vertices which build a polyline. l v1 v2 v3 v4 v5 v6 ... Other geometry formats 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. Reference materials Materials that describe the visual aspects of the polygons are stored in external files. More than one external MTL material file may be referenced from within the OBJ file. The file may contain one or more named material definitions. mtllib [external .mtl file name] ... This tag specifies the material name for the element following it. The material name matches a named material definition in an external file. usemtl [material name] ... Named objects and polygon groups are specified via the following tags. o [object name] ... g [group name] ... Smooth shading across polygons is enabled by smoothing groups. s 1 ... # Smooth shading can be disabled as well. s off ... Relative and absolute indices 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. Material template library 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 The MTL format, although still widely used, is outdated and does not fully support later technologies such as specular maps and parallax maps.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