Firstly: Remember the hack method I mentioned in class.  Forget it.  It does not work well and I know you all can do better.

Plus the hack method totally doesn't work if your window is resizable.



So here's how we do this properly.





Goals:  

	- we want to select a control point by clicking the mouse on the screen.



Our Assets:

	- we can easily find the coordinates on the gl widget that our mouse has clicked (e->x(); and e->y();)

	- we know where in 3D space our control points are.



Problem:  

	- how do we convert between these two coordinate systems?  



Answer:

	- By doing a projection from 3D space to viewspace.



What do we need to do this projection? 

	- get the viewport matrix

	- get the projection matrix

	- get the model view matrix



How do you get these?

	- look in the blue book documention under glGet.



So how do we do the actual projection?

	- from lectures you should know how to multiply all these matrices with your points to get your projected point (on the 2D window).

	- if you're lazy (aren't we all) and want OpenGL to do this for you, take a look in the blue book documentation at gluProject.



So this will give you an x, y, and z coordinate that refer to the x and y pixel values on the screeni (it also gives you a z

value that indicates the "depth" of this screen coordinate, you can ignore this).



Is that it?

	- not quite, the last thing you have to do is remember that your mouse coordinates for the gl windows have 0,0 in the top left corner.

	- the coordinates you receive from the projection have 0,0 in the bottom left hand corner

	- adjust for this difference by changing the y value.