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 parameter of NurbsProperty: GLU_NURBS_MODE_EXT 100160 Accepted by the parameter of NurbsProperty: GLU_NURBS_TESSELLATOR_EXT 100161 GLU_NURBS_RENDERER_EXT 100162 Accepted by the 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 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 will be used. 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 callback indicates the start of a primitive. 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 callback indicates a vertex of the primitive. The coordinates of the vertex are stored in the parameter 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 callback is invoked as the vertex normal is generated. The components of the normal are stored in the parameter . 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 callback is invoked as the color of a vertex is generated. The components of the color are stored in the parameter . This callback is effective only when the user provides a color map (GL_MAP1_COLOR_4 or GL_MAP2_COLOR_4). contains four components: R,G,B,A. The default color callback function is NULL. The callback is invoked as the texture coordinates of a vertex are generated. These coordinates are stored in the parameter . 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 callback is invoked at the end of a primitive. The default end callback function is NULL. The 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.