改变gluOrtho2D()函数的参数，实现图形的伸缩。

注：源代码参照《computer graphics with opengl》英文版第四版272页。
1、关键是如何控制gluOrtho2D()的参数

gluOrtho2D(-size, size, -size, size);

2、改变参数

GLint flag = 0;     //to control '+' or '-'
...
if (flag == 0)
        size++;
    else if (flag == 1)
        size--;
    if (size > 1000)
        flag = 1;
    if (size < 100)
        flag = 0;

3、完整代码

#include "pch.h"
#include <GL/glut.h>

GLfloat size = 100.0;
GLsizei size1 = 100, size2 = 100;
GLint flag = 0;     //to control '+' or '-'
class wcPt2D {
public:
    GLfloat x, y;
};

void init(void) {
    /* set color of display window to white */
    glClearColor(1.0, 1.0, 1.0, 1.0);
    gluOrtho2D(-size, size, -size, size);

    /*set parameters for world-cooridinate clipping window*/
    glMatrixMode(GL_PROJECTION);

    /*set mode for constructiong geometric transformation matrix*/
    glMatrixMode(GL_MODELVIEW);
}

void triangle(wcPt2D *verts) {
    GLint k;

    glBegin(GL_TRIANGLES);
    for (k = 0; k < 3; k++) {
        glVertex2f(verts[k].x, verts[k].y);
        }
    glEnd();
}

void changeSize() {
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    if (flag == 0)
        size++;
    else if (flag == 1)
        size--;
    if (size > 1000)
        flag = 1;
    if (size < 100)
        flag = 0;
    gluOrtho2D(-size, size, -size, size);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glutPostRedisplay();
}

void displayFcn(void) {
    /*define initial position for triangle*/
    wcPt2D verts[3] = {
        {-50.0,-25.0},
        {50.0,-25.0},
        {0.0,50.0}
    };

    /*clear diaplay window*/
    glClear(GL_COLOR_BUFFER_BIT);

    /*set fill color to blue*/
    glColor3f(0.0, 0.0, 1.0);

    /*set left viewport*/
    glViewport(0, 0, 300, 300);

    /*diplay triangle*/
    triangle(verts);

    /*rotate Triangle and display in right half of diaplay windows*/
    /*set fill color to red*/
    glColor3f(1.0, 0.0, 0.0);

    /*set rught viewport*/
    glViewport(300, 0, 300, 300);

    /*rotate about a z axis*/
    glRotatef(90.0, 0.0, 0.0, 1.0);

    /*display red rotated triangle */
    triangle(verts);

    glFlush();
}

void main(int argc,char ** argv) {
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowPosition(50, 50);
    glutInitWindowSize(600, 300);
    glutCreateWindow("split-screen example");

    init();
    glutDisplayFunc(displayFcn);
    glutIdleFunc(changeSize);
    glutMainLoop();
}

4、实际效果