求助OpenGL的问题，怎么去掉glutSwapBuffers()就显示不出来图像了？变成一片空白？
代码如下：
#ifdef _WIN32#include <windows.h>#endif#include <assert.h>#include <iostream>#include <math.h>#if 0#include <GL/gl.h>#else#include <GL/glew.h> #endif#include <GL/glut.h>#include <stdlib.h>float angleX = 2.0;float angle = 2.0, deltaAngle = 1.0;float red=1.0, blue=1.0, green=1.0;float width,height;int specialKey;//draw circleconst int n = 20;const GLfloat R = 0.5f;const GLfloat Pi = 3.1415926536f; void JShape(GLsizei w,GLsizei h) { glClearColor(1.0,1.0,1.0,0.0); glMatrixMode(GL_PROJECTION); gluOrtho2D(0,800,0,800); }void Primitives(void) { glClear(GL_COLOR_BUFFER_BIT); glColor3f(1,0,0); glBegin(GL_LINES); glVertex2i(800,15); glVertex2i(10,800); glEnd(); glFlush(); // glutSwapBuffers(); }void main(int argc, char **argv) { glutInit(&argc, argv); glutInitDisplayMode(GLUT_DEPTH | GLUT_DOUBLE); glutInitWindowPosition(80,80); glutInitWindowSize(800,800); glutCreateWindow("FIVE PIMITIVES"); glutDisplayFunc(Primitives); glutReshapeFunc(JShape); glutMainLoop(); }