/************************************************************
  From: Alex Gleyzer <alex@boylesoftware.com>
  To: Chee Yap <yap@cs.nyu.edu>
  CC: g22_3033_010_fa01@cs.nyu.edu
  Subject: Re: [G22_3033_010_fa01] Re: hw2: opengl draws only convex polygons
  Date: Fri, 09 Nov 2001 18:15:48 -0500
  
  Apparently GLUT has a special api to draw non-convex polygons:
  
  http://ask.ii.uib.no/ebt-bin/nph-dweb/dynaweb/SGI_Developer/OpenGL_PG/@ebt-link;cs=fullhtml;pt=23505?target=%25N%15_23934_START_RESTART_N%25#X
  
  1. Attached code demonstrates how it works (version of GLUT matters - my 
  mesa seems  to have version 1.1)
  2. Attached code also is leaking memory by creating new vecto for every 
  x,yr in myVertex. The idea is that array v must stay in memory for a 
  while, I guess GLUT stores pointer to it in some internal data 
  structures. Ideally you have an array of vectors and you pass pointer to 
  current point, as it is done in the manual's example.

************************************************************/

// Date: Sat, 03 Nov 2001 13:26:00 -0500
// From: Alex Gleyzer <alex@boylesoftware.com>
// To: yap@cs.nyu.edu
// Subject: hw2: opengl draws only convex polygons
//
// Professor,
//
// We discussed this last Tuesday after class, but I had time to implement
// this only today.
//
// Attached program demonstrates that OpenGL (I use Mesa) draws only convex
// polygons with GL_POLYGON. Therefore some of the Tiger polygons will not be
// displayed properly -- I can see it when using Manhattan county data.
//
// Thanks,
// Alex
//
// Chee's Note: main modification is to try several alternative
//	drawing of rotated/flipped versions of the polygons to see
//	how GLUT manage it.  The orignal polygon in in the first
//	quadrant, the rest occur in ccw order.
/*
 * This program demostrates that opengl can draw only convex polygons.
 */


// #include <cstdlib>
#include <stdlib.h>
// #include <cstdio>
#include <stdio.h>
// #include <string>
#include <string.h>
#include <GL/glut.h>

/*  the callback routines registered by gluTessCallback() */

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

void endCallback(void)
{
   glEnd();
}

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

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


GLUtriangulatorObj *to;


void initTess()  {
    glClear(GL_COLOR_BUFFER_BIT);

    to = gluNewTess();

    gluTessCallback(to, GLU_TESS_VERTEX,
		    (GLvoid (*) ()) &glVertex3dv);
    gluTessCallback(to, GLU_TESS_BEGIN,
		    (GLvoid (*) ()) &beginCallback);
    gluTessCallback(to, GLU_TESS_END,
		    (GLvoid (*) ()) &endCallback);
    gluTessCallback(to, GLU_TESS_ERROR,
		    (GLvoid (*) ()) &errorCallback);

}


void myVertex(GLdouble x, GLdouble y) {
    GLdouble *v = new GLdouble[3];
    v[0] = x;
    v[1] = y;
    v[2] = 0;
    gluTessVertex(to, v, v);
}


void myDisplay() {
    initTess();

    glClear(GL_COLOR_BUFFER_BIT);

    // THIS IS THE ORIGINAL polygon of Gleyzer:
    glColor3f(0, 0, 1);
    gluBeginPolygon(to);
    myVertex(0, 0); myVertex(0, 2); myVertex(2, 2);
    myVertex(2, 1); myVertex(1, 1); myVertex(1, 0);
    gluEndPolygon(to);

    glColor3f(1, 0, 0);
    glBegin(GL_LINE_LOOP);
    glVertex2f(0, 0); glVertex2f(0, 2); glVertex2f(2, 2);
    glVertex2f(2, 1); glVertex2f(1, 1); glVertex2f(1, 0);
    glEnd();

    // THIS IS THE REVERSE of the polygon of Gleyzer:
    glTranslated(-3, 0, 0);

    glColor3f(0, 0, 1);
    gluBeginPolygon(to);
    myVertex(0, 0); myVertex(1, 0); myVertex(1, 1);
    myVertex(2, 1); myVertex(2, 2); myVertex(0, 2);
    gluEndPolygon(to);

    glColor3f(1, 0, 0);
    glBegin(GL_LINE_LOOP);
    glVertex2f(0, 0); glVertex2f(1, 0); glVertex2f(1, 1);
    glVertex2f(2, 1); glVertex2f(2, 2); glVertex2f(0, 2);
    glEnd();

    // THIS IS THE Horizontal Flip of the polygon of Gleyzer:
    glTranslated(0, -3, 0);

    glColor3f(0, 0, 1);
    gluBeginPolygon(to);
    myVertex(0, 0); myVertex(2, 0); myVertex(2, 1);
    myVertex(1, 1); myVertex(1, 2); myVertex(0, 2);
    gluEndPolygon(to);

    glColor3f(1, 0, 0);
    glBegin(GL_LINE_LOOP);
    glVertex2f(0, 0); glVertex2f(2, 0); glVertex2f(2, 1);
    glVertex2f(1, 1); glVertex2f(1, 2); glVertex2f(0, 2);
    glEnd();

    // THIS IS THE Vertical Flip of the polygon of Gleyzer:
    glTranslated(3, 0, 0);

    glColor3f(0, 0, 1);
    gluBeginPolygon(to);
    myVertex(1, 0); myVertex(2, 0); myVertex(2, 2);
    myVertex(0, 2); myVertex(0, 1); myVertex(1, 1);
    gluEndPolygon(to);

    glColor3f(1, 0, 0);
    glBegin(GL_LINE_LOOP);
    glVertex2f(1, 0); glVertex2f(2, 0); glVertex2f(2, 2);
    glVertex2f(0, 2); glVertex2f(0, 1); glVertex2f(1, 1);
    glEnd();

    // get back to original position
    glTranslated(0, 3, 0);
    glFlush();
}


int main(int argc, char *argv[]) {
    // init GL
    glutInit(&argc, argv);          // initialize the toolkit
    cerr << gluGetString(GLU_VERSION) << endl;

    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB); // set display mode
    glutInitWindowSize(640,480);     // set window size
    glutInitWindowPosition(100, 150); // set window position on screen
    glutCreateWindow("Polygon test"); // open the screen window

    glutDisplayFunc(myDisplay);     // register redraw function

    glClearColor(1.0, 1.0, 1.0, 0.0);       // set white background color
    glColor3f(0.0f, 0.0f, 0.0f);         // set the drawing color

    glLineWidth(2);
    glPointSize(3);

    // Reset the coordinate system before modifying
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(-5, 5, -5, 5);
    glMatrixMode(GL_MODELVIEW);

    glutMainLoop();
}




--------------060708010403010300000104--

_______________________________________________
G22_3033_010_fa01 mailing list
G22_3033_010_fa01@cs.nyu.edu
http://www.cs.nyu.edu/mailman/listinfo/g22_3033_010_fa01