opengl sphere clipping - opengl

Question

I've been experimenting with programs from my text book that involved clipping 2-D polygons using glOrtho and then creating glutWireSpheres in gluPerspective. My goal is to clip half the sphere with a plane, however, I am having trouble clipping 3-D objects. I created a toggle button to show the sphere clipped and unclipped, however, the button instead shows the sphere moving in a ellipse motion I believe.
Here is my drawscene for creating the sphere
double eqn0[4] = {1, 0, 0.0, -60}; // Data for clipping plane 0.
// Choose window.
glutSetWindow(id2);
gluLookAt(0.0, 3.0, 12.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
glClear(GL_COLOR_BUFFER_BIT);
glClipPlane(GL_CLIP_PLANE0, eqn0); // Specify clipping plane 0.
if (isClip0) glEnable(GL_CLIP_PLANE0); // Clip points s.t. z > 0.25.
else glDisable(GL_CLIP_PLANE0);
glPushMatrix();
glColor3f(1.0, 0.0, 0.0);
glutWireSphere(1.0, 10, 10);
glPopMatrix();
glFlush();
And here is my toggle
case '0':
if (isClip0 == 0) isClip0 = 1;
else isClip0 = 0;
glutPostRedisplay();
break;
Can someone help me get in the right direction for clipping 3-D objects? Because this will work on 2-D polygons, yet when I try to apply it to spheres, the toggle button doesn't even act like a toggle.
EDIT: Full code:
#include <cmath>
#include <iostream>
#ifdef __APPLE__
# include <GLUT/glut.h>
#else
# include <GL/glut.h>
#endif
#define PI 3.14159265
using namespace std;
// Globals.
static int id1, id2; // Window identifiers.
static int isClip0 = 0; // Is clipping plane 0 enabled?
static int isClip1 = 0; // Is clipping plane 1 enabled?
static int isClip3 = 0; // Is clipping plane 0 enabled?
static int isClip4 = 0; // Is clipping plane 1 enabled?
// Drawing routine for first window.
void drawScene1(void)
{
// Choose window.
glutSetWindow(id1);
glClear(GL_COLOR_BUFFER_BIT);
// A red square.
glColor3f(1.0, 0.0, 0.0);
glBegin(GL_POLYGON);
glVertex3f(10.0, 10.0, 0.0);
glVertex3f(40.0, 10.0, 0.0);
glVertex3f(40.0, 40.0, 0.0);
glVertex3f(10.0, 40.0, 0.0);
glEnd();
glFlush();
}
// Drawing routine for second window.
void drawScene2(void)
{
double eqn0[4] = {1, 0, 0.0, -1000}; // Data for clipping plane 0.
// Choose window.
glutSetWindow(id2);
gluLookAt(0.0, 3.0, 12.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
glClear(GL_COLOR_BUFFER_BIT);
glClipPlane(GL_CLIP_PLANE0, eqn0); // Specify clipping plane 0.
if (isClip0) glEnable(GL_CLIP_PLANE0); // Clip points s.t. z > 0.25.
else glDisable(GL_CLIP_PLANE0);
glPushMatrix();
glColor3f(1.0, 0.0, 0.0);
glutWireSphere(1.0, 10, 10);
glPopMatrix();
glFlush();
}
// Initialization routine for first window.
void setup1(void)
{
// Black background.
glClearColor(0.0, 0.0, 0.0, 0.0);
}
// Initialization routine for second window.
void setup2(void)
{
// Green background.
glClearColor(1.0, 1.0, 1.0, 0.0);
}
// Reshape routine for first window.
void resize1(int w, int h)
{
glViewport(0, 0, (GLsizei) w, (GLsizei) h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
// Non-square aspect ratio squashes the square.
glOrtho(0.0, 50.0, 0.0, 100.0, -1.0, 1.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
// Reshape routine for second window.
void resize2(int w, int h)
{
glViewport (0, 0, (GLsizei)w, (GLsizei)h);
glMatrixMode (GL_PROJECTION);
glLoadIdentity();
gluPerspective(60.0, (float)w/(float)h, 1.0, 50.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
// Keyboard input processing routine shared by both windows.
void keyInput(unsigned char key, int x, int y)
{
switch(key)
{
case 27:
exit(0);
break;
case '0':
if (isClip0 == 0) isClip0 = 1;
else isClip0 = 0;
glutPostRedisplay();
break;
default:
break;
}
}
// Main routine.
int main(int argc, char **argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
// First top-level window definition.
glutInitWindowSize(250, 500);
glutInitWindowPosition(100, 100);
// Create the first window and return id.
id1 = glutCreateWindow("windows.cpp - window 1");
// Initialization, display, and other routines of the first window.
setup1();
glutDisplayFunc(drawScene1);
glutReshapeFunc(resize1);
glutKeyboardFunc(keyInput); // Routine is shared by both windows.
// Second top-level window definition.
glutInitWindowSize(250, 500);
glutInitWindowPosition(400, 100);
// Create the second window and return id.
id2 = glutCreateWindow("windows.cpp - window 2");
// Initialization, display, and other routines of the second window.
setup2();
glutDisplayFunc(drawScene2);
glutReshapeFunc(resize2);
glutKeyboardFunc(keyInput); // Routine is shared by both windows.
glutMainLoop();
return 0;
}
Sphere is dealt with in drawScene2

Answer 1

So, after adding a glLoadIdentity() right before the gluLookAt(), the movements will go away (as I already had suggested...). And when one sets a useful clip plane equation, the clipping works as expected, too. As you define a sphere with radius 1 around the object space center, setting
GLdouble eqn0[4] = {1, 0, 0.0, 0.5};
will result in the sphere being clipped at x=-0.5, so 3/4 of it is still visible, as one would expect.