined by Wavefront Technologies, that describes surface shading (material) properties of objects within one or more OBJ files. A OBJ file references one or more files (called "material libraries"), and from there, references one or more material descriptions by name. files are ASCII text that define the light reflecting properties of a surface for the purposes of computer rendering, and according to the Phong reflection model. The standard has widespread support among different computer software packages, making it a useful format for interchange of materials. The MTL format, although still widely used, is outdated and does not fully support later technologies such as specular maps and parallax maps. However, due to the open and intuitive nature of the format, these can easily be added with a custom MTL file generator. The MTL format defines a number of formats.

Because of the ease in parsing the files, and the unofficial spreading of the file format, files may contain vendor specific alterations. According to the spec, options are supposed to precede the texture filename. However, at least one vendor generates files with options at the end. # bump multiplier of 0.2
bump texbump. tga bm 0.2