nurb:Callback(which, func)

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. 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.

All callbacks receive a handle to the NURBS object as their first parameter. The second parameter depends on the callback type specified in which.

The legal callbacks are as follows:

#GLU_NURBS_BEGIN

The begin callback indicates the start of a primitive. The function receives an integer argument, which can be one of #GLU_LINES, #GLU_LINE_STRIP, #GLU_TRIANGLE_FAN, #GLU_TRIANGLE_STRIP, #GLU_TRIANGLES, or #GLU_QUAD_STRIP. The default begin callback function is Nil.

#GLU_NURBS_VERTEX

The vertex callback indicates a vertex of the primitive. The coordinates of the vertex are passed in a table parameter as four floating-point values. All the generated vertices have dimension 3; that is, homogeneous coordinates have been transformed into affine coordinates. The default vertex callback function is Nil.

#GLU_NURBS_NORMAL

The normal callback is invoked as the vertex normal is generated. The components of the normal are passed in a table parameter as three floating-point values. In the case of a NURBS curve, the callback function is effective only when the user provides a normal map (#GLU_MAP1_NORMAL). In the case of a NURBS surface, if a normal map (#GLU_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 #GLU_AUTO_NORMAL is enabled. The default normal callback function is Nil.

#GLU_NURBS_COLOR

The color callback is invoked as the color of a vertex is generated. The components of the color are passed in a table parameter. This callback is effective only when the user provides a color map (#GLU_MAP1_COLOR_4 or #GLU_MAP2_COLOR_4). The table passed to this callback contains four floating-point components: R, G, B, A. The default color callback function is Nil.

#GLU_NURBS_TEXTURE_COORD

The texture callback is invoked as the texture coordinates of a vertex are generated. These coordinates are passed in a table parameter. The number of texture coordinates can be 1, 2, 3, or 4 depending on which type of texture map is specified (#GLU_MAP1_TEXTURE_COORD_1, #GLU_MAP1_TEXTURE_COORD_2, #GLU_MAP1_TEXTURE_COORD_3, #GLU_MAP1_TEXTURE_COORD_4, #GLU_MAP2_TEXTURE_COORD_1, #GLU_MAP2_TEXTURE_COORD_2, #GLU_MAP2_TEXTURE_COORD_3, #GLU_MAP2_TEXTURE_COORD_4). If no texture map is specified, this callback function will not be called. The default texture callback function is Nil.

#GLU_NURBS_END

The end callback is invoked at the end of a primitive. The default end callback function is Nil. This callback doesn't receive any parameters except a handle to the NURBS object.

Please consult an OpenGL reference manual for more information.

which

specifies the callback being defined (see above)

func

specifies the function that the callback calls