Question

Using SCREEN_WIDTH = 1200, and SCREEN_HEIGHT = 800.

First, I draw a box to the screen at (x = 0, y = 0, w = 40, h = 40);

Then I use the handleMouse Function to return the x and y coordinates of where the mouse is clicked.

The problem is, when the program starts in non-Maximized windowed mode, when I click on (What appears to be) the very bottom right corner of the box the coordinates returned are x = 40, y = 32. When I think it should be returning x = 40, y = 40.

I don’t know whether the problem is if its not being drawn right, or the functions is returning the wrong x/y.

I believe I understand how openGL rendering, transformation and glOrth work, but I could be completely wrong. I have seen a few suggestions online saying that the Windows Decor(Using windows 7) can cause this problem, but have done very little explaining and provided no solution.

This is my entire source code. I have stripped off everything from my game down to the basics, and the problem still persists 🙁 . I added two pictures so people could see my problem. In NON-MAXIMIZED WINDOW(the top picture), when clicking the bottom-right corner, the coordinates returned are 41,32; The y coordinate is smaller than it should be. And in the MAXIMIZED WINDOW(the bottom picture), when clicking the same corner, It returns the correct coordinates 40, 40. These results occur for both my original source code and genpfault’s suggested code.

//Turns out I can’t post Pictures :(, links instead.

non-Maximized Windowed!

Maximized Windowed!

main.cpp
int main( int argc, char* args[] )
{
    //Initialize FreeGLUT
    glutInit( &argc, args );

    //Create OpenGL 2.1 context
    glutInitContextVersion( 2, 1 );

    //Create Double Buffered Window
    glutInitDisplayMode( GLUT_DOUBLE );
    glutInitWindowSize( SCREEN_WIDTH, SCREEN_HEIGHT );
    glutCreateWindow( "OpenGL" );

    //Do post window/context creation initialization
    if( !initGL() )
    {
        printf( "Unable to initialize graphics library!\n" );
        return 1;
    }
    //initGame();


    glutMouseFunc(handleMouse);


    glutDisplayFunc( render );


    glutMainLoop();

    return 0;
}

Functions.h

#ifndef FUNCTIONS_H
#define FUNCTIONS_H

bool initGL();

void render();

void handleMouse(int button, int state, int x, int y);

#endif

Functions.cpp

bool initGL()
{

    //Initialize clear color
    glClearColor( 0.f, 0.f, 0.f, 1.f );

    //Check for error
    GLenum error = glGetError();
    if( error != GL_NO_ERROR )
    {
        //cout <<"Error initializing OpenGL! " << gluErrorString( error ) << endl;
        return false;
    }

    return true;
}

void render()
{

    //Clear color buffer
    glClear( GL_COLOR_BUFFER_BIT );
    glColor3f(1.f,1.f,1.f);

    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    gluOrtho2D(0, SCREEN_WIDTH, SCREEN_HEIGHT, 0);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glBegin(GL_QUADS);
        glVertex2f(0,0);
        glVertex2f(0, 41);
        glVertex2f(41, 41);
        glVertex2f(41, 0);
    glEnd();

    glutSwapBuffers();
}

void handleMouse(int button, int state, int x, int y)
{

    std::cout << x << '\t' << y << std::endl;

}

constants.h

const int SCREEN_WIDTH = 1200;
const int SCREEN_HEIGHT = 800;

Answer 1

This works fine for me on Windows 7 on Aero and Classic:

#include <GL/glut.h>
#include <iostream>

void render()
{
    glClearColor( 0, 0, 0, 1 );
    glClear( GL_COLOR_BUFFER_BIT );

    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    double w = glutGet( GLUT_WINDOW_WIDTH );
    double h = glutGet( GLUT_WINDOW_HEIGHT );
    glOrtho(0, w, h, 0, -1, 1);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glColor3ub( 255, 255, 255 );
    glBegin(GL_QUADS);
        glVertex2f(0,0);
        glVertex2f(41, 0);
        glVertex2f(41, 41);
        glVertex2f(0, 41);
    glEnd();

    glutSwapBuffers();
}

void handleMouse(int button, int state, int x, int y)
{
    std::cout << x << '\t' << y << std::endl;
}

int main( int argc, char* args[] )
{
    glutInit( &argc, args );

    glutInitDisplayMode( GLUT_DOUBLE );
    glutInitWindowSize( 640, 480 );
    glutCreateWindow( "OpenGL" );

    glutMouseFunc(handleMouse);
    glutDisplayFunc( render );
    glutMainLoop();
    return 0;
}

Most notably I added the runtime glutGet() window size queries to render().