Re: [Algorithms] On 2D and 3D Sprites

[Doug C. <dc...@d-...>]

Thanks for the response, what I did to solve it was leave the view alone and
stick the view transpose into the worlds rotation components and it works
fine now. I was setting both of the rotational components ( in world and
view ) to identity to cancel out all rotations in both coordinate spaces but
thats not what I wanted to do ( obviously ).

Doug Chism
----- Original Message -----
From: "Stephen J Baker" <sj...@li...>
To: <gda...@li...>
Sent: Thursday, August 03, 2000 1:51 PM
Subject: Re: [Algorithms] On 2D and 3D Sprites


> On Thu, 3 Aug 2000, Doug Chism wrote:
>
> > I thought of an easy way to do 2D and 3D sprites but as of yet I cant
get it
> > to work well. The method involves simply grabbing the current world and
view
> > matrices and simply cancelling out the rotational components so the
object
> > can translate in world/view space but is always facing the camera.
>
> That's certainly what most people do.
>
> It's not *perfect* though because it keeps the object parallel to the
> screen - which is not the same thing as "facing the camera".
>
> The difference is usually unimportant unless you have a wide
field-of-view.
>
> > It works
> > great if the object is just sitting there at the origin, moving the
camera
> > in any direction still appears as the object is facing right at you
> >  whether its a 2D sprite or a 3D complex object ). Billboards are easy
> > because you can just allow rotations on one or more of the axes and it
> > appears correct also. The weird thing is when you transform the object
so it
> > is no longer centered at the origin. The object still faces you
perfectly,
> > but it moves in relation to the camera - i.e. if you translate it
along -x
> > it moves along the -x axis with respect to the camera, not the world.
For
> > instance it would always move to the left with a -x translation matrix
> > applied no matter what direction the camera faces it from.
>
> The model will always rotate about its local origin.  If you move it away
> from there (by changing the vertex coordinates) it's still going to rotate
> around the origin - I suppose that might *look* like a translation if
> it's sufficiently far from the origin.
>
> You have to position it billboards using the matrix and THEN nuke the
> rotation parts.
>
> > I cant figure out
> > why it would do this since I am still passing though the world
translation
> > as well as the view, just not their rotations. Any ideas on this, has or
> > anyone else tried doing 2D and 3D sprites in a similar fashon
successfully?
>
> I don't quite understand your explanation of the bug - but I can tell you
that
> I've done this successfully *many* times in the past and if you have it
> implemented right, it will work.
>
> Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
> L3Com/Link Simulation & Training (817)619-2466 (Fax)
> Work: sj...@li...           http://www.link.com
> Home: sjb...@ai...       http://web2.airmail.net/sjbaker1
>
>
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