gluNurbsCallback
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## Name

gluNurbsCallback — define a callback for a NURBS object

## C Specification

`void `gluNurbsCallback`` | `(` | `GLUnurbs* `nurb`, GLenum `which`, _GLfuncptr `CallBackFunc`);`

## Python Specification

``gluNurbsCallback`` | `(` | ``nurb`, `which`, `CallBackFunc`) → `None``

## Parameters

`nurb`
  Specifies the NURBS object (created with `gluNewNurbsRenderer`).
`which`
  Specifies the callback being defined. Valid values are `GLU_NURBS_BEGIN`, `GLU_NURBS_VERTEX`, `GLU_NURBS_NORMAL`, `GLU_NURBS_COLOR`, `GLU_NURBS_TEXTURE_COORD`, `GLU_NURBS_END`, `GLU_NURBS_BEGIN_DATA`, `GLU_NURBS_VERTEX_DATA`, `GLU_NURBS_NORMAL_DATA`, `GLU_NURBS_COLOR_DATA`, `GLU_NURBS_TEXTURE_COORD_DATA`, `GLU_NURBS_END_DATA`, and `GLU_NURBS_ERROR`.
`CallBackFunc`
  Specifies the function that the callback calls.

## Description

`gluNurbsCallback` is used to define a callback to be used by a NURBS object. If the specified callback is already defined, then it is replaced. If `CallBackFunc` is `NULL`, then this callback will not get invoked and the related data, if any, will be lost.

Except the error callback, these callbacks are used by NURBS tessellator (when `GLU_NURBS_MODE` is set to be `GLU_NURBS_TESSELLATOR`) to return back the OpenGL polygon primitives resulting from the tessellation. Note that there are two versions of each callback: one with a user data pointer and one without. If both versions for a particular callback are specified then the callback with the user data pointer will be used. Note that ``userData'' is a copy of the pointer that was specified at the last call to `gluNurbsCallbackData`.

The error callback function is effective no matter which value that `GLU_NURBS_MODE` is set to. All other callback functions are effective only when `GLU_NURBS_MODE` is set to `GLU_NURBS_TESSELLATOR`.

The legal callbacks are as follows:

`GLU_NURBS_BEGIN`
  The begin callback indicates the start of a primitive. The function takes a single argument of type GLenum, which can be one of `GL_LINES`, `GL_LINE_STRIP`, `GL_TRIANGLE_FAN`, `GL_TRIANGLE_STRIP`, `GL_TRIANGLES`, or `GL_QUAD_STRIP`. The default begin callback function is `NULL`. The function prototype for this callback looks like: void begin ( GLenum type );
`GLU_NURBS_BEGIN_DATA`
  The same as the `GLU_NURBS_BEGIN` callback except that it takes an additional pointer argument. This pointer is a copy of the pointer that was specified at the last call to `gluNurbsCallbackData`. The default callback function is `NULL`. The function prototype for this callback function looks like: void beginData (GLenum type, void *userData);
`GLU_NURBS_VERTEX`
  The vertex callback indicates a vertex of the primitive. The coordinates of the vertex are stored in the parameter ``vertex''. All the generated vertices have dimension 3, that is, homogeneous coordinates have been transformed into affine coordinates. The default vertex callback function is `NULL`. The function prototype for this callback function looks like: void vertex ( GLfloat *vertex );
`GLU_NURBS_VERTEX_DATA`
  This is the same as the `GLU_NURBS_VERTEX` callback, except that it takes an additional pointer argument. This pointer is a copy of the pointer that was specified at the last call to `gluNurbsCallbackData`. The default callback function is `NULL`. The function prototype for this callback function looks like: void vertexData ( GLfloat *vertex, void *userData );
`GLU_NURBS_NORMAL`
  The normal callback is invoked as the vertex normal is generated. The components of the normal are stored in the parameter ``normal''. In the case of a NURBS curve, the callback function is effective only when the user provides a normal map (`GL_MAP1_NORMAL`). In the case of a NURBS surface, if a normal map (`GL_MAP2_NORMAL`) is provided, then the generated normal is computed from the normal map. If a normal map is not provided then a surface normal is computed in a manner similar to that described for evaluators when `GL_AUTO_NORMAL` is enabled. The default normal callback function is `NULL`. The function prototype for this callback function looks like: void normal ( GLfloat *normal );
`GLU_NURBS_NORMAL_DATA`
  The same as the `GLU_NURBS_NORMAL` callback except that it takes an additional pointer argument. This pointer is a copy of the pointer that was specified at the last call to `gluNurbsCallbackData`. The default callback function is `NULL`. The function prototype for this callback function looks like: void normalData ( GLfloat *normal, void *userData );
`GLU_NURBS_COLOR`
  The color callback is invoked as the color of a vertex is generated. The components of the color are stored in the parameter ``color''. This callback is effective only when the user provides a color map (`GL_MAP1_COLOR_4` or `GL_MAP2_COLOR_4`). ``color'' contains four components: R,G,B,A. The default color callback function is `NULL`. The prototype for this callback function looks like: void color ( GLfloat *color );
`GLU_NURBS_COLOR_DATA`
  The same as the `GLU_NURBS_COLOR` callback except that it takes an additional pointer argument. This pointer is a copy of the pointer that was specified at the last call to `gluNurbsCallbackData`. The default callback function is `NULL`. The function prototype for this callback function looks like: void colorData ( GLfloat *color, void *userData );
`GLU_NURBS_TEXTURE_COORD`
  The texture callback is invoked as the texture coordinates of a vertex are generated. These coordinates are stored in the parameter ``texCoord''. The number of texture coordinates can be 1, 2, 3, or 4 depending on which type of texture map is specified (`GL_MAP1_TEXTURE_COORD_1`, `GL_MAP1_TEXTURE_COORD_2`, `GL_MAP1_TEXTURE_COORD_3`, `GL_MAP1_TEXTURE_COORD_4`, `GL_MAP2_TEXTURE_COORD_1`, `GL_MAP2_TEXTURE_COORD_2`, `GL_MAP2_TEXTURE_COORD_3`, `GL_MAP2_TEXTURE_COORD_4`). If no texture map is specified, this callback function will not be called. The default texture callback function is `NULL`. The function prototype for this callback function looks like: void texCoord ( GLfloat *texCoord );
`GLU_NURBS_TEXTURE_COORD_DATA`
  This is the same as the `GLU_NURBS_TEXTURE_COORD` callback, except that it takes an additional pointer argument. This pointer is a copy of the pointer that was specified at the last call to `gluNurbsCallbackData`. The default callback function is `NULL`. The function prototype for this callback function looks like: void texCoordData (GLfloat *texCoord, void *userData);
`GLU_NURBS_END`
  The end callback is invoked at the end of a primitive. The default end callback function is `NULL`. The function prototype for this callback function looks like: void end ( void );
`GLU_NURBS_END_DATA`
  This is the same as the `GLU_NURBS_END` callback, except that it takes an additional pointer argument. This pointer is a copy of the pointer that was specified at the last call to `gluNurbsCallbackData`. The default callback function is `NULL`. The function prototype for this callback function looks like: void endData ( void *userData );
`GLU_NURBS_ERROR`
  The error function is called when an error is encountered. Its single argument is of type GLenum, and it indicates the specific error that occurred. There are 37 errors unique to NURBS named `GLU_NURBS_ERROR1` through `GLU_NURBS_ERROR37`. Character strings describing these errors can be retrieved with `gluErrorString`.

## Notes

`gluNurbsCallback` is available only if the GLU version is 1.2 or greater.

GLU version 1.2 supports only the `GLU_ERROR` parameter for `which`. The `GLU_ERROR` value is deprecated in GLU version 1.3 in favor of `GLU_NURBS_ERROR`. All other accepted values for `CallBackFunc` are available only if the GLU version is 1.3 or greater.

## See Also

gluErrorString, gluNewNurbsRenderer, gluNurbsCallbackData, gluNurbsProperty

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