Figure 60

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Figure 60
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/*
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*/

// select.cpp : Defines the entry point for the console application.
//



/* */

/*
* select.c
* This is an illustration of the selection mode and
* name stack, which detects whether objects which collide
* with a viewing volume. First, four triangles and a
* rectangular box representing a viewing volume are drawn
* (drawScene routine). The green triangle and yellow
* triangles appear to lie within the viewing volume, but
* the red triangle appears to lie outside it. Then the
* selection mode is entered (selectObjects routine).
* Drawing to the screen ceases. To see if any collisions
* occur, the four triangles are called. In this example,
* the green triangle causes one hit with the name 1, and
* the yellow triangles cause one hit with the name 3.
*/
#include
#include
#include

/* draw a triangle with vertices at (x1, y1), (x2, y2)
* and (x3, y3) at z units away from the origin.
*/
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 ();
}

/* draw a rectangular box with these outer x, y, and z values */
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 ();
}

/* drawScene draws 4 triangles and a wire frame
* which represents the viewing volume.
*/
void drawScene (void)
{
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
gluPerspective (40.0, 4.0/3.0, 1.0, 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, -1.0);
drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -9.0);
drawViewVolume (0.0, 5.0, 0.0, 5.0, 0.0, 10.0);
}

/* processHits prints out the contents of the selection array
*/
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");
}
}

/* selectObjects "draws" the triangles in selection mode,
* assigning names for the triangles. Note that the third
* and fourth triangles share one name, so that if either
* or both triangles intersects the viewing/clipping volume,
* only one hit will be registered.
*/
#define BUFSIZE 512

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

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

glInitNames();
glPushName(0);

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, -1.0);
drawTriangle (2.0, 2.0, 3.0, 2.0, 2.5, 3.0, -9.0);
glPopMatrix ();
glFlush ();

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

void init (void)
{
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();
}

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

/* Main Loop */
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();
glutDisplayFunc(display);
glutKeyboardFunc(keyboard);
glutMainLoop();
return 0;
}