Vertex (computer graphics)

A vertex (plural vertices) in computer graphics is a data structure that describes certain attributes, like the position of a point in 2D or 3D space, at multiple points on a surface.

Application to 3D models

3D models are most-often represented as triangulated polyhedra forming a triangle mesh. Non triangular surfaces can be converted to an array of triangles through tessellation. Attributes from the vertices are typically interpolated across mesh surfaces.

Vertex attributes

The vertices of triangles are associated not only with spatial position but also with other values used to render the object correctly. Most attributes of a vertex represent vectors in the space to be rendered. These vectors are typically 1 (x), 2 (x, y), or 3 (x, y, z) dimensional and can include a fourth homogeneous coordinate (w). These values are given meaning by a material description. In realtime rendering these properties are used by a vertex shader or vertex pipeline.

Such attributes can include:

Position

2D or 3D coordinates representing a position in space

Color

Typically diffuse or specular RGB values, either representing surface colour or precomputed lighting information.

Reflectance

of the surface at the vertex, e.g. specular exponent, metallicity, frenel values.

Texture coordinates

Also known as UV coordinates,these control the texture mapping of the surface, possibly for multiple layers.

normal vectors

These define an approximated curved surface at the location of the vertex, used for lighting calculations (such as Phong shading), normal mapping, or displacement mapping, and to control subdivision.

tangent vectors

These define an approximated curved surface at the location of the vertex, used for lighting calculations (such as Phong shading), normal mapping, or displacement mapping, and to control subdivision.

Blend weights

Bone weights

Weighting for assignment to bones to control deformation in skeletal animation.

Blend shapes

Multiple position vectors may be specified to be blended over time, especially for facial animation.

Vertex attributes in OpenGL

The following is a table of built in attributes of vertices in the OpenGL standard.

OpenGL vertex attributes^[1]

GL attribute name	attribute defined (data value size)
gl_Vertex	Position (vec4)
gl_Normal	Normal (vec4)
gl_Color	Primary color of vertex (vec4)
gl_MultiTexCoord0	Texture coordinate of texture unit 0 (vec4)
gl_MultiTexCoord1	Texture coordinate of texture unit 1 (vec4)
gl_MultiTexCoord2	Texture coordinate of texture unit 2 (vec4)
gl_MultiTexCoord3	Texture coordinate of texture unit 3 (vec4)
gl_MultiTexCoord4	Texture coordinate of texture unit 4 (vec4)
gl_MultiTexCoord5	Texture coordinate of texture unit 5 (vec4)
gl_MultiTexCoord6	Texture coordinate of texture unit 6 (vec4)
gl_MultiTexCoord7	Texture coordinate of texture unit 7 (vec4)
gl_FogCoord	Fog Coord (float)

See also

For how vertices are processed on 3D graphics cards, see shader.

References

1. Christen, Martin. "Clockworkcoders Tutorials: Vertex Attributes". Khronos Group. Retrieved 26 January 2009.