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OpenGL Question about Tesselator,need help.

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I've write some test code for learning OpenGL tesselator,but i can not get the right result.The real polygon is a simple concave polygon: //vertex array,simple 2D convave polygon const int NUM_POINTS = 5; Gldouble vPoly[NUM_POINTS][2]= { {100,100}, {300,100}, {200,200}, {300,300}, {100,300} }; GLUTesselator m_tessObj; m_=gluNewTess(); // CALLBACK functions gluTessCallback(m_tessObj,GLU_TESS_BEGIN,(void (CALLBACK*)())&BeginCallback); gluTessCallback(m_tessObj,GLU_TESS_VERTEX,(void (CALLBACK*)())&VertexCallback); gluTessCallback(m_tessObj,GLU_TESS_END,(void (CALLBACK*)())&EndCallback); gluTessCallback(m_tessObj,GLU_TESS_COMBINE,(void (CALLBACK*)())CombineCallback); gluTessCallback(m_tessObj,GLU_TESS_ERROR,(void (CALLBACK*)())&ErrorCallback); //tesselating gluTessBeginPolygon(m_tessObj,NULL); gluTessBeginContour(m_tessObj); for(i = 0; i < NUM_POINTS; i++) { gluTessVertex(m_tessObj,vPoly,vPoly); } gluTessEndContour(m_tessObj); gluTessEndPolygon(m_tessObj); glFinish(); gluDeleteTess(m_tessObj); //callback functions void CALLBACK ErrorCallback(GLenum errorCode) { const GLubyte *estring; CString mexstr; estring = gluErrorString(errorCode); mexstr.Format("Tessellation/Quadric Error: %s\n", estring); AfxMessageBox(mexstr,MB_OK | MB_ICONEXCLAMATION); TRACE("Tessellation Error: %s\n", estring); TRACE("\n\n\n\n----------------error callback\n\n\n\n"); } void CALLBACK BeginCallback(GLenum which) { glBegin(which); }; void CALLBACK EndCallback(void) { glEnd(); }; void CALLBACK VertexCallback(GLvoid* vertex) { glVertex2dv((const double *)vertex); TRACE("----------------glVertex called\n"); }; void CALLBACK CombineCallback(GLdouble coords[3], GLdouble *data[4], GLfloat weight[4], GLdouble **dataOut ) { GLdouble *vertex; vertex = new GLdouble[3]; vertex[0] = coords[0]; vertex[1] = coords[1]; vertex[2] = coords[2]; *dataOut = vertex; }; But the tesselatiing result is wrong,unexpected new vertex created near (180,140) in tesselating. "*" is the result, "-" is the real polygon: ---------------------- -***************- ---***********- ----********- -----******- ------****- -------***--- --------**----- ---------*------- ------------------- if move the real polygon to left bottom: GLdouble vPoly[NUM_POINTS][2] = { {0,0}, {200,0}, {100,100}, {200,200}, {0,200} }; the result polygon changed,but is still wrong. Changing the order of the vertex: GLdouble vPoly[NUM_POINTS][2] = { {200,0}, {100,100}, {200,200}, {0,200}, {0,0} }; the result polygon is also wrong. After working on it for many hours,i stil don't know why,anyone could help me?

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