Question

i have the following code which draws nothing.
If i use glBegin(GL_POINTS) it draws a circle but with polygon mode it doesn’t.

int WXSIZE=500,WYSIZE=500;

//Coordinate system
float Xmin=-8, Xmax=8, Ymin=-8, Ymax=8;

void setupmywindow()
{
    glClearColor(0,0,0,0);  
    gluOrtho2D(Xmin, Xmax, Ymin, Ymax);
}


void mypolygon(float radius) //test object
{

   glColor3f(1,0,0);
   int numPoints=20;
   float x,y;
   float centerx,centery=0;

    for (int i = 0; i < numPoints; i++)
{
    x = centerx + radius * sin(2.0*PI*i/numPoints);
    y = centery + radius * cos(2.0*PI*i/numPoints);

     glPolygonMode(GL_FRONT_AND_BACK,GL_LINE);
     glBegin(GL_POLYGON);
     glVertex2f(x, y);
     glEnd();

}

}


void myDisplay()
//single object
{
    glClear(GL_COLOR_BUFFER_BIT);
    mypolygon(2.0);
    glutSwapBuffers();
}


int main(int argc, char **argv)
{

    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGB);
    glutInitWindowSize(WXSIZE,WYSIZE);
    glutCreateWindow("My graphic window");
    setupmywindow();
    glutDisplayFunc(myDisplay);
    glutMainLoop();
} 

Any suggestions?

EDIT———————-

glBegin(GL_POLYGON);

    for (int i = 0; i < numPoints; i++)
{
    x = centerx + radius * sin(2.0*PI*i/numPoints);
    y = centery + radius * cos(2.0*PI*i/numPoints);

    glVertex2f(x, y);

}
   glEnd();

I messed it with the loop.

Answer 1

In every loop you are drawing a polygon, that consists of a single vertex, so nothing. Just put the glBegin/glEnd (and the glPolygonMode) outside of the for loop and only draw glVertex in the loop. Of course it works with points, as a n times a single point is the same as n points. But n polygon consisting of one point each is not the same as one polygon consisting of n points.