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. Display objects are composed of arrays of flat surfaces (typically triangles) and vertices define the location and other attributes of the corners of the surfaces.
The attributes can represent pretty much anything, e.g. temperature, velocity or direction.
Application to object models
In computer graphics, objects are most-often represented as triangulated polyhedra. Non triangular surfaces can be converted to an array of triangles through tessellation. The vertices of triangles are associated not only with position but also with other graphical attributes used to render the object correctly. Such attributes can include color at the vertex point, reflectance of the surface at the vertex, textures of the surface at the vertex, and the normal of an approximated curved surface at the location of the vertex. These properties are used in rendering by a vertex shader or vertex pipeline.
The normal can be used to determine a surface's orientation toward a light source for flat shading using Lambert's cosine law, or the orientation of each of the vertices to mimic a curved surface with Phong shading.
Vertex attributes
Most attributes of a vertex represent vectors in the space to be rendered. Vectors can be 1 (x), 2 (x, y), or 3 (x, y, z) dimensional and can include a fourth homogeneous coordinate (w).
The following is a table of built in attributes of vertices in the OpenGL standard.
| 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
- ↑ Christen, Martin. "Clockworkcoders Tutorials: Vertex Attributes". Khronos Group. Retrieved 26 January 2009.