example2.c

#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
#include <stdlib.h>
#include <stdio.h>

void init(void)
{
   glClearColor(0.0, 0.0, 0.0, 0.0);
   glEnable(GL_DEPTH_TEST);
   glShadeModel(GL_FLAT);
   glDepthRange(0.0, 1.0);  /* The default z mapping */
}

void drawRects(GLenum mode)
{
   if (mode == GL_SELECT)
      glLoadName(1);
   glBegin(GL_QUADS);
   glColor3f(1.0, 1.0, 0.0);
   glVertex3i(2, 0, 0);
   glVertex3i(2, 6, 0);
   glVertex3i(6, 6, 0);
   glVertex3i(6, 0, 0);
   glEnd();
   if (mode == GL_SELECT)
      glLoadName(2);
   glBegin(GL_QUADS);
   glColor3f(0.0, 1.0, 1.0);
   glVertex3i(3, 2, -1);
   glVertex3i(3, 8, -1);
   glVertex3i(8, 8, -1);
   glVertex3i(8, 2, -1);
   glEnd();
   if (mode == GL_SELECT)
      glLoadName(3);
   glBegin(GL_QUADS);
   glColor3f(1.0, 0.0, 1.0);
   glVertex3i(0, 2, -2);
   glVertex3i(0, 7, -2);
   glVertex3i(5, 7, -2);
   glVertex3i(5, 2, -2);
   glEnd();
}

void processHits(GLint hits, GLuint buffer[])
{
   unsigned int i, j;
   GLuint names, *ptr;

   printf("hits = %d\n", hits);
   ptr = (GLuint *) buffer;
   for (i = 0; i < hits; i++) {  /* for each hit  */
      names = *ptr;
      printf(" number of names for hit = %d\n", names); ptr++;
      printf("  z1 is %g;", (float) *ptr/0x7fffffff); ptr++;
      printf(" z2 is %g\n", (float) *ptr/0x7fffffff); ptr++;
      printf("   the name is ");
      for (j = 0; j < names; j++) {  /* for each name */
         printf("%d ", *ptr); ptr++;
      }
      printf("\n");
   }
}

#define BUFSIZE 512

void pickRects(int button, int state, int x, int y)
{
   GLuint selectBuf[BUFSIZE];
   GLint hits;
   GLint viewport[4];

   if (button != GLUT_LEFT_BUTTON || state != GLUT_DOWN)
      return;
   glGetIntegerv(GL_VIEWPORT, viewport);

   glSelectBuffer(BUFSIZE, selectBuf);
   (void) glRenderMode(GL_SELECT);

   glInitNames();
   glPushName(0);

   glMatrixMode(GL_PROJECTION);
   glPushMatrix();
   glLoadIdentity();
   
/*  create 5x5 pixel picking region near cursor location */
   gluPickMatrix((GLdouble) x, (GLdouble) (viewport[3] - y),
                 5.0, 5.0, viewport);
   glOrtho(0.0, 8.0, 0.0, 8.0, -0.5, 2.5);
   drawRects(GL_SELECT);
   glPopMatrix();
   glFlush();

   hits = glRenderMode(GL_RENDER);
   processHits(hits, selectBuf);
}

void display(void)
{
   glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
   drawRects(GL_RENDER);
   glFlush();
}

void reshape(int w, int h)
{
   glViewport(0, 0, (GLsizei) w, (GLsizei) h);
   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();
   glOrtho(0.0, 8.0, 0.0, 8.0, -0.5, 2.5);
   glMatrixMode(GL_MODELVIEW);
   glLoadIdentity();
}

int main(int argc, char **argv)
{
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);
   glutInitWindowSize (200, 200);
   glutInitWindowPosition (100, 100);
   glutCreateWindow(argv[0]);
   init();
   glutMouseFunc(pickRects);
   glutReshapeFunc(reshape);
   glutDisplayFunc(display);
   glutMainLoop();
   return 0; 
}