opengl出现的不惑.该怎么处理

opengl出现的不惑...
下面是我写的一个小的Opengl程序，原本是打算是在白色的矩形区域内画四个不同颜色的点，但是却出现了一个绿色区域内的四个点，求解释啊。。下面是我的代码：
#include <gl/glut.h>
void display(){
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1.0,0.0,0.0);
  glBegin(GL_POLYGON);
glVertex2f(-2,-1);
glVertex2f(-2,-2);
glVertex2f(-1,0);
glVertex2f(-4,-2);
glEnd();
  glPointSize(2.0);
glBegin(GL_POINTS);
glColor3f(1.0,1.0,1.0);
glVertex2f(-0.5,-0.5);
glColor3f(1.0,0.0,0.0);
glVertex2f(-0.5,0.5);  
glColor3f(0.0,0.0,0.0);
glVertex2f(0.5,0.5);
glColor3f(0.0,1.0,0.0);
glVertex2f(0.5,-0.5);
glEnd();
glFlush();
}
void init()
{
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-1.0,1.0,-1.0,1.0);
}
int main(int argc,char **argv){
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowPosition(0,0);
glutInitWindowSize(500,500);
glutCreateWindow("simple");
glutDisplayFunc(display);
init();
glutMainLoop();
}

。。。各位大虾给个解释吧。。。。

[解决办法]
你画的矩形不在屏幕坐标范围内.
可改为

C/C++ code

    //白色区域    glColor3f(1.0,1.0,1.0);            glBegin(GL_POLYGON);    glVertex2f(-0.75,-0.75);    glVertex2f(0.75,-0.75);    glVertex2f(0.75,0.75);    glVertex2f(-0.75,0.75);    glEnd();    glPointSize(2.0);    glBegin(GL_POINTS);    glColor3f(0.0,0.0,0.0);            //黑    glVertex2f(-0.5,-0.5);    glColor3f(1.0,0.0,0.0);            //红    glVertex2f(-0.5,0.5);           glColor3f(0.0,0.0,1.0);            //蓝    glVertex2f(0.5,0.5);    glColor3f(0.0,1.0,0.0);            //绿    glVertex2f(0.5,-0.5);    glEnd();    glFlush();
[解决办法]
gluOrtho2D(-1.0,1.0,-1.0,1.0)

有效区域是(-1.0,1.0,-1.0,1.0)