glClipPlane函数

1.OGL 一般最多支持6个裁剪平面。glGetIntegerv(GL_MAX_CLIP_PLANES, &clipPlanes);
2.裁剪平面是在视图空间中进行计算的，而不是裁剪空间中，和通常的视锥体裁剪，图像裁剪不同；所以如果是正交投影，裁剪平面也是在3D视图坐标系中裁剪。

3.平面的方程为Ax + By + Cz  + （- D）= 0 等于VecP * VecNormal + (-D) = 0,glClipPlane中定义的平面就是：

 GLdouble eqn[4] = {0.0, -1.0, 0.0, 0.0}; A, B, C的定义是VecNormal上的平面，x,y, z却是平面上的点，D是平面上到原点距离的相反数。

4.法向量(A,B,C)指向的方向是平面的正方向，裁剪出来的空间也是法向量指向的空间，多个裁剪平面相互裁剪下是他们裁剪出来的空间的交集。

实例代码：

/*
* Copyright (c) 1993-2003, Silicon Graphics, Inc.
* All Rights Reserved
*
* Permission to use, copy, modify, and distribute this software for any
* purpose and without fee is hereby granted, provided that the above
* copyright notice appear in all copies and that both the copyright
* notice and this permission notice appear in supporting documentation,
* and that the name of Silicon Graphics, Inc. not be used in
* advertising or publicity pertaining to distribution of the software
* without specific, written prior permission.
*
* THE MATERIAL EMBODIED ON THIS SOFTWARE IS PROVIDED TO YOU "AS-IS" AND
* WITHOUT WARRANTY OF ANY KIND, EXPRESS, IMPLIED OR OTHERWISE,
* INCLUDING WITHOUT LIMITATION, ANY WARRANTY OF MERCHANTABILITY OR
* FITNESS FOR A PARTICULAR PURPOSE.  IN NO EVENT SHALL SILICON
* GRAPHICS, INC.  BE LIABLE TO YOU OR ANYONE ELSE FOR ANY DIRECT,
* SPECIAL, INCIDENTAL, INDIRECT OR CONSEQUENTIAL DAMAGES OF ANY KIND,
* OR ANY DAMAGES WHATSOEVER, INCLUDING WITHOUT LIMITATION, LOSS OF
* PROFIT, LOSS OF USE, SAVINGS OR REVENUE, OR THE CLAIMS OF THIRD
* PARTIES, WHETHER OR NOT SILICON GRAPHICS, INC.  HAS BEEN ADVISED OF
* THE POSSIBILITY OF SUCH LOSS, HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, ARISING OUT OF OR IN CONNECTION WITH THE POSSESSION, USE
* OR PERFORMANCE OF THIS SOFTWARE.
*
* US Government Users Restricted Rights
* Use, duplication, or disclosure by the Government is subject to
* restrictions set forth in FAR 52.227.19(c)(2) or subparagraph
* (c)(1)(ii) of the Rights in Technical Data and Computer Software
* clause at DFARS 252.227-7013 and/or in similar or successor clauses
* in the FAR or the DOD or NASA FAR Supplement.  Unpublished - rights
* reserved under the copyright laws of the United States.
*
* Contractor/manufacturer is:
*	Silicon Graphics, Inc.
*	1500 Crittenden Lane
*	Mountain View, CA  94043
*	United State of America
*
* OpenGL(R) is a registered trademark of Silicon Graphics, Inc.
*/

/*
*  clip.c
*  This program demonstrates arbitrary clipping planes.
*/
#include <GL/glut.h>
/*#include <stdlib.h>  */

void init(void)
{
glClearColor (0.0, 0.0, 0.0, 0.0);
glShadeModel (GL_FLAT);
GLint clipPlanes;
// 6,OGL中一般也是最多支持6个裁剪平面
glGetIntegerv(GL_MAX_CLIP_PLANES, &clipPlanes);
}

void display(void)
{
GLdouble eqn[4] = {0.0, 1.0, 0.0, -.5};
GLdouble eqn2[4] = {1.0, 0.0, 0.0, 0.0};

glClear(GL_COLOR_BUFFER_BIT);

glColor3f (1.0, 1.0, 1.0);
glPushMatrix();
glTranslatef (0.0, 0.0, -5.0);

/*    clip lower half -- y < 0          */
//glRotatef(45, 1, 0, 0);
glClipPlane (GL_CLIP_PLANE0, eqn);
glEnable (GL_CLIP_PLANE0);
/*    clip left half -- x < 0           */
glClipPlane (GL_CLIP_PLANE1, eqn2);
glEnable (GL_CLIP_PLANE1);

glRotatef (90.0, 1.0, 0.0, 0.0);
glutWireSphere(1.0, 20, 16);
glPopMatrix();

glFlush ();
}

void reshape (int w, int h)
{
glViewport (0, 0, (GLsizei) w, (GLsizei) h);
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
gluPerspective(60.0, (GLfloat) w/(GLfloat) h, 1.0, 20.0);
glMatrixMode (GL_MODELVIEW);
}

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

int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize (500, 500);
glutInitWindowPosition (100, 100);
glutCreateWindow (argv[0]);
init ();
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutKeyboardFunc(keyboard);
glutMainLoop();
return 0;
}