OpenGL Programming Guide

Chapter 12Selection and Feedback

Selection

A Selection Example

In Example 12-2 , four triangles (green, red, and two yellow ones) are drawn in selection mode, and the corresponding hit records are processed. The first triangle generates a hit, the second one doesn't, and the third and fourth ones together generate a single hit. Routines are defined to draw a triangle (drawTriangle()) and to draw a wireframe box representing the viewing volume (drawViewVolume()). The processHits() routine prints out the selection array. Finally, selectObjects() draws the triangles in selection mode to generate the hit records.

Example 12-2 : A Selection Example: select.c

#include <GL/gl.h>
#include <GL/glu.h>
#include "aux.h"

void drawTriangle (GLfloat x1, GLfloat y1, GLfloat x2, 
    GLfloat y2, GLfloat x3, GLfloat y3, GLfloat z)
{
    glBegin (GL_TRIANGLES);
    glVertex3f (x1, y1, z);
    glVertex3f (x2, y2, z);
    glVertex3f (x3, y3, z);
    glEnd ();
}

void drawViewVolume (GLfloat x1, GLfloat x2, GLfloat y1, 
    GLfloat y2, GLfloat z1, GLfloat z2)
{
    glColor3f (1.0, 1.0, 1.0);
    glBegin (GL_LINE_LOOP);
        glVertex3f (x1, y1, -z1);
        glVertex3f (x2, y1, -z1);
        glVertex3f (x2, y2, -z1);
        glVertex3f (x1, y2, -z1);
    glEnd ();

    glBegin (GL_LINE_LOOP);
        glVertex3f (x1, y1, -z2);
        glVertex3f (x2, y1, -z2);
        glVertex3f (x2, y2, -z2);
        glVertex3f (x1, y2, -z2);
    glEnd ();

    glBegin (GL_LINES);   /*  4 lines   */
        glVertex3f (x1, y1, -z1);
        glVertex3f (x1, y1, -z2);
        glVertex3f (x1, y2, -z1);
        glVertex3f (x1, y2, -z2);
        glVertex3f (x2, y1, -z1);
        glVertex3f (x2, y1, -z2);
        glVertex3f (x2, y2, -z1);
        glVertex3f (x2, y2, -z2);
    glEnd ();
}

void drawScene (void)
{
    glMatrixMode (GL_PROJECTION);
    glLoadIdentity ();
    gluPerspective (40.0, 4.0/3.0, 0.01, 100.0);

    glMatrixMode (GL_MODELVIEW);
    glLoadIdentity ();
    gluLookAt (7.5, 7.5, 12.5, 2.5, 2.5, -5.0, 0.0, 1.0, 0.0);
    glColor3f (0.0, 1.0, 0.0);     /*  green triangle */
    drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -5.0);
    glColor3f (1.0, 0.0, 0.0);     /*  red triangle */
    drawTriangle (2.0, 7.0, 3.0, 7.0, 2.5, 8.0, -5.0);
    glColor3f (1.0, 1.0, 0.0);     /*  yellow triangles */
    drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, 0.0);
    drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -10.0);
    drawViewVolume (0.0, 5.0, 0.0, 5.0, 0.0, 10.0);
}

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 %u;", *ptr); ptr++;
        printf (" z2 is %u\n", *ptr); ptr++;
        printf ("   the name is ");
        for (j = 0; j < names; j++) {      /* for each name */
            printf ("%d ", *ptr); ptr++;
        }
        printf ("\n");
    }
}

#define BUFSIZE 512

void selectObjects(void)
{
    GLuint selectBuf[BUFSIZE];
    GLint hits, viewport[4];

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

    glInitNames();
    glPushName(-1);

    glPushMatrix ();
        glMatrixMode (GL_PROJECTION);
        glLoadIdentity ();
        glOrtho (0.0, 5.0, 0.0, 5.0, 0.0, 10.0);
        glMatrixMode (GL_MODELVIEW);
        glLoadIdentity ();
        glLoadName(1);
        drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -5.0);
        glLoadName(2);
        drawTriangle (2.0, 7.0, 3.0, 7.0, 2.5, 8.0, -5.0);
        glLoadName(3);
        drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, 0.0);
        drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -10.0);
    glPopMatrix ();
    glFlush ();

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

void myinit (void) 
{
    glDepthFunc(GL_LEQUAL);
    glEnable(GL_DEPTH_TEST);
    glShadeModel(GL_FLAT);
}

void display(void)
{
    glClearColor (0.0, 0.0, 0.0, 0.0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    drawScene ();
    selectObjects ();
    glFlush();
}


int main(int argc, char** argv)
{
    auxInitDisplayMode (AUX_SINGLE | AUX_RGBA | AUX_DEPTH);
    auxInitPosition (0, 0, 200, 200);
    auxInitWindow (argv[0]);
    myinit ();
    auxMainLoop(display);
}

OpenGL Programming Guide