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dabeav
12-04-2003, 07:00 AM
Does anyone know of an algorithm, or a library, that can be used to triangulate a concave poly w/out any holes in it. I know how to triangulate convex w/out any problems, but concave polys (once again without any holes) are causing me headacks. Any ideas??

Tom Nuydens
12-04-2003, 07:12 AM
Use the GLU tessellator.

-- Tom

dabeav
12-04-2003, 07:27 AM
But, with the glu tesselator, doesnt that just display the triangles? I want to be able to pull them into memory, for manipulator later. Can I do that with glu as well, or do I need something else?

ScottManDeath
12-04-2003, 08:45 AM
you give the glu tesselator call back functions for beginning/ending a primitive, for vertices .... so you can do whatever you want, store them, render them directly ....

dabeav
12-04-2003, 08:56 AM
Well, then that is very cool. But, do you happen to know where I could find a good example of how to use it. (No i am not a code stealer, i am just a visual learner and if i can see it done, i can probly do it). Thanx for any more help you can give.

ScottManDeath
12-04-2003, 09:21 AM
just copy & paste from a sample from the red book



/*
* Copyright (c) 1993-1997, Silicon Graphics, Inc.
* ALL RIGHTS RESERVED
* Permission to use, copy, modify, and distribute this software for
* any purpose and without fee is hereby granted, provided that the above
* copyright notice appear in all copies and that both the copyright notice
* and this permission notice appear in supporting documentation, and that
* the name of Silicon Graphics, Inc. not be used in advertising
* or publicity pertaining to distribution of the software without specific,
* written prior permission.
*
* THE MATERIAL EMBODIED ON THIS SOFTWARE IS PROVIDED TO YOU "AS-IS"
* AND WITHOUT WARRANTY OF ANY KIND, EXPRESS, IMPLIED OR OTHERWISE,
* INCLUDING WITHOUT LIMITATION, ANY WARRANTY OF MERCHANTABILITY OR
* FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL SILICON
* GRAPHICS, INC. BE LIABLE TO YOU OR ANYONE ELSE FOR ANY DIRECT,
* SPECIAL, INCIDENTAL, INDIRECT OR CONSEQUENTIAL DAMAGES OF ANY
* KIND, OR ANY DAMAGES WHATSOEVER, INCLUDING WITHOUT LIMITATION,
* LOSS OF PROFIT, LOSS OF USE, SAVINGS OR REVENUE, OR THE CLAIMS OF
* THIRD PARTIES, WHETHER OR NOT SILICON GRAPHICS, INC. HAS BEEN
* ADVISED OF THE POSSIBILITY OF SUCH LOSS, HOWEVER CAUSED AND ON
* ANY THEORY OF LIABILITY, ARISING OUT OF OR IN CONNECTION WITH THE
* POSSESSION, USE OR PERFORMANCE OF THIS SOFTWARE.
*
* US Government Users Restricted Rights
* Use, duplication, or disclosure by the Government is subject to
* restrictions set forth in FAR 52.227.19(c)(2) or subparagraph
* (c)(1)(ii) of the Rights in Technical Data and Computer Software
* clause at DFARS 252.227-7013 and/or in similar or successor
* clauses in the FAR or the DOD or NASA FAR Supplement.
* Unpublished-- rights reserved under the copyright laws of the
* United States. Contractor/manufacturer is Silicon Graphics,
* Inc., 2011 N. Shoreline Blvd., Mountain View, CA 94039-7311.
*
* OpenGL(R) is a registered trademark of Silicon Graphics, Inc.
*/

/*
* tess.c
* This program demonstrates polygon tessellation.
* Two tesselated objects are drawn. The first is a
* rectangle with a triangular hole. The second is a
* smooth shaded, self-intersecting star.
*
* Note the exterior rectangle is drawn with its vertices
* in counter-clockwise order, but its interior clockwise.
* Note the combineCallback is needed for the self-intersecting
* star. Also note that removing the TessProperty for the
* star will make the interior unshaded (WINDING_ODD).
*/
#include <GL/glut.h>
#include <stdlib.h>
#include <stdio.h>

#ifdef GLU_VERSION_1_2

/* Win32 calling conventions. */
#ifndef CALLBACK
#define CALLBACK
#endif

GLuint startList;

void display (void) {
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1.0, 1.0, 1.0);
glCallList(startList);
glCallList(startList + 1);
glFlush();
}

void CALLBACK beginCallback(GLenum which)
{
glBegin(which);
}

void CALLBACK errorCallback(GLenum errorCode)
{
const GLubyte *estring;

estring = gluErrorString(errorCode);
fprintf(stderr, "Tessellation Error: %s\n", estring);
exit(0);
}

void CALLBACK endCallback(void)
{
glEnd();
}

void CALLBACK vertexCallback(GLvoid *vertex)
{
const GLdouble *pointer;

pointer = (GLdouble *) vertex;
glColor3dv(pointer+3);
glVertex3dv(vertex);
}

/* combineCallback is used to create a new vertex when edges
* intersect. coordinate location is trivial to calculate,
* but weight[4] may be used to average color, normal, or texture
* coordinate data. In this program, color is weighted.
*/
void CALLBACK combineCallback(GLdouble coords[3],
GLdouble *vertex_data[4],
GLfloat weight[4], GLdouble **dataOut )
{
GLdouble *vertex;
int i;

vertex = (GLdouble *) malloc(6 * sizeof(GLdouble));

vertex[0] = coords[0];
vertex[1] = coords[1];
vertex[2] = coords[2];
for (i = 3; i < 6; i++)
vertex[i] = weight[0] * vertex_data[0][i]
+ weight[1] * vertex_data[1][i]
+ weight[2] * vertex_data[2][i]
+ weight[3] * vertex_data[3][i];
*dataOut = vertex;
}

void init (void)
{
GLUtesselator *tobj;
GLdouble rect[4][3] = {50.0, 50.0, 0.0,
200.0, 50.0, 0.0,
200.0, 200.0, 0.0,
50.0, 200.0, 0.0};
GLdouble tri[3][3] = {75.0, 75.0, 0.0,
125.0, 175.0, 0.0,
175.0, 75.0, 0.0};
GLdouble star[5][6] = {250.0, 50.0, 0.0, 1.0, 0.0, 1.0,
325.0, 200.0, 0.0, 1.0, 1.0, 0.0,
400.0, 50.0, 0.0, 0.0, 1.0, 1.0,
250.0, 150.0, 0.0, 1.0, 0.0, 0.0,
400.0, 150.0, 0.0, 0.0, 1.0, 0.0};

glClearColor(0.0, 0.0, 0.0, 0.0);

startList = glGenLists(2);

tobj = gluNewTess();
gluTessCallback(tobj, GLU_TESS_VERTEX,
(GLvoid (CALLBACK*) ()) &amp;glVertex3dv);
gluTessCallback(tobj, GLU_TESS_BEGIN,
(GLvoid (CALLBACK*) ()) &amp;beginCallback);
gluTessCallback(tobj, GLU_TESS_END,
(GLvoid (CALLBACK*) ()) &amp;endCallback);
gluTessCallback(tobj, GLU_TESS_ERROR,
(GLvoid (CALLBACK*) ()) &amp;errorCallback);

/* rectangle with triangular hole inside */
glNewList(startList, GL_COMPILE);
glShadeModel(GL_FLAT);
gluTessBeginPolygon(tobj, NULL);
gluTessBeginContour(tobj);
gluTessVertex(tobj, rect[0], rect[0]);
gluTessVertex(tobj, rect[1], rect[1]);
gluTessVertex(tobj, rect[2], rect[2]);
gluTessVertex(tobj, rect[3], rect[3]);
gluTessEndContour(tobj);
gluTessBeginContour(tobj);
gluTessVertex(tobj, tri[0], tri[0]);
gluTessVertex(tobj, tri[1], tri[1]);
gluTessVertex(tobj, tri[2], tri[2]);
gluTessEndContour(tobj);
gluTessEndPolygon(tobj);
glEndList();

gluTessCallback(tobj, GLU_TESS_VERTEX,
(GLvoid (CALLBACK*) ()) &amp;vertexCallback);
gluTessCallback(tobj, GLU_TESS_BEGIN,
(GLvoid (CALLBACK*) ()) &amp;beginCallback);
gluTessCallback(tobj, GLU_TESS_END,
(GLvoid (CALLBACK*) ()) &amp;endCallback);
gluTessCallback(tobj, GLU_TESS_ERROR,
(GLvoid (CALLBACK*) ()) &amp;errorCallback);
gluTessCallback(tobj, GLU_TESS_COMBINE,
(GLvoid (CALLBACK*) ()) &amp;combineCallback);

/* smooth shaded, self-intersecting star */
glNewList(startList + 1, GL_COMPILE);
glShadeModel(GL_SMOOTH);
gluTessProperty(tobj, GLU_TESS_WINDING_RULE,
GLU_TESS_WINDING_POSITIVE);
gluTessBeginPolygon(tobj, NULL);
gluTessBeginContour(tobj);
gluTessVertex(tobj, star[0], star[0]);
gluTessVertex(tobj, star[1], star[1]);
gluTessVertex(tobj, star[2], star[2]);
gluTessVertex(tobj, star[3], star[3]);
gluTessVertex(tobj, star[4], star[4]);
gluTessEndContour(tobj);
gluTessEndPolygon(tobj);
glEndList();
gluDeleteTess(tobj);
}

void reshape (int w, int h)
{
glViewport(0, 0, (GLsizei) w, (GLsizei) h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0.0, (GLdouble) w, 0.0, (GLdouble) h);
}

/* ARGSUSED1 */
void keyboard(unsigned char key, int x, int y)
{
switch (key) {
case 27:
exit(0);
break;
}
}

int main(int argc, char** argv)
{
glutInit(&amp;argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(500, 500);
glutCreateWindow(argv[0]);
init();
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutKeyboardFunc(keyboard);
glutMainLoop();
return 0;
}

#else
int main(int argc, char** argv)
{
fprintf (stderr, "This program demonstrates the new tesselator API in GLU 1.2.\n");
fprintf (stderr, "Your GLU library does not support this new interface, sorry.\n");
return 0;
}
#endif

chief
12-04-2003, 10:45 PM
Try this http://www.codeproject.com/useritems/polygon_tesselation.asp

Eric Lengyel
12-05-2003, 02:56 PM
My article, "T-Junction Elimination and Retriangulation" in Game Programming Gems 3 contains code that will triangulate an arbitrary concave polygon. (The code is on the CD.)

This also appears in Section 9.6 of the second edition of Mathematics for 3D Game Programming and Computer Graphics:

http://www.terathon.com/books/mathgames2.html