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Dan
03-07-2000, 03:49 PM
I was wondering how to use the billboarding effect (where a certain poly always faces the viewer), i am using the LookAt function. So i dont really know the roation values...so does anybody know how to make a certain polygon always face the viewer, while using the LookAt function?

P.S. any code would be greatly appreciated http://www.opengl.org/discussion_boards/ubb/biggrin.gif

Gorg
03-07-2000, 10:10 PM
First I would suggest that you post that question in the beginner or advanced forum http://www.opengl.org/discussion_boards/ubb/smile.gif

It's just simple trigonometry.

In glulookat, the 3(eyex,eyey,eyez) first point are the position of the camera. The other 3(centerx,centery,centerz) are the point where the eye look at. The 3 last or the position of the up-vector.

So, the "looking" direction vector is :

(centerx - eyex, centery - eyey, centerz - eyez).

Let's say you only want the billboarding to happen when there is a y rotation(there is billboarding when you are horizontaly moving around the object)

Now let's say that your billboarded polygon was on the x axis first at (X1,0,0) for center. you calculate the X angle between that point and the vector:

Xangle = atan (X1 / (centerx - eyex)

Rotangle = 90 - Xangle;

this is just one case. depending if you are in wich quadrant around the object. I'm sure you can easely figure it out by taking a sheet of paper and doing some tests.