Inaccurate Mouse Coordinates Returned

The issue is that the further the mouse click is from the top left origin (0,0) the greater the height inaccuracy when the vertex is plotted. Any ideas?

int WindowWidth = 19;
int WindowHeight = 13;
int mouseClickCount = 0;
int rectPlotted;
GLint x1;
GLint y1;
GLint x2;
GLint y2;

//Declare our functions with prototypes:
void display(void);
void init (void);
void processNormalKeys(unsigned char key, int x, int y);
void on_vertex_selected(GLint x, GLint y);
void on_mouse_event(int button, int state, int x, int y);
/////////////////////////////////// MAIN ////////////////////////////////
int main(int argc, char** argv)
{
  glutInit(&argc, argv);
  glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
  //start calculation of max window size available at 19:13 aspect ratio
  int screenWidth = glutGet(GLUT_SCREEN_WIDTH);
  int screenHeight = glutGet(GLUT_SCREEN_HEIGHT);
  screenWidth = screenWidth/WindowWidth;
  WindowWidth = WindowWidth*screenWidth;
  screenHeight = screenHeight/WindowHeight;
  WindowHeight = screenHeight*WindowHeight;
  //end calculation
  glutInitWindowSize (WindowWidth, WindowHeight);
  glutInitWindowPosition (0, 0);
  glutCreateWindow ("Plot a rectangle!");
  glutDisplayFunc(display);
  glutKeyboardFunc(processNormalKeys);
  glutMouseFunc(on_mouse_event);
  init();
  glutMainLoop();
  return 0;
}
/////////////////////////////////////////////////////////////////////////

void display(void)
{
 glClear (GL_COLOR_BUFFER_BIT); //clear all pixels
 glFlush();
}

void init (void)
{
  /* select clearing (background) color */
  glClearColor (1.0, 1.0, 1.0, 0.0);

  /* initialize viewing values */
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  glOrtho(0, WindowWidth, 0, WindowHeight, -1.0, 1.0);
}

void processNormalKeys(unsigned char key, int x, int y) {
 if (key == 27) //esc
  exit(0);
//Allows color change of most recently plotted rectangle
 if (key == 98){ //b
  glColor3f(0.0, 0.0, 1.0);
  glBegin(GL_QUADS);
  glVertex2i(x1, y1);
  glVertex2i(x1, y2);
  glVertex2i(x2, y2);
  glVertex2i(x2, y1);
  glEnd();
  glFlush();
 }
 if (key == 114){ //r
  glColor3f(1.0, 0.0, 0.0);
  gl开发者_开发技巧Begin(GL_QUADS);
  glVertex2i(x1, y1);
  glVertex2i(x1, y2);
  glVertex2i(x2, y2);
  glVertex2i(x2, y1);
  glEnd();
  glFlush();
 }
 if (key == 103){ //g
  glColor3f(0.0, 1.0, 0.0);
  glBegin(GL_QUADS);
  glVertex2i(x1, y1);
  glVertex2i(x1, y2);
  glVertex2i(x2, y2);
  glVertex2i(x2, y1);
  glEnd();
  glFlush();
 }
}

void on_vertex_selected(GLint x, GLint y){
 if(mouseClickCount == 0){
  x1 = x;
  y1 = y;
  glColor3f(0.0, 0.0, 0.0);
  glEnable(GL_POINT_SMOOTH);
  glPointSize(5.0);
  glBegin(GL_POINTS);
  glVertex2i(x1, y1);
  glEnd();
  glFlush();
 }
 else{
  x2 = x;
  y2 = y;
  glBegin(GL_POINTS);
  glColor3f(1.0, 1.0, 1.0);
  glVertex2i(x1,y1); //"clears" previous point to make way for the rectangle
  glEnd();
  glColor3f(0.0, 0.0, 0.0);
  glBegin(GL_QUADS);
  glVertex2i(x1, y1);
  glVertex2i(x1, y2);
  glVertex2i(x2, y2);
  glVertex2i(x2, y1);
  glEnd();
  glFlush();
 }
}

void on_mouse_event(int button, int state, int x, int y){
 if(button==GLUT_LEFT_BUTTON && state ==GLUT_DOWN && mouseClickCount == 0){
  //y = y+20; //adjusts for VM mouse tracking error
  on_vertex_selected(x, WindowHeight - y);
  rectPlotted = 0;
    }
 if(button==GLUT_LEFT_BUTTON && state ==GLUT_UP && mouseClickCount == 0){
  if(rectPlotted == 1){
   return;
  }
  else{
   mouseClickCount++;
  }
 }
 if(button==GLUT_LEFT_BUTTON && state ==GLUT_DOWN && mouseClickCount == 1){
   //y = y+20; //adjusts for VM mouse tracking error
   on_vertex_selected(x, WindowHeight - y);
   mouseClickCount = 0;
   rectPlotted = 1;
 }
}

Your glOrtho() call is almost certainly wrong. You pass the window size, not the window client area size. The difference is the size of the borders and the height of the caption.

Be aware that your actual window size will most probably be slightly different than what you asked at init time.

Simply said, implement a glutReshapeFunc()

I was compiling and running my code within Parallels VM v.6. This was the issue leading to inaccurate mouse coordinates being returned. I compiled and ran the exact same code on OS X without problem.