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GLU_EXT_nurbs_tessellator
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);
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.
Implementation Support
List of OpenGL implementations supporting the GLU_EXT_nurbs_tessellator extension
Original File
Original text file for the GLU_EXT_nurbs_tessellator extension
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