关于opengl：将透视投影转换为正交投影

Converting perspective projection to orthogonal projection

我创建了这个程序，用户单击鼠标右键，他会得到一个菜单，他可以在其中选择在透视投影中绘制立方体或球体

我希望他能够将透视投影更改为正交投影。我希望仅使用正交投影将形状放置在同一位置。

我希望他能够通过单击键盘上的"O"来执行此操作，然后通过单击"p"返回透视图。

如何在这段代码中混合正交投影？

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#include <windows.h>
#include <stdio.h>
#include<math.h>

#include <gl\\glut.h> // glut.h must come before gl.h and glu.h
#include <gl\\gl.h>
#include <gl\\glu.h>

void menu(int value);
static int win;
static int menid;
static int submenid;
static int left_click = GLUT_UP;
static int right_click = GLUT_UP;
static int xold;
static int yold;
static int width;
static int height;
static float rotate_x = 30;
static float rotate_y = 15;
static float alpha = 0;
static float beta = 0;
static int primitive = 0;
static int orth =0;
static int flag =0;

void sphere (void)
{
glBegin (GL_LINES);
glColor3f (1, 0, 0); glVertex3f (-1, -1, -1); glVertex3f ( 1, -1, -1);
glColor3f (0, 1, 0); glVertex3f (-1, -1, -1); glVertex3f (-1, 1, -1);
glColor3f (0, 0, 1); glVertex3f (-1, -1, -1); glVertex3f (-1, -1, 1);

glEnd ();
glRotatef (beta, 1, 0, 0);
glRotatef (alpha, 0, 1, 0);
glColor3f (0, 1, 0);
glutWireSphere(1.0, 20, 16);
}

void cube (void)
{
glBegin (GL_LINES);
glColor3f (1, 0, 0); glVertex3f (-1, -1, -1); glVertex3f ( 1, -1, -1);
glColor3f (0, 1, 0); glVertex3f (-1, -1, -1); glVertex3f (-1, 1, -1);
glColor3f (0, 0, 1); glVertex3f (-1, -1, -1); glVertex3f (-1, -1, 1);

glEnd ();
glRotatef (beta, 1, 0, 0);
glRotatef (alpha, 0, 1, 0);
glColor3f (1, 0, 0);
glutWireCube(1);

}

void KeyboardFunc (unsigned char key, int x, int y)
{
xold = x; /* Has no effect: just to avoid a warning */
yold = y;
if ('q' == key || 'Q' == key || 27 == key)
exit (0);
if(key=='O' || key =='o')
//does something to change the perspective to orthogonal...
// you would want to redraw now
glutPostRedisplay();

}
void createMenu(void){

// MENU //

submenid = glutCreateMenu(menu);

glutAddMenuEntry("cube", 2);
glutAddMenuEntry("sphere", 3);

menid = glutCreateMenu(menu);

// Create an entry
glutAddMenuEntry("Clear", 1);

glutAddSubMenu("Draw", submenid);

glutAddMenuEntry("Quit", 0);

glutAttachMenu(GLUT_RIGHT_BUTTON);

}

void menu(int value)
{
if(value == 0)
{
glutDestroyWindow(win);
exit(0);
}
else{
primitive=value;
}
glutPostRedisplay();
}

void DisplayFunc (void)
{
const float a = height / (float) width;
const float b = width / (float) height;

glClear (GL_COLOR_BUFFER_BIT);

glMatrixMode(GL_PROJECTION);

/* Perspective projection */
glLoadIdentity();
gluPerspective (20 * sqrt (1 + a * a), b, 8, 12);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

/* Perspective view */
glViewport (0, 0, width , height);
glPushMatrix ();
glTranslatef (0, 0, -10);
glRotatef (rotate_y, 1, 0, 0);
glRotatef (rotate_x, 0, 1, 0);

switch(primitive)
{

case 2:
cube();
break;

case 3:
sphere();
break;
}

glPopMatrix ();

glFlush ();
glutSwapBuffers ();
}

void ReshapeFunc (int new_width, int new_height)
{
width = new_width;
height = new_height;
glutPostRedisplay();
}

void MouseFunc (int button, int state, int x, int y)
{
if (GLUT_LEFT_BUTTON == button)
left_click = state;
if (GLUT_RIGHT_BUTTON == button)
right_click = state;
xold = x;
yold = y;
}

void MotionFunc (int x, int y)
{
if (GLUT_DOWN == left_click)
{
rotate_y = rotate_y + (y - yold) / 5.f;
rotate_x = rotate_x + (x - xold) / 5.f;
if (rotate_y > 90)
rotate_y = 90;
if (rotate_y < -90)
rotate_y = -90;
glutPostRedisplay ();
}
if (GLUT_DOWN == right_click)
{
beta = beta + (y - yold) / 2.f;
alpha = alpha + (x - xold) / 2.f;
glutPostRedisplay ();
}
xold = x;
yold = y;
}

int main (int argc, char **argv)
{
/* Creation of the window */
glutInit (&argc, argv);
glutInitDisplayMode (GLUT_RGB | GLUT_DOUBLE);
glutInitWindowSize (500, 500);
glutCreateWindow ("perspective");

/* OpenGL settings */
glClearColor (0, 0, 0, 0);
glEnable (GL_CULL_FACE);
glCullFace (GL_BACK);
glEnable (GL_BLEND);
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

/* Declaration of the callbacks */
glutDisplayFunc (&DisplayFunc);
glutReshapeFunc (&ReshapeFunc);
glutKeyboardFunc (&KeyboardFunc);
glutMouseFunc (&MouseFunc);
glutMotionFunc (&MotionFunc);

// put all the menu functions in one nice procedure
createMenu();

/* Loop */
glutMainLoop ();
return 0;
}