OpenGL Nothing will Display

Question

Why can't I get anything to display with this code?

#include <iostream>
#include "GL/glfw.h"
#ifndef MAIN
#define MAIN
#include "GL/gl.h"
#include "GL/glu.h"
#endif
using namespace std;

void display();

int main()
{
    int running = GL_TRUE;
    glfwInit();

    if( !glfwOpenWindow( 640,480, 0,0,0,0,0,0, GLFW_WINDOW ) )
    {
        glfwTerminate();
        return 0;
    }

    while( running )
    {
        //GL Code here
        display();

        glfwSwapBuffers();
        // Check if ESC key was pressed or window was closed
        running = !glfwGetKey( GLFW_KEY_ESC ) &&
        glfwGetWindowParam( GLFW_OPENED );
    }

    glfwTerminate();

    return 0;
}

void display()
{
    glClearColor(0,0,0,0);
    glClear(GL_COLOR_BUFFER_BIT);
    glViewport(0,0,640,480);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    //glTranslatef(0, 0, -2);
    glBegin(GL_POLYGON);
        glColor3f(1.0, 0.2, 0.2);
        glVertex3f(0.25, 0.25, 0.0);
        glVertex3f(0.75, 0.25, 0.0);
        glVertex3f(0.75, 0.75, 0.0);
        glVertex3f(0.25, 0.75, 0.0);
    glEnd();
    glFlush();
}

Answer 1

You haven't set the viewport with glViewport nor the projection matrix. If you change the clear color to something other than black do you get this color in your window?

Try changing the code to:

void display()
{
    glClearColor(0,0,0,0);
    glClear(GL_COLOR_BUFFER_BIT);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(-1, 1, -1, 1);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    //glTranslatef(0, 0, -2);
    glBegin(GL_POLYGON);
        glColor3f(1.0, 0.2, 0.2);
        glVertex3f(0.25, 0.25, 0.0);
        glVertex3f(0.75, 0.25, 0.0);
        glVertex3f(0.75, 0.75, 0.0);
        glVertex3f(0.25, 0.75, 0.0);
    glEnd();
    glFlush();
}

(The viewport may have been set correctly when you created your OpenGL window, otherwise initialise it with the window width and height)

Also disable backface culling incase your polygon has incorrect winding, glDisable(GL_CULLFACE)

Answer 2

I think the issue is that when you don't specify a coordinate system eg

void glOrtho(GLdouble   left,
             GLdouble   right,
             GLdouble   bottom,
             GLdouble   top,
             GLdouble   nearVal,
             GLdouble   farVal);

or

void glFrustum(GLdouble  left,
               GLdouble  right,
               GLdouble  bottom,
               GLdouble  top,
               GLdouble  nearVal,
               GLdouble  farVal);

the default coordinate system is:

-1, 1, -1, 1, -1, 1

so the glTranslate moves the object outside the coordinate system and openGL clips the object.

If you change the glTranslate() call to translate by 1.0 the object is visible.