OpenGL多视口

#include <gl/glew.h>
#include <gl/freeglut.h>
#include <iostream>

int window_width = ;
int window_height = ;
float rtri = 0.0;
float rquad = 0.0;
void init();
void display();
void resize(int w, int h);
int main(int argc, char ** argv)
{
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE);
   glutInitWindowSize(, );
   glutCreateWindow("OpenGL");
   GLenum res = glewInit();
   if (res != GLEW_OK)
   {
       std::cout << glewGetErrorString(res) << std::endl;
   }

   init();
   glutDisplayFunc(display);
   glutReshapeFunc(resize);
   glutMainLoop();
   return EXIT_SUCCESS;
}

void init()
{
    glClearColor(0.0, 0.0, 0.0, 1.0);
}

void display()
{
    glClear(GL_COLOR_BUFFER_BIT);
    glLoadIdentity();
    for (int loop = ; loop < ; ++loop)
    {
        if ( == loop)
        {
           glViewport(, , window_width/, window_height/);
           glMatrixMode(GL_PROJECTION);
           glLoadIdentity();
           gluOrtho2D(-, , -, );
        }
        if ( == loop)
        {
            glViewport(window_width/, , window_width, window_height/);
            glMatrixMode(GL_PROJECTION);
            glLoadIdentity();
            gluOrtho2D(-, , -, );
        }
        if ( == loop)
        {
            glViewport(, window_height/, window_width/, window_height);
            glMatrixMode(GL_PROJECTION);
            glLoadIdentity();
            gluOrtho2D(-, , -, );
        }
        if ( == loop)
        {
           glViewport(window_width/, window_height/, window_width, window_height) ;
           glMatrixMode(GL_PROJECTION);
           glLoadIdentity();
           gluOrtho2D(-, , -, );
        }
        glMatrixMode(GL_MODELVIEW);
        glLoadIdentity();
        glClear(GL_DEPTH_BUFFER_BIT);
        if ( == loop)
        {
            glBegin(GL_QUADS);
                glVertex2f(-, -);
                glVertex2f(, -);
                glVertex2f(, );
                glVertex2f(-, );
            glEnd();
        }
        if ( == loop)
        {
            glBegin(GL_QUADS);
                glVertex2f(-, -);
                glVertex2f(, -);
                glVertex2f(, );
                glVertex2f(-, );
            glEnd();
        }
        if ( == loop)
        {
            glBegin(GL_QUADS);
                glVertex2f(-, -);
                glVertex2f(, -);
                glVertex2f(, );
                glVertex2f(-, );
            glEnd();
        }
        if ( == loop)
        {
            glBegin(GL_QUADS);
                glVertex2f(-, -);
                glVertex2f(, -);
                glVertex2f(, );
                glVertex2f(-, );
            glEnd();
        }
    }
    glutSwapBuffers();
}

void resize(int w, int h)
{
     window_height = h;
     window_width = w;
}