openGL编程指南中逆变换的有关问题

openGL编程指南中逆变换的问题
编程指南中逆变换和模拟变换着一节中的一个例子，有的地方还不太明白，请大神们给指点一下，以下是这个例子的源码
///////////////////////////////////////////////////////////////////////////////
#include <stdlib.h>
#include <GL/glut.h>
#include <stdio.h>

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

/* Change these values for a different transformation  */
void reshape(int w, int h)
{
   glViewport (0, 0, (GLsizei) w, (GLsizei) h);
   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();
   gluPerspective (45.0, (GLfloat) w/(GLfloat) h, 1.0, 100.0);
   glMatrixMode(GL_MODELVIEW);
   glLoadIdentity();
}

void mouse(int button, int state, int x, int y)
{
   GLint viewport[4];
   GLdouble mvmatrix[16], projmatrix[16];
   GLint realy;  /*  OpenGL y coordinate position  */
   GLdouble wx, wy, wz;  /*  returned world x, y, z coords  */

   switch (button) {
      case GLUT_LEFT_BUTTON:
         if (state == GLUT_DOWN) {
            glGetIntegerv (GL_VIEWPORT, viewport);
            glGetDoublev (GL_MODELVIEW_MATRIX, mvmatrix);
            glGetDoublev (GL_PROJECTION_MATRIX, projmatrix);
/*  note viewport[3] is height of window in pixels  */
            realy = viewport[3] - (GLint) y - 1;//为什么要这么计算
            printf ("Coordinates at cursor are (%4d, %4d)\n", x, realy);
            gluUnProject ((GLdouble) x, (GLdouble) realy, 0.0,
               mvmatrix, projmatrix, viewport, &wx, &wy, &wz);
            printf ("World coords at z=0.0 are (%f, %f, %f)\n",
               wx, wy, wz);
            gluUnProject ((GLdouble) x, (GLdouble) realy, 1.0,
               mvmatrix, projmatrix, viewport, &wx, &wy, &wz);
            printf ("World coords at z=1.0 are (%f, %f, %f)\n",
               wx, wy, wz);
         }
         break;
      case GLUT_RIGHT_BUTTON:
         if (state == GLUT_DOWN)
            exit(0);
         break;
      default:
         break;
   }
}

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

/*
*  Open window, register input callback functions
*/
int main(int argc, char** argv)
{
   glutInit(&argc, argv);
   glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
   glutInitWindowSize (500, 500);
   glutInitWindowPosition (100, 100);
   glutCreateWindow (argv[0]);
   glutDisplayFunc(display);
   glutReshapeFunc(reshape);
   glutKeyboardFunc (keyboard);
   glutMouseFunc(mouse);
   glutMainLoop();
   return 0;
}
/////////////////////////////////////////////////////////////////////////////////
实现的功能就是读取鼠标位置，并确定鼠标光标位置经过变换之后在三维空间的近侧裁剪平面和远侧裁剪平面上的点，问题在mouse这个函数中被红色标出：
1、这个计算是什么原理，为什么要这么算？
2、红色标记的变量各代表什么意思？x,realy   wx,wy,wz

openGL编程指南中逆变换的有关问题

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