3D Circles

[Malohkan]

I want to be able to draw a circle under any transformation and have it still face the screen.  I can translate the circles anywhere, and rotate the camera, and make the circles face the camera.  My challenge now is that sometimes I rotate the circles about an arbitrary point, which as a result moves the circle from its original location.  Now I don't know what direction the circle is facing.  Is there a way to do a "glLoadRotationIdentity()" or something of the sort so that the current global translation is still in the same place, but the orientation is still in the "identity" direction?  Or is there a better way to figure out what direction I'm facing?

[turquoise3232]

Hi, the fast and easy way to answer I guess : google for "billboards"...

[Malohkan]

Well after 2 hours of searching and applying, I haven't gotten it yet.  I found this:

http://www.opengl.org/resources/tutorials/advanced/advanced97/programs/billboard.c

and that seemed like it was doing what I want.  It takes the modelview matrix and thus all of the transformations I've done into account.  However, after implementing it my circles are still never facing the scene and I can't make out any pattern to recognize what it's doing wrong.  It's simply all over the place.

I also searched the forums and found some code Tom wrote here:
http://192.18.37.44/forums/index.php?topic=3029.0
I tried that, but it didn't take into account the fact that I'm doing multiple transformations AFTER the camera transform.  His code did exactly what mine was doing before, making the circles face the screen assuming the gl(u)LookAt was the only transformation.

Any other ideas for me?  Maybe some other examples that take into account any arbitrary number of transformations after the camera?

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[Malohkan]

can I do something like... take the modelview matrix and set 3 of its entries to 0 to get rid of all rotation or something?  Hmm or will that not work?