I am trying to write a code which selects graphic entities when I move them through the centre of the screen. I am using gluproject to determine the screen coordinate of each graphic entity. However I find that the boundary coordinates of the screen seem to change when I do any rotation. Why does this happen and how can I determine the current screen boundary coordinates

To select something on screen the best method is to use selection buffer. For more explanation and examples, you can check out the NeHe tutorials at