plib-users

RE: [Plib-users] ssg coordinate system convention

[Paul B. <pbl...@di...>]

Hey Steve,

Where does the "Z is up" affect code?  i.e. "Z is up" to me 
means that there are assumptions in the code like "the 'floor' 
is in the XY plane", "gravity has the unit vector (0,0,-1)",
and such.  It could also affect what roll, pitch and yaw mean.
(But I thought there were de facto standards for that in the
vizsim industry.  Are they Z is up?)

It's been a while since I was in the ssg code, so this may
seem obvious to others.  


Thanks,
Paul


> -----Original Message-----
> From: Steve Baker [mailto:sjb...@ai...]
> Sent: Wednesday, February 23, 2000 11:50 PM
> To: pli...@li...
> Subject: Re: [Plib-users] ssg coordinate system convention
> 
> 
> Rahul Choudhury wrote:
> > 
> > Just started looking into the ssg src code. I was
> > wondering whether the coordinate system convention
> > used in SSG is same as OpenGL or something different.
> 
> SSG's coordinate system is the same 'handedness' as OpenGL,
> but uses Z-is-up instead of Y-is-up.   I'm afraid that's
> because I come from a flight simulation background (where
> Z-is-up has always been the convention) - and there are really
> too many users to change it at this late stage.
> 
> SSG is not alone in this - SGI's Performer scene graph library
> is also a Z-is-up system that's based on OpenGL.
> 
> -- 
> Steve Baker                  http://web2.airmail.net/sjbaker1
> sjb...@ai... (home)  http://www.woodsoup.org/~sbaker
> sj...@ht...      (work)
> 
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> 

Re: [Plib-users] ssg coordinate system convention

[Steve B. <sjb...@ai...>]

Paul Bleisch wrote:
> 
> Hey Steve,
> 
> Where does the "Z is up" affect code?  i.e. "Z is up" to me
> means that there are assumptions in the code like "the 'floor'
> is in the XY plane", "gravity has the unit vector (0,0,-1)",
> and such.  It could also affect what roll, pitch and yaw mean.

That's the main difference.  There are a couple of routines
in SSG that know which way is up....but the noticable difference
is that an unrotated camera looks down the Y axis with Z pointing
up.

Note though that when I load a file from (say) a '.ac' file (which
has a marked Y-is-up bias), I do the necessary conversions such
that things look the same in AC3D as they do in SSG.

> (But I thought there were de facto standards for that in the
> vizsim industry.  Are they Z is up?)

Yes.  YAW means rotation around the Z axis, PITCH means rotation
about X and ROLL means rotation about Y.  In SSG and SG, I talk
about 'Heading' instead of 'Yaw' because the word Yaw starts with
a 'Y' and that can get confused with the Y axis. H,P and R are
mercifully unused for other common graphics coordinates.

-- 
Steve Baker                  http://web2.airmail.net/sjbaker1
sjb...@ai... (home)  http://www.woodsoup.org/~sbaker
sj...@ht...      (work)