"Ron Parker" <ron### [at] povrayorg> wrote in message
news:slr### [at] fwicom...
> On Tue, 4 Dec 2001 15:20:45 -0500, Bill DeWitt wrote:
> >
> >     Probably a really screwy way of doing it, but it makes sense to me.
>
> Almost exactly what I do, except that I see z as into the monitor (i.e.
> z increases as the distance from the viewer does) and so I use a
left-handed
> system (yours is right-handed.)

    But ... but.. how do you see through the graph paper?

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On Tue, 4 Dec 2001 16:19:42 -0500, Bill DeWitt wrote:
> 
> "Ron Parker" <ron### [at] povrayorg> wrote in message
> news:slr### [at] fwicom...
>> On Tue, 4 Dec 2001 15:20:45 -0500, Bill DeWitt wrote:
>> >
>> >     Probably a really screwy way of doing it, but it makes sense to me.
>>
>> Almost exactly what I do, except that I see z as into the monitor (i.e.
>> z increases as the distance from the viewer does) and so I use a
> left-handed
>> system (yours is right-handed.)
> 
>     But ... but.. how do you see through the graph paper?

I just use a sharpie to draw the lines right on the screen.

-- 
#macro R(L P)sphere{L F}cylinder{L P F}#end#macro P(V)merge{R(z+a z)R(-z a-z)R(a
-z-z-z a+z)torus{1F clipped_by{plane{a 0}}}translate V}#end#macro Z(a F T)merge{
P(z+a)P(z-a)R(-z-z-x a)pigment{rgbf 1}hollow interior{media{emission 3-T}}}#end 
Z(-x-x.2x)camera{location z*-10rotate x*90normal{bumps.02scale.05}}

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"Ron Parker" <ron### [at] povrayorg> wrote :
> >
> >     But ... but.. how do you see through the graph paper?
>
> I just use a sharpie to draw the lines right on the screen.

    I've done that. Mostly marking positions during an animation.

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On Tue, 4 Dec 2001 16:27:34 -0500, Bill DeWitt wrote:
> 
> "Ron Parker" <ron### [at] povrayorg> wrote :
>> >
>> >     But ... but.. how do you see through the graph paper?
>>
>> I just use a sharpie to draw the lines right on the screen.
> 
>     I've done that. Mostly marking positions during an animation.

I usually use electrical tape for that; it comes off easier.  I taught
my son that trick for video games, too, so he'd know just exactly which
point on the screen to jump/shoot/whatever from when going up against 
some obstacle or other.

-- 
#macro R(L P)sphere{L F}cylinder{L P F}#end#macro P(V)merge{R(z+a z)R(-z a-z)R(a
-z-z-z a+z)torus{1F clipped_by{plane{a 0}}}translate V}#end#macro Z(a F T)merge{
P(z+a)P(z-a)R(-z-z-x a)pigment{rgbt 1}hollow interior{media{emission T}}finish{
reflection.1}}#end Z(-x-x.2y)Z(-x-x.4x)camera{location z*-10rotate x*90}

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My math teacher projects graphs from her calculator onto a screen.  So y
is up and down and z is in and out.  But with our calculators on our
desks, y is toward and away from us and z is up and down.  I read
somewhere that the reason that POV-Ray uses the left-handed coordinate
system is because people are used to seeing graphs on boards and
pointing the y axis up.  But positive rotation is clockwise with POV and
counterclockwise with math classes.  So if POV-Ray adopted y as up, why
not positive rotation as being counterclockwise?  I hope that I'm wrong
about this otherwise there would be a self-contraction.
Brendan

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On Tue, 04 Dec 2001 20:31:12 -0500, Andrea Ryan wrote:
> pointing the y axis up.  But positive rotation is clockwise with POV and
> counterclockwise with math classes.  So if POV-Ray adopted y as up, why
> not positive rotation as being counterclockwise?  I hope that I'm wrong
> about this otherwise there would be a self-contraction.

Because POV also adopted z as into the screen, which is not how it works
in math.

--
#macro R(L P)sphere{L __}cylinder{L P __}#end#macro P(_1)union{R(z+_ z)R(-z _-z)
R(_-z*3_+z)torus{1__ clipped_by{plane{_ 0}}}translate z+_1}#end#macro S(_)9-(_1-
_)*(_1-_)#end#macro Z(_1 _ __)union{P(_)P(-_)R(y-z-1_)translate.1*_1-y*8pigment{
rgb<S(7)S(5)S(3)>}}#if(_1)Z(_1-__,_,__)#end#end Z(10x*-2,.2)camera{rotate x*90}

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Ron Parker wrote:
> On Tue, 04 Dec 2001 20:31:12 -0500, Andrea Ryan wrote:
> > pointing the y axis up.  But positive rotation is clockwise with POV and
> > counterclockwise with math classes.  So if POV-Ray adopted y as up, why
> > not positive rotation as being counterclockwise?  I hope that I'm wrong
> > about this otherwise there would be a self-contraction.
>
> Because POV also adopted z as into the screen, which is not how it works
> in math.
>

When I was developing my person model (see it in p.b.a), I did the right
arm/leg and scaled it by <-1,1,1> to get the left.  What this means is that
when I'm figuring out the rotations for positioning the right side, I use my
left
hand, and when I'm figuring out the rotations for positioning the left side,
I use
my right hand.  Do you think I get confused?  You bet.

My point:  I need a point?

Tom Bates.

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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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