OpenGL Programming Guide (Addison-Wesley Publishing Company)

OpenGL Programming Guide

Picking

As an extension of the process described in the previous section, you can use selection mode to determine if objects are picked. To do this, you use a special picking matrix in conjunction with the projection matrix to restrict drawing to a small region of the viewport, typically near the cursor. Then you allow some form of input, such as clicking a mouse button, to initiate selection mode. With selection mode established and with the special picking matrix used, objects that are drawn near the cursor cause selection hits. Thus, during picking you're typically determining which objects are drawn near the cursor.

Picking is set up almost exactly like regular selection mode is, with the following major differences:

Picking is usually triggered by an input device. In the following code examples, pressing the left mouse button invokes a function that performs picking.
You use the utility routine gluPickMatrix() to multiply a special projection matrix onto the current matrix. This routine should be called prior to multiplying a projection matrix onto the stack.

Another, completely different way to perform picking is described in "Object Selection Using the Back Buffer." This technique uses color values to identify different components of an object. void gluPickMatrix(GLdouble x, GLdouble y, GLdouble width, GLdouble height, GLint viewport[4]);

Creates a projection matrix that restricts drawing to a small region of the viewport and multiplies that matrix onto the current matrix stack. The center of the picking region is (x, y) in window coordinates, typically the cursor location. width and height define the size of the picking region in screen coordinates. (You can think of the width and height as the sensitivity of the picking device.) viewport[] indicates the current viewport boundaries, which can be obtained by calling

glGetIntegerv(GL_VIEWPORT, GLint *viewport);

Advanced

The net result of the matrix created by gluPickMatrix() is to transform the clipping region into the unit cube -1 ≤ (x, y, z) ≤ 1 (or -w ≤ (wx, wy, wz) ≤ w). The picking matrix effectively performs an orthogonal transformation that maps a subregion of this unit cube to the unit cube. Since the transformation is arbitrary, you can make picking work for different sorts of regions - for example, for rotated rectangular portions of the window. In certain situations, you might find it easier to specify additional clipping planes to define the picking region.

Example 12-3 illustrates simple picking. It also demonstrates how to use multiple names to identify different components of a primitive, in this case the row and column of a selected object. A 3 × 3 grid of squares is drawn, with each square a different color. The board[3][3] array maintains the current amount of blue for each square. When the left mouse button is pressed, the pickSquares() routine is called to identify which squares were picked by the mouse. Two names identify each square in the grid - one identifies the row, and the other the column. Also, when the left mouse button is pressed, the color of all squares under the cursor position changes.

Example 12-3 : A Picking Example: picksquare.c

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

int board[3][3];     /*  amount of color for each square */

/* Clear color value for every square on the board */
void myinit(void)
{
    int i, j;
    for (i = 0; i < 3; i++) 
        for (j = 0; j < 3; j ++)
            board[i][j] = 0;
    glClearColor (0.0, 0.0, 0.0, 0.0);
}

void drawSquares(GLenum mode)
{
    GLuint i, j;
    for (i = 0; i < 3; i++) {
        if (mode == GL_SELECT)
            glLoadName (i);
        for (j = 0; j < 3; j ++) {
            if (mode == GL_SELECT)
                glPushName (j);
            glColor3f ((GLfloat) i/3.0, (GLfloat) j/3.0, 
                (GLfloat) board[i][j]/3.0);
            glRecti (i, j, i+1, j+1);
            if (mode == GL_SELECT)
            glPopName ();
        }
    }
}

void processHits (GLint hits, GLuint buffer[])
{
    unsigned int i, j;
    GLuint ii, jj, 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 this hit = %d\n", names);
            ptr++;
        printf ("  z1 is %u;", *ptr); ptr++;
        printf (" z2 is %u\n", *ptr); ptr++;
        printf ("   names are ");
        for (j = 0; j < names; j++) {   /*  for each name */
            printf ("%d ", *ptr);
            if (j == 0)   /*  set row and column  */
                ii = *ptr;
            else if (j == 1)
                jj = *ptr;
            ptr++;
        }
    printf ("\n");
    board[ii][jj] = (board[ii][jj] + 1) % 3;
    }
}

#define BUFSIZE 512

void pickSquares(AUX_EVENTREC *event)
{
    GLuint selectBuf[BUFSIZE];
    GLint hits;
    GLint viewport[4];
    int x, y;

    x = event->data[AUX_MOUSEX];
    y = event->data[AUX_MOUSEY];
    glGetIntegerv (GL_VIEWPORT, viewport);

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

    glInitNames();
    glPushName(-1);

    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);
        gluOrtho2D (0.0, 3.0, 0.0, 3.0);
        drawSquares (GL_SELECT);
    glPopMatrix ();
    glFlush ();

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

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT);
    drawSquares (GL_RENDER);
    glFlush();
}

void myReshape(GLsizei w, GLsizei h)
{
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D (0.0, 3.0, 0.0, 3.0);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}


int main(int argc, char** argv)
{
    auxInitDisplayMode (AUX_SINGLE | AUX_RGBA);
    auxInitPosition (0, 0, 100, 100);
    auxInitWindow (argv[0]);
    myinit ();
    auxMouseFunc (AUX_LEFTBUTTON, AUX_MOUSEDOWN, pickSquares);
    auxReshapeFunc (myReshape);
    auxMainLoop(display);
}

Picking with Multiple Names and a Hierarchical Model

Multiple names can also be used to choose parts of a hierarchical object in a scene. For example, if you were rendering an assembly line of automobiles, you might want the user to move the mouse to pick the third bolt on the left front tire of the third car in line. A different name can be used to identify each level of hierarchy: which car, which tire, and finally which bolt. As another example, one name can be used to describe a single molecule among other molecules, and additional names can differentiate individual atoms within that molecule.

Example 12-4 is a modification of Example 3-4 that draws an automobile with four identical wheels, each of which has five identical bolts. Code has been added to manipulate the name stack with the object hierarchy.

Example 12-4 : Creating Multiple Names

draw_wheel_and_bolts()
{
    long i;

    draw_wheel_body();
    for (i = 0; i < 5; i++) {
        glPushMatrix();
            glRotate(72.0*i, 0.0, 0.0, 1.0);
            glTranslatef(3.0, 0.0, 0.0);
            glPushName(i);
                draw_bolt_body();
            glPopName();
        glPopMatrix();
    }
 }

draw_body_and_wheel_and_bolts()
{
    draw_car_body();
    glPushMatrix();
        glTranslate(40, 0, 20);  /* first wheel position*/
        glPushName(1);           /* name of wheel number 1 */
            draw_wheel_and_bolts();
        glPopName();
    glPopMatrix();
    glPushMatrix();
        glTranslate(40, 0, -20); /* second wheel position */
        glPushName(2);           /* name of wheel number 2 */
            draw_wheel_and_bolts();
        glPopName();
    glPopMatrix();

    /* draw last two wheels similarly */
 }

Example 12-5 uses the routines in Example 12-4 to draw three different cars, numbered 1, 2, and 3.

Example 12-5 : Using Multiple Names

draw_three_cars()
{
    glInitNames();
    glPushMatrix();
        translate_to_first_car_position();
        glPushName(1);
            draw_body_and_wheel_and_bolts();
        glPopName();
    glPopMatrix();

    glPushMatrix();
        translate_to_second_car_position();
        glPushName(2);
            draw_body_and_wheel_and_bolts();
        glPopName();
    glPopMatrix();

    glPushMatrix();
        translate_to_third_car_position();
        glPushName(3);
            draw_body_and_wheel_and_bolts();
        glPopName();
    glPopMatrix();
}

Assuming that picking is performed, the following are some possible name-stack return values and their interpretations. In these examples, at most one hit record is returned; also, d1 and d2 are depth values.

empty The pick was outside all cars

2 d1d2 2 1 Car 2, wheel 1

1 d1d2 3 Car 3 body

3 d1d2 1 1 0 Bolt 0 on wheel 1 on car 1

The last interpretation assumes that the bolt and wheel don't occupy the same picking region. A user might well pick both the wheel and the bolt, yielding two hits. If you receive multiple hits, you have to decide which hit to process, perhaps by using the depth values to determine which picked object is closest to the viewpoint. The use of depth values is explored further in the next section.

Picking and Depth Values

Example 12-6 demonstrates how to use depth values when picking to determine which object is picked. This program draws three overlapping rectangles in normal rendering mode. When the left mouse button is pressed, the pickRects() routine is called. This routine returns the cursor position, enters selection mode, initializes the name stack, and multiplies the picking matrix onto the stack before the orthographic projection matrix. A selection hit occurs for each rectangle the cursor is over when the left mouse button is clicked. Finally, the contents of the selection buffer is examined to identify which named objects were within the picking region near the cursor.

The rectangles in this program are drawn at different depth, or z, values. Since only one name is used to identify all three rectangles, only one hit can be recorded. However, if more than one rectangle is picked, that single hit has different minimum and maximum z values.

Example 12-6 : Picking with Depth Values: pickdepth.c

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

void myinit(void)
{
    glClearColor (0.0, 0.0, 0.0, 0.0);
    glDepthFunc(GL_LEQUAL);
    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);
        glColor3f (0.0, 1.0, 1.0);
        glVertex3i (3, 2, -1);
        glVertex3i (3, 8, -1);
        glVertex3i (8, 8, -1);
        glVertex3i (8, 2, -1);
        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 %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 pickRects(AUX_EVENTREC *event)
{
    GLuint selectBuf[BUFSIZE];
    GLint hits;
    GLint viewport[4];
    int x, y;

    x = event->data[AUX_MOUSEX];
    y = event->data[AUX_MOUSEY];
    glGetIntegerv (GL_VIEWPORT, viewport);

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

    glInitNames();
    glPushName(-1);

    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, 0.0, 2.0);
        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 myReshape(GLsizei w, GLsizei h)
{
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho (0.0, 8.0, 0.0, 8.0, 0.0, 2.0);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}


int main(int argc, char** argv)
{
    auxInitDisplayMode (AUX_SINGLE | AUX_RGBA | AUX_DEPTH);
    auxInitPosition (0, 0, 100, 100);
    auxInitWindow (argv[0]);
    myinit ();
    auxMouseFunc (AUX_LEFTBUTTON, AUX_MOUSEDOWN, pickRects);
    auxReshapeFunc (myReshape);
    auxMainLoop(display);
}

Try This

Modify Example 12-6 to add additional calls to glPushName() so that multiple names are on the stack when the selection hit occurs. What will the contents of the selection buffer be?
By default, glDepthRange() sets the mapping of the z values to [0.0,1.0]. Try modifying the glDepthRange() values and see how it affects the z values that are returned in the selection array.

OpenGL Programming Guide

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