View Full Version : Detecting the mouse on rotated geometry

Nathan Warden
02-21-2005, 01:07 AM
Hey all,

Just wondering if anybody here can hook me up with how to detect the mouse on geometry that has been rotated.

In other words, before I call glRotatef(), most of the geometry is offscreen and I need to detect if the mouse is over that geometry when it is drawn to the screen in its rotated state.

Do I need to rotate it myself using the rotational matrix (which I don't have or know anymore), or is there a function that will return coordinates that have been rotated.

I know about glProject() and glUnproject() but, they only tell me of the current unrotated state of everything.

I'm trying to get hotspots to work with a cubic panorama if that helps at all.

Thanks for any help,
Nathan Warden

02-21-2005, 01:33 AM
You're probably supplying the modelviewmatrix to the gluUnproject function after you have popped the modelviewmatrix. You have to supply the modelviewmatrix with the rotation.

If you only want to now if the object covers the mouse position, I suggest using a stencil test and reading out the stencil value at the mouse position.


Nathan Warden
02-21-2005, 02:09 AM
Thanks for your reply!!!

Does this mean use glLoadIdentity() after I do a mouse test instead of before? If so... that sounds easy enough.

What do you mean by stencil test? If this is the same thing I used in REALbasic in the 3D environment, I would rather use that!!! Can you tell me more (or give an example) of this?

Thanks a ton!!!
Nathan Warden

02-23-2005, 01:29 PM
I just gave a similar answer to another question in the Math forum, but it applies well to interactive hotspots.

I use the "Object Selection Using the Back Buffer" method as described in The Red Book:

Basically, render the rotated cubic panorama hot spots as filled polys, each as a unique flat color, into the back buffer. Do not swap at the end of the render unless you are debugging what is drawn. Then read the single pixel of the mouse location from the back buffer. Not only can the pixel color tell you if a hot spot was hit, but if you rendered each with a unique color you can tell which hotspot the mouse is over.

Hope that helps.

Nathan Warden
02-23-2005, 05:42 PM
That sounds like it's exactly what I'm looking for. Plus, it sounds very efficient seeing that I won't have to see if the point lies within a specific square.

Thank You,
Nathan Warden