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
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