opengl中gluLookAt函数的疑问

[8435123]

首先代码如下：

#include "pch.h"
#include <gl/GLUT.H>

GLsizei ww = 500, wh = 500;

void display()
{
	glClear(GL_COLOR_BUFFER_BIT);   //清空颜色缓冲池
	glColor3f(1.0f, 0.0f, 0.0f);    //设置绘图颜色
	//gluLookAt(0.0, 0.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
	glRectf(-0.5f, -0.5f, 0.5f, 0.5f);  //绘制矩形
	glFlush();  //执行OpenGL指令列表中的指令
}

void myReshape(int w, int h)
{
	ww = w, wh = h;
	glViewport(0, 0, w, h);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	if (w <= h) glOrtho(-2.0, 2.0, -2.0 * (GLfloat)h / (GLfloat)w, 2.0* (GLfloat)h / (GLfloat)w, 0.0, 20.0);
	else glOrtho(-2.0 * (GLfloat)w / (GLfloat)h, 2.0* (GLfloat)w / (GLfloat)h, -2.0, 2.0, 0.0, 20.0);
}

int main(int argc, char **argv)
{
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
	glutInitWindowSize(ww, wh);
	glutCreateWindow("Rec");
	glutReshapeFunc(myReshape);
	glutDisplayFunc(display);
	glutMainLoop();
}

当使用gluLookAt()函数时，运行程序并且在窗口中点击3次鼠标左键，绘制的红色矩形消失了。不使用gluLookAt()时不会有这种情况，小白求教。