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OpenGL 1.1 for AIX: Reference Manual

gluTessCallback Subroutine

Purpose

Defines a callback for a tessellation object.

Library

OpenGL C bindings library: libGL.a

C Syntax

void gluTessCallback(GLUtriangulatorObj *tobj, 
   GLenum Which, 
   void (*Function)())

Description

The gluTessCallback subroutine defines a new callback for use by a tessellation object. If the specified callback is already defined, it is replaced. If the Function parameter is set to null, the existing callback is erased.

These callbacks are used by the tessellation object to describe how a polygon specified by the user is broken into triangles.

The acceptable callbacks are as follows:

GLU_TESS_BEGIN The begin callback is invoked like glBegin to indicate the start of a (triangle) primitive. The function takes a single argument of type GLenum. If the GLU_TESS_BOUNDARY_ONLY property is set to GL_FALSE, then the argument is set to either GL_TRIANGLE_FAN, GL_TRIANGLE_STRIP, or GL_TRIANGLES. If the GLU_TESS_BOUNDARY_ONLY property is set to GL_TRUE, then the argument will be set to GL_LINE_LOOP. The function prototype for this callback is:
  void begin(GLenum type);
GLU_TESS_EDGE_FLAG Similar to the glEdgeFlag subroutine. It takes a single Boolean flag that indicates which edges of the created triangles were part o f the original polygon defined by the user and which were created by the tessellation process. If the flag is GL_TRUE, each vertex that follows begins an edge that was part of the original polygon. If the flag is GL_FALSE, each vertex that follows begins an edge that was generated by the tessellator. To avoid confusion with the first few edges, the edge flag callback is started before the first vertex callback is made.

Because triangle fans and strips do not support edge flags, the GLU_BEGIN callback cannot be called with GL_TRIANGLE_FAN or GL_TRIANGLE_STRIP if the GLU_EDGE_FLAG callback is provided. Instead, fans and strips are converted into independent triangles. The function prototype for this callback is:

  void edgeFlag(GLboolean flag);
GLU_TESS_EDGE_FLAG_DATA The same as the GLU_TESS_EDGE_FLAG callback except that it takes an additional pointer argument. This pointer is identical to the opaque pointer provided when gluTessBeginPolygon was called. The function prototype for this callback is:
  void edgeFlagData(GLboolean flag, void *polygon_data);
GLU_TESS_VERTEX Started between the GLU_BEGIN and GLU_END callbacks. It is similar to the glVertex subroutine and defines the vertices of the triangles created by the tessellation process. It takes a single pointer as its only argument. This pointer is identical to the opaque pointer provided by the user when the vertex was described. (See the gluTessVertex subroutine for details on specifying a polygon vertex.) The function prototype for this callback is:
  void vertex (void *vertex_data);
GLU_TESS_END Serves the same purpose as the glEnd subroutine and indicates the end of a primitive. It takes no arguments. The function prototype for this callback is:
  void end(void);
GLU_TESS_ERROR Called when an error is encountered. The GLenum type is the only argument and indicates the specific error that occurred. There are eight errors unique to polygon tessellation. They are named GLU_TESS_ERROR1 through GLU_TESS_ERROR8. Character strings describing these errors can be retrieved with the gluErrorString subroutine. The function prototype for this callback is:
void error(GLenum errno);

Parameters

tobj Specifies the tessellation object created with the gluNewTess subroutine.
Which Specifies the callback being defined. The following values are valid:
  • GLU_TESS_BEGIN
  • GLU_TESS_EDGE_FLAG
  • GLU_TESS_EDGE_FLAG_DATA
  • GLU_TESS_VERTEX
  • GLU_TESS_END
  • GLU_TESS_ERROR
Function Specifies the new callback.

Examples

Tessellated polygons can be rendered directly as in the following example:

gluTessCallback(tobj, GLU_TESS_BEGIN, glBegin);
gluTessCallback(tobj, GLU_TESS_VERTEX, glVertex3dv);
gluTessCallback(tobj, GLU_TESS_END, glEnd);
gluBeginPolygon(tobj); 
gluTessVertex(tobj, v, v);
...
gluEndPolygon(tobj);

Typically, the tessellated polygon should be stored in a display list so that is does need to be retessellated every time it is rendered.

Files

/usr/include/GL/gl.h Contains C language constraints, variable type definitions, and ANSI function prototypes for OpenGL.

Related Information

The glBegin subroutine, glEdgeFlag subroutine, glVertex subroutine, gluDeleteTess subroutine, gluErrorString subroutine, gluNewTess subroutine, gluTessVertex subroutine, gluErrorString subroutine.

OpenGL Overview.


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