Name
	EXT_nurbs_tessellator

Name Strings

	GLU_EXT_nurbs_tessellator

Version

	Date: 1996/04/19

Number
	
	100

Dependencies

	None

Overview

       This extension provides a mechanism for applications to retrieve a
       tessellation of a GLU NURBS curve or surface.  The tessellation consists of
       OpenGL Begin,End,Color,Normal,Texture, and Vertex data.

       This feature is useful for those applications which need to cache
       the primitives to use their own advanced shading model, or to
       accelerate frame rate or to perform other computations on the surface or
       curve data.

Issues

   * Is it a good idea to transform all the homogeneous coordinates into
     affine ones? Are there any circumstances where the user prefers
     to have the homogeneous coordinates?

New Procedures and Functions	

     gluNurbsCallbackDataEXT(GLUnurbsObj* theNurb, 
	             void* userData);

New Tokens

      Accepted by the <property> parameter of NurbsProperty:

	GLU_NURBS_MODE_EXT             100160

      Accepted by the <value> parameter of NurbsProperty:

        GLU_NURBS_TESSELLATOR_EXT      100161
        GLU_NURBS_RENDERER_EXT         100162
  

      Accepted by the <which> parameter of NurbsCallback:

	GLU_NURBS_BEGIN_EXT	               100164
	GLU_NURBS_VERTEX_EXT                   100165
	GLU_NURBS_NORMAL_EXT                   100166
	GLU_NURBS_COLOR_EXT                    100167
	GLU_NURBS_TEXTURE_COORD_EXT            100168
	GLU_NURBS_END_EXT                      100169

	GLU_NURBS_BEGIN_DATA_EXT	       100170
	GLU_NURBS_VERTEX_DATA_EXT              100171
	GLU_NURBS_NORMAL_DATA_EXT              100172
	GLU_NURBS_COLOR_DATA_EXT               100173
	GLU_NURBS_TEXTURE_COORD_DATA_EXT       100174
	GLU_NURBS_END_DATA_EXT                 100175


Additions to the GLU Specification version 1.2, section 7.1

    NURBS curves and surfaces are converted to OpenGL primitives
    by the function described in this section.  The interface employs
    a NURBS object to describe the curves and surfaces and to specify how
    they should be rendered. Basic trimming support is included to allow
    more flexible definition of surfaces.

    There are two ways to handle a NURBS object (curve or surface), to render or to tessellate.
    In rendering mode, the objects are converted or
    tessellated to a sequence of OpenGL primitives such as evaluators and
    triangles and sent to the OpenGL pipeline for rendering.  In tessellation
    mode, objects are converted to a sequence of triangles and triangle
    strips and returned back to the application through a callback
    interface for further processing.  The decomposition algorithm used for
    rendering and for returning tessellations are not guaranteed to produce
    identical results.

    
Additions to the GLU Specification version 1.2, section 7.2

    7.2 Callbacks

    To define a callback for a NURBS object use:
    void gluNurbsCallback(GLUnurbsObj *nurbsObj, GLenum which, void (*fn)());

    The parameter <which> can be one of the following:
    GLU_NURBS_BEGIN_EXT, GLU_NURBS_VERTEX_EXT, GLU_NORMAL_EXT, 
    GLU_NURBS_COLOR_EXT, GLU_NURBS_TEXTURE_COORD_EXT, GLU_END_EXT,
    GLU_NURBS_BEGIN_DATA_EXT, GLU_NURBS_VERTEX_DATA_EXT, GLU_NORMAL_DATA_EXT, 
    GLU_NURBS_COLOR_DATA_EXT, GLU_NURBS_TEXTURE_COORD_DATA_EXT, GLU_END_DATA_EXT, 
    and GLU_ERROR,

    These callbacks have the following prototypes:

    void begin(GLenum type);
    void vertex(GLfloat *vertex);
    void normal(GLfloat *normal);
    void color(GLfloat *color);
    void texCoord(GLfloat *tex_coord);
    void end(void);

    void beginData(GLenum type, void* userData);
    void vertexData(GLfloat *vertex, void* userData);
    void normalData(GLfloat *normal, void* userData);
    void colorData(GLfloat *color, void* userData);
    void texCoordData(GLfloat *tex_coord, void* userData);
    void endData(void* userData);

    void error(GLenum errno);
    
    The first 12 callbacks are for the user to get the primitives back from the
    NURBS  tessellator when the GLU_NURBS_MODE_EXT is set to GLU_NURBS_TESSELLATOR_EXT. 
    These callbacks have no effects when the GLU_NURBS_MODE_EXT is set to GLU_NURBS_RENDERER_EXT.

    There are two forms of each callback: one with a pointer to application
    supplied data and one without. If both versions of a particular callback
    are specified then the callback with <userData> will be used. <userData>
    is a copy of the pointer that was specified at the last call to 
    gluNurbsCallbackDataEXT.

    All callback functions can be set to NULL even when GLU_NURBS_MODE_EXT is set to
    GLU_NURBS_TESSELLATOR_EXT.  When a callback function is set to NULL,
    this callback function will not get invoked and the related data, if any, will be lost.
    
    The <begin> callback indicates the start of a primitive. <type> is
    one of GL_LINES, GL_LINE_STRIPS, GL_TRIANGLE_FAN, GL_TRIANGLE_STRIP, GL_TRIANGLES, 
    or GL_QUAD_STRIP.
    The default begin callback function is NULL.

    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 <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 (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 AUTO_NORMAL
    is enabled. The  default normal callback function is NULL.

    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 <texCoord> callback is invoked as the texture coordinates 
    of a vertex are generated. These coordinates are stored in the parameter
    <tex_coord>. The number of texture coordinates can be 1,2,3,or 4 depending
    on which type of texture map is specified (GL_MAP*_TEXTURE_COORD_1,
    GL_MAP*_TEXTURE_COORD_2, GL_MAP*_TEXTURE_COORD_3, GL_MAP*_TEXTURE_COORD_4 where
    * can be either 1 or 2).
    If no texture map is specified, this callback function will not be called.
    The default texCoord callback function is NULL.

    The <end> callback is invoked at the end of a primitive. 
    The default end callback function is NULL.

    The <error> callback is invoked when a NURBS function detects an error
    condition. There are 37 errors specific to NURBS functions, and they are 
    named GLU_NURBS_ERROR1 through GLU_NURBS_ERROR37. Strings describing
    the meaning of these error codes can be retrieved with gluErrorString.

Additions to the GLU Specification version 1.2, section 7.6

     NURBS Property
     --------------

     A set of properties associated with a NURBS object affects
     the way that NURBS are rendered or tessellated. These properties can be
     adjusted by the user.

     void gluNurbsProperty(GLnurbsObj *nurbsObj, GLenum property,
                           GLfloat value);

    allows the user to set one of the following properties: GLU_CULLING,
    GLU_SAMPLING_TOLERANCE, GLU_SAMPLING_METHOD, GLU_PARAMETRIC_TOLERANCE,
    GLU_DISPLAY_MODE, GLU_AUTO_LOAD_MATRIX, GLU_U_STEP,  GLU_V_STEP, and 
    GLU_NURBS_MODE_EXT. property indicates the property to be
    modified, and value specifies the new value.  

    GLU_NURBS_MODE_EXT property should be set to be either GLU_NURBS_RENDERER_EXT
    or GLU_NURBS_TESSELLATOR_EXT. When set to GLU_NURBS_RENDERER_EXT, NURBS
    objects are tessellated into openGL primitives and sent to the pipeline for
    rendering. When set to GLU_NURBS_TESSELLATOR_EXT, NURBS objects are
    tessellated into a sequence of primitives such as lines, triangles and triangle strips, 
    but the vertices,
    normals, colors, and/or textures are retrieved back through a callback interface 
    as specified in Section 7.2. This allows the user to cache the tessellated results 
    for further processing.

    When GLU_NURBS_MODE_EXT is set to be GLU_NURBS_RENDERER_EXT,
    GLU_DISPLAY_MODE specifies how a NURBS surface should be rendered. value
    may be set to one of GLU_FILL, GLU_OUTLINE_POLY, or GLU_OUTLINE_PATCH. When set to GLU_FILL,
    the surface is rendered as a set of polygons. GLU_OUTLINE_POLY instructs the NURBS library to 
    draw only the outlines of the polygons created by tessellation. GLU_OUTLINE_PATCH will cause
    just the outlines of patches and trim curves defined by the user to be drawn.
    The default is GLU_FILL. 
    When GLU_NURBS_MODE_EXT is set to be GLU_NURBS_TESSELLATOR_EXT,
    if GLU_DISPLAY_MODE is set to GLU_FILL or GLU_OUTLINE_POLY, the NURBS surface is tessellated
    into triangles and triangle strips and these polygon primitives are retrieved back 
    through callback functions. If GLU_DISPLAY_MODE is set to GLU_OUTLINE_PATCH, only the outlines
    of the patches and trim curves are generated as a sequence of line strips and can be
    retrieved back through callback functions.


