Question

This code is from the red book, example 2-15 (well, the code is not exactly the one in the book). Take care of the note I noted.

#include <fstream>
#include <stdlib.h>
#include <GL/glew.h>
#include <GL/glut.h>
#pragma comment(lib,"glew32.lib")
using namespace std;
#define BUFFER_OFFSET(offset) ((GLubyte *)NULL+offset)
#define XStart              -0.8
#define XEnd                0.8
#define YStart              -0.8
#define YEnd                0.8
#define NumXPoints          11
#define NumYPoints          11
#define NumPoints           (NumXPoints * NumYPoints)
#define NumPointsPerStrip   (2*NumXPoints)
#define NumStrips           (NumYPoints-1)
#define RestartIndex        0xffff

void display(void)
{
    int i,start;
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glColor3f(1,1,1);
    glDrawElements(GL_TRIANGLE_STRIP,NumStrips*(NumPointsPerStrip+1),GL_UNSIGNED_SHORT,BUFFER_OFFSET(0));
    //glFlush();//it works,show a white square with black backgroud
    glutSwapBuffers();///it doesn't work,show what tha area looked like before

}

void init (void) 
{
    GLuint vbo,ebo;
    GLfloat *vertices;
    GLushort *indices;
    glewInit();
    glGenBuffers(1,&vbo);
    glBindBuffer(GL_ARRAY_BUFFER,vbo);
    glBufferData(GL_ARRAY_BUFFER,2*NumPoints*sizeof(GLfloat),NULL,GL_STATIC_DRAW);
    vertices=(GLfloat *)glMapBuffer(GL_ARRAY_BUFFER,GL_WRITE_ONLY);
    if(vertices==NULL)
    {
        fprintf(stderr,"Unable to map vertex buffer\n");
        exit(EXIT_FAILURE);
    }
    else
    {
        int i,j;
        GLfloat dx=(XEnd-XStart)/(NumXPoints-1);
        GLfloat dy=(YEnd-YStart)/(NumYPoints-1);
        GLfloat *tmp=vertices;
        int n=0;
        for(j=0;j<NumYPoints;++j)
        {
            GLfloat y=YStart+j*dy;
            for(i=0;i<NumXPoints;++i)
            {
                GLfloat x=XStart + i*dx;
                *tmp++=x;
                *tmp++=y;
            }
        }
        glUnmapBuffer(GL_ARRAY_BUFFER);
        glVertexPointer(2,GL_FLOAT,0,BUFFER_OFFSET(0));
        glEnableClientState(GL_VERTEX_ARRAY);
    }
    glGenBuffers(1,&ebo);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,ebo);

    glBufferData(GL_ELEMENT_ARRAY_BUFFER,NumStrips*(NumPointsPerStrip+1)*sizeof(GLushort),NULL,GL_STATIC_DRAW);
    indices=(GLushort *)glMapBuffer(GL_ELEMENT_ARRAY_BUFFER,GL_WRITE_ONLY);
    if(indices==NULL)
    {
        fprintf(stderr,"Unable to map index buffer\n");
        exit(EXIT_FAILURE);
    }
    else
    {
        int i,j;
        GLushort *index=indices;
        for(j=0;j<NumStrips;++j)
        {
            GLushort bottomRow=j*NumYPoints;
            GLushort topRow=bottomRow+NumYPoints;
            for(i=0;i<NumXPoints;++i)
            {
                *index++=topRow+i;
                *index++=bottomRow+i;
            }
            *index++=RestartIndex;
        }
        glUnmapBuffer(GL_ELEMENT_ARRAY_BUFFER);

    }
    glPrimitiveRestartIndex(RestartIndex);
    glEnable(GL_PRIMITIVE_RESTART);
}


void reshape (int w, int h)
{

    glMatrixMode (GL_PROJECTION);
    glLoadIdentity ();
    gluOrtho2D (-1,1,-1,1);
    glViewport (0,0,w,h);
}

void keyboard(unsigned char key, int x, int y)
{
    switch (key) {
      case 27:
          exit(0);
          break;
    }
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize (200, 200);
    glutInitWindowPosition (100, 100);
    glutCreateWindow (argv[0]);
    init ();
    glutDisplayFunc(display);
    glutReshapeFunc(reshape);
    glutKeyboardFunc (keyboard);
    glutMainLoop();
    return 0;  
}

as the http://www.opengl.org/resources/libraries/glut/spec3/node21.html says:
An implicit glFlush is done by glutSwapBuffers before it returns. Subsequent OpenGL commands can be issued immediately after calling glutSwapBuffers, but are not executed until the buffer exchange is completed.
If the layer in use is not double buffered, glutSwapBuffers has no effect.

How do I know whether the layer in use is double buffered?give an double buffered example.is this code I wrote is double buffered?

Answer 1

In your main function, you call

glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);

That needs to be GLUT_DOUBLE. See the documentation for glutInitDisplayMode().