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.
 Anything following a hash character (#) is a comment.
 An OBJ file may contain vertex data, free-form curve/surface attributes, elements, free-form curve/surface body statements, connectivity between free-form surfaces, grouping and display/render attribute information.   The most common elements are geometric vertices, texture coordinates, vertex normals and polygonal faces:
 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.
 Optionally, texture coordinate indices can be used to specify texture coordinates when defining a face.
To add a texture coordinate index to a vertex index when defining a face,
one must put a slash immediately after the vertex index and then put the 
texture coordinate index.  No spaces are permitted before or after the slash.
A valid texture coordinate index starts from 1 and matches the corresponding
element in the previously defined list of texture coordinates. 
Each face can contain three or more elements.
 Optionally, normal indices can be used to specify normal vectors for vertices when defining a face.
To add a normal index to a vertex index when defining a face, 
one must put a second slash after the texture coordinate index and then put
the normal index.
A valid normal index starts from 1 and matches the corresponding element
in the previously defined list of normals.
Each face can contain three or more elements.
 As texture coordinates are optional, one can define geometry without them, but
one must put two slashes after the vertex index before putting the normal index.
 Records starting with the letter "l" (lowercase L) specify the order of the vertices which build a polyline.
 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.
 Materials that describe the visual aspects of the polygons are stored in
external .mtl files.  More than one external MTL material file may be referenced from within the OBJ file.  The .mtl file may contain one or more named material definitions.
 This tag specifies the material name for the element following it. The material name matches a named material definition in an external .mtl file.
 Named objects and polygon groups are specified via the following tags.
 Smooth shading across polygons is enabled by smoothing groups.
 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 .MTL 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 .MTL files (called "material libraries"), and from there, references one or more material descriptions by name. .MTL 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.
 A single .mtl file may define multiple materials.  Materials are defined one after another in the file, each starting with the newmtl command:
 The ambient color of the material is declared using Ka.  Color definitions are in RGB where each channel's value is between 0 and 1.
 Similarly, the diffuse color is declared using Kd.
 The specular color is declared using Ks, and weighted using the specular exponent Ns.
 Materials can be transparent.  This is referred to as being dissolved.  Unlike real transparency, the result does not depend upon the thickness of the object. A value of 1.0 for "d" (dissolve) is the default and means fully opaque, as does a value of 0.0 for "Tr". Dissolve works on all illumination models.
 Transparent materials can additionally have a Transmission Filter Color, specified with "Tf".
 A material can also have an optical density for its surface.  This is also known as index of refraction.
 Values can range from 0.001 to 10.  A value of 1.0 means that light does not bend as it passes through an object.  Increasing the optical density 
increases the amount of bending.  Glass has an index of refraction of 
about 1.5.  Values of less than 1.0 produce bizarre results and are not 
recommended.
 Multiple illumination models are available, per material.  Note that it is not required to set a transparent illumination model in order to achieve transparency with "d" or "Tr", and in modern usage illum models are often not specified, even with transparent materials. The illum models are enumerated as follows:
 Textured materials use the same properties as above, and additionally define texture maps. Below is an example of a common material file. See the full Wavefront file format reference for more details.
 Texture map statements may also have option parameters (see full spec).
 For example,
 For reflection maps...
 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.
 The creators of the online 3D editing and modeling tool, Clara.io, proposed extending the MTL format to enable specifying physically-based rendering (PBR) maps and parameters.  This extension has been subsequently adopted by Blender and TinyObjLoader.  The extension PBR maps and parameters are:
 
 Further proposed extensions come from the DirectXMesh toolkit for Microsoft's DirectX engine, allowing the ability to define a model's pre-compiled RMA material.