如何反转OpenGL代码的输出？

Question

我编写了以下代码来使用OpenGL打印Koch曲线。但我得到一个倒置的输出。在OpenGL中反转输出需要哪些修改？另外，还有其他公式可以找到Koch曲线的等边三角形的顶点坐标吗？

#include<iostream>
#include<GL/glut.h>
#define COS 0.5
#define SIN 0.866
int x1=10,x2=550,y1=200,y2=200;

void myinit()
{
    glClearColor(0.0,1.0,0.0,1.0);
    glColor3f(0.0,0.0,1.0);
    glPointSize(3.0);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0,640,0,480);
}

void koch(int x1, int y1, int x2, int y2, int m)
{
        int x[5],y[5],xx,yy,lx,ly;
        lx=(x2-x1)/3;
        ly=(y2-y1)/3;
        x[0]=x1;
        y[0]=y1;
        x[4]=x2;
        y[4]=y2;
        x[1]=x[0]+lx;
        y[1]=y[0]+ly;
        x[3]=x[0]+2*lx;
        y[3]=y[0]+2*ly;
        xx=x[3]-x[1];
        yy=y[3]-y[1];
        x[2]=xx*(COS)+yy*(SIN);
        y[2]=-xx*(SIN)+yy*(COS);
        x[2]=x[1]+x[2];
        y[2]=y[1]+y[2];

        if(m>0)
        {
            koch(x[0],y[0],x[1],y[1],m-1);
            koch(x[1],y[1],x[2],y[2],m-1);
            koch(x[2],y[2],x[3],y[3],m-1);
            koch(x[3],y[3],x[4],y[4],m-1);
        }
        else
        {
            glBegin(GL_LINES);
                glVertex2d(x[0],y[0]);
                glVertex2d(x[1],y[1]);
            glEnd();
            glBegin(GL_LINES);
                glVertex2d(x[1],y[1]);
                glVertex2d(x[2],y[2]);
            glEnd();
            glBegin(GL_LINES);
                glVertex2d(x[2],y[2]);
                glVertex2d(x[3],y[3]);
            glEnd();
            glBegin(GL_LINES);
                glVertex2d(x[3],y[3]);
                glVertex2d(x[4],y[4]);
            glEnd();    
        }
}

void display()
{
    glClear(GL_COLOR_BUFFER_BIT);
    glBegin(GL_LINES);
        koch(x1,y1,x2,y2,2);
    glEnd();
    glFlush();  
}

int main(int argc, char ** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize(640,480);
    glutInitWindowPosition(10,10);
    glutCreateWindow("Grllp");
    glutDisplayFunc(display);
    myinit();
    glutMainLoop(); 
}