2D OpenGL with QMouseEvents

Look at the updated Qt OpenGL example program (introduced in the previous lab).

You can find it at: /home/jungle/pj/www/453/box/2DwithMouseEvents

• The changes listed allow you to use the mouse to rotate the object, as well as the slider.
  You use the left mouse button to rotate.
  
• You can also use the mouse to move the object on the screen.
  You use the right mouse button to move
  
• You can use the middle mouse button to output the mouse position to the terminal.

  mouse position: 221, 200
  

  
  Remember that Qt screen coordinates and OpenGL coordinates are not the same. You will need to perform a conversion to get the correct values for OpenGL. This, you can do yourselves!
• The
glbox.cpp and glbox.h files are the only ones to have changed.
The changes are as follows:
• glbox.h
  • 3 new slots have been added:
    • void mousePressEvent( QMouseEvent* event );
    • void mouseReleaseEvent( QMouseEvent* event );
    • void mouseMoveEvent( QMouseEvent* event );

    Each of these slots for handling the events use a QMouseEvent.
    This is a Qt class that is used to describe (wee?!) moose events.
    Events occur when a mouse button is pressed and/or released and also when the mouse is moving.

  • 2 new private variables have been defined; these are:
    • QPoint lastPos;
      The QPoint is from a Qt class that defines a point in a plane; in this context the position of the mouse on the screen.
    • int moving;
      To indicate whether the mouse is moving, usually while a mouse button is pressed.
• glbox.cpp
  • mousePressEvent( QMouseEvent* event )

    //get the information about the position of the mouse.
    //using Qt's QMouseEvent
    lastPos = event->pos();
    //if the middle mouse button is pressed ...
    if(event->button()==MidButton)
    {
       float dx = event->pos().x();
       float dy = event->pos().y();
       //get the x and y (screen) coordinates of the mouse
       //and output the value.
       cout<<"mouse position: "<< dx<<", "<< dy<< endl;
    }
    

  • mouseReleaseEvent( QMouseEvent* event )

    //update the OpenGL, this is an OpenGL command.
    updateGL();
    

  • mouseMoveEvent( QMouseEvent* event )

    
    //these values use the distance the mouse has moved
    //using the current position and the last position.
    float dx = (event->pos().x()-lastPos.x());
    float dy = (event->pos().y()-lastPos.y());
    
    // ...while the left button is presssed
    if(event->state()&LeftButton)
    {
       zRot -= dy;
    	zRot += dx;
    	
    	//set the rotations values appropriately (rotate the scene)
    	// using the values from the dx dy calculated from mouse positions.
    
    }
    
    // ...while the middle button is pressed
    else if(event->state()&MidButton)
    {	
    	//nothing at the moment
    }
    
    // ...while the right button is pressed
    else if(event->state()&RightButton)
    {
    	xTrans -= dx/100.0;
    	yTrans -= dy/100.0;	
    	//translate the scene
    }
    
    //update the last position so we can use it to find the distance the mouse 
    //  has travelled.
    
    lastPos = event->pos();
    
    //If the mouse is moving we want to keep a note of this, will be
    //useful for other interactions and events in future.
    moving = true;
    updateGL();
    moving = false;
    

• Exercise: write the conversion function, as per the lecture notes, to convert Qt coordintes to OpenGL ones.
• Hints for a conversion function:
  • The Resize function has a function call:
    • glViewport( 0, 0, (GLint)w, (GLint)h );
    • The parameter values are (x, y, width, height).
    • x, y specifies (in pixels) the lower left corner of the viewport.
    • width, height specifies the width and height of the viewport.
    • You can store these values in a class variable to use in your conversion function.
      For example:
      • viewwidth = w;
      • viewheight = h;
  • The Resize function has a function call:
    • gluOrtho2D( -1.5, 1.5, -1.5, 1.5);
    • This means that the view you have will have the origin centered in the middle of the OpenGL viewport and an object on the screen will be shown if it has coordinates that lie within this range. For example look at the square's coordinates.
    • You can change these to any values you wish. I chose them just to be able to view the square at a sensible size.
    • NB remember this means that the viewport is 3 OpenGL units wide and 3 OpenGL units high.
  • You can calculate the x ratio using:
    • viewwidth (approx 300 in the box example; the same width as in the glViewport() function)
    • OpenGLwidth (3 in the above example, as in the gluOrtho2D() function).
  • Calculate the y-ratio in exactly the same way.
  • Apply these ratios to the mouse position x and y values to get your OpenGL coordinate values.
  • This is all the information you need to write your conversion function.
  • Try not to make it more complicated for yourself than need be.
  
  Remember to check the documentation.

More OpenGL ~ 3D objects

Useful References

OpenGL Programming Guide (The Red Book).

OpenGL Reference Manual (Blue Book)

The OpenGL Website

The OpenGL Website - tutorials

Contact me
email: pj@cpsc.ucalgary.ca
Tel:  (403) 220 7041.