On Tue, 04 Dec 2001 07:06:15 -0500, "Timothy R. Cook"
<tim### [at] scifi-fantasycom> wrote:

>I still don't quite get why math (and hence POV) 3D coords use
>left-hand rule; apart from most people being right-handed, CAD
>programs use right-hand, being based on architectural design which
>draws a floorplan along the XY plane with Z being the elevation,
>which is what you get when you draw a floorplan on a standard
>cartesian grid with 0,0 being the origin in the lower-left.

3D computer graphics were first employed in maths where x points to
the right and y up (as on a blackboard, drawing board etc.) The
direction of z was chosen the way it is (away from user) so that with
the origin in the lower-left corner, all coordinates of visible points
in pseudo-perspective (axonometry?) are positive, therefore easier to
deal with.


Peter Popov ICQ : 15002700
Personal e-mail : pet### [at] vipbg
TAG      e-mail : pet### [at] tagpovrayorg

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Timothy R. Cook wrote:
> 
> I still don't quite get why math (and hence POV) 3D coords use
> left-hand rule; apart from most people being right-handed, CAD
> programs use right-hand, being based on architectural design which
> draws a floorplan along the XY plane with Z being the elevation,
> which is what you get when you draw a floorplan on a standard
> cartesian grid with 0,0 being the origin in the lower-left.

While working on my modeller I briefly toyed with the idea of
expressing all coordinates in <right,up,forward> and make the
handness an issue for file exporting.  I'm back with left-handed
<x,y,z> code for now.  If it isn't too difficult I'll make the
handedness a user option.

Regards,
John

-- 
ICQ: 46085459

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"Timothy R. Cook" <tim### [at] scifi-fantasycom> wrote in message
news:3C0CD18A.968961E9@scifi-fantasy.com...
> "Greg M. Johnson" wrote:
> > One may represent the three axes as they appear in povray with
> > one's right hand, but your coworkers and family members would
> > complain about you flipping them the bird.
>
> ...actually I never understood why people even needed to
> represent the axes with their hand except for demonstrating
> to people who didn't know about them; I have no trouble
> picturing the axes in my head.

It's to specify the directions of rotation about the axes.

...Chambers

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