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

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.