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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.
--
Tim Cook
http://empyrean.scifi-fantasy.com
-----BEGIN GEEK CODE BLOCK-----
Version: 3.12
GFA dpu- s: a?-- C++(++++) U P? L E--- W++(+++)>$
N++ o? K- w(+) O? M-(--) V? PS+(+++) PE(--) Y(--)
PGP-(--) t* 5++>+++++ X+ R* tv+ b++(+++) DI
D++(---) G(++) e*>++ h+ !r--- !y--
------END GEEK CODE BLOCK------
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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;
Because when you have the pencil in the right hand, only
the left hand is available for fancy manipulation !
Moreover, on computer, you need the right hand for the numeric pad.
(especially when typing pov-vector in script)
And last: Pov is not left-handed. It is half-ambidextrous* :
default to left hand, but can work without problem with right hand.
* half-ambidextrous: just similar to half-duplex, can be both but not
at the same time.
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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.
>
It's no problem at all to do right-handed geometry in POV, just have a
look at what Moray generates for example.
Christoph
--
Christoph Hormann <chr### [at] gmx de>
IsoWood include, radiosity tutorial, TransSkin and other
things on: http://www.schunter.etc.tu-bs.de/~chris/
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Christoph Hormann wrote:
> It's no problem at all to do right-handed geometry in POV,
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.
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On Tue, 04 Dec 2001 07:06:15 -0500, 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
Math uses the right-hand rule too. POV uses the left-hand rule by
default because that's how it was created. Call it a historical
accident. To change it now would just be wrong.
--
#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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"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.
--
Tim Cook
http://empyrean.scifi-fantasy.com
-----BEGIN GEEK CODE BLOCK-----
Version: 3.12
GFA dpu- s: a?-- C++(++++) U P? L E--- W++(+++)>$
N++ o? K- w(+) O? M-(--) V? PS+(+++) PE(--) Y(--)
PGP-(--) t* 5++>+++++ X+ R* tv+ b++(+++) DI
D++(---) G(++) e*>++ h+ !r--- !y--
------END GEEK CODE BLOCK------
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"Timothy R. Cook" <tim### [at] scifi-fantasycom> wrote :
>
> ...actually I never understood why people even needed to
> represent the axes with their hand
It's a learning tool... there was a time when you needed to learn it
too, whether you remember it or not.
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On Tue, 04 Dec 2001 08:37:14 -0500, Timothy R. Cook wrote:
> "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.
The "right hand rule" is most useful for visualizing which way a
cross product (or, by extension, a rotation) goes. If you can do that
in your head, great, but many otherwise talented people can't.
--
plane{-z,-3normal{crackle scale.2#local a=5;#while(a)warp{repeat x flip x}rotate
z*60#local a=a-1;#end translate-9*x}pigment{rgb 1}}light_source{-9red 1rotate 60
*z}light_source{-9rgb y rotate-z*60}light_source{9-z*18rgb z}text{ttf"arial.ttf"
"RP".01,0translate-<.6,.4,.02>pigment{bozo}}light_source{-z*3rgb-.2}//Ron Parker
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"Timothy R. Cook" <tim### [at] scifi-fantasycom> wrote in message
news:3C0CD18A.968961E9@scifi-fantasy.com...
>I have no trouble
> picturing the axes in my head.
>
Sounds painful, have you been arguing with Bill?
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> And last: Pov is not left-handed. It is half-ambidextrous* :
> default to left hand, but can work without problem with right hand.
This isn't quite true. Lots of the primitives have the y-axis hardcoded as
the "vertical" axis.
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Tom Melly wrote:
> >I have no trouble picturing the axes in my head.
> Sounds painful, have you been arguing with Bill?
...
Er, that would be plural axis, not plural axe. ;)
--
Tim Cook
http://empyrean.scifi-fantasy.com
-----BEGIN GEEK CODE BLOCK-----
Version: 3.12
GFA dpu- s: a?-- C++(++++) U P? L E--- W++(+++)>$
N++ o? K- w(+) O? M-(--) V? PS+(+++) PE(--) Y(--)
PGP-(--) t* 5++>+++++ X+ R* tv+ b++(+++) DI
D++(---) G(++) e*>++ h+ !r--- !y--
------END GEEK CODE BLOCK------
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Bill DeWitt wrote:
> It's a learning tool... there was a time when you needed
> to learn it too, whether you remember it or not.
The way I learned the axes was by drawing stuff on graph
paper (cheesy game maps mostly), I understood that as XY.
Z was then obviously the elevation.
--
Tim Cook
http://empyrean.scifi-fantasy.com
-----BEGIN GEEK CODE BLOCK-----
Version: 3.12
GFA dpu- s: a?-- C++(++++) U P? L E--- W++(+++)>$
N++ o? K- w(+) O? M-(--) V? PS+(+++) PE(--) Y(--)
PGP-(--) t* 5++>+++++ X+ R* tv+ b++(+++) DI
D++(---) G(++) e*>++ h+ !r--- !y--
------END GEEK CODE BLOCK------
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Ron Parker wrote:
> Math uses the right-hand rule too. POV uses the left-hand rule by
> default because that's how it was created. Call it a historical
> accident. To change it now would just be wrong.
Hmmm... I don't think on it as an accident. Tim has really given the
answer with the question: CAD programs were done for designers, people
that thinks on floorplans with implicit x-y axis. But which was the
public POV-Ray was created for? IMHO, this is the key...
--
Jaime Vives Piqueres
La Persistencia de la Ignorancia
http://www.ignorancia.org/
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"bob h" <omn### [at] charternet> wrote in message
news:3c0ceef3@news.povray.org...
<snip>
What I've never really understood is why Moray works with right-handedness. It's
not a major problem, but it does seem odd...
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"Jaime Vives Piqueres" <jai### [at] ignoranciaorg> wrote in message
news:3C0### [at] ignoranciaorg...
> Ron Parker wrote:
>
> > Math uses the right-hand rule too. POV uses the left-hand rule by
> > default because that's how it was created. Call it a historical
> > accident. To change it now would just be wrong.
>
> Hmmm... I don't think on it as an accident. Tim has really given the
> answer with the question: CAD programs were done for designers, people
> that thinks on floorplans with implicit x-y axis. But which was the
> public POV-Ray was created for? IMHO, this is the key...
Hopefully I cancelled my reply to Ron's message before too many people saw
it.
8-o
Don't know what I was thinking, saying that image_map projected onto the xy
plane. It of course projects onto the xz plane by default. Thought POV-Ray
had some logic there before I realized my mistake.
There is still the gaming industry which uses the z axis as in/out, away
near, z-buffer and all that. That's how I learned it during using Basic to
draw things in 2D or 3D. x and y were always the plots on the screen
surface, z being depth.
--
text{ttf"arial","bob h",.1,0pigment{rgb 9}translate<-1,-.2,3>}
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Tom Melly <tom### [at] tomandlucouk> wrote:
: What I've never really understood is why Moray works with right-handedness. It's
: not a major problem, but it does seem odd...
That's not odd at all. What is odd is the strange principle that support for
left-handedness will never be added.
--
#macro N(D,I)#if(I<6)cylinder{M()#local D[I]=div(D[I],104);M().5,2pigment{
rgb M()}}N(D,(D[I]>99?I:I+1))#end#end#macro M()<mod(D[I],13)-6,mod(div(D[I
],13),8)-3,10>#end blob{N(array[6]{11117333955,
7382340,3358,3900569407,970,4254934330},0)}// - Warp -
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"Tom Melly" <tom### [at] tomandlucouk> wrote in message
news:3c0cef40$1@news.povray.org...
> "bob h" <omn### [at] charternet> wrote in message
> news:3c0ceef3@news.povray.org...
>
> <snip>
>
> What I've never really understood is why Moray works with
right-handedness. It's
> not a major problem, but it does seem odd...
Um, sorry Tom, I tried to Cancel that message because I thought I goofed on
image_map going onto xy plane. Now I have checked on it in the Docs to be
sure. :-) So now maybe I'm unconfused again. It does say xy plane. This
talk of coordinate orientation always turns me through an infinite loop!
Yeah, about Moray, it must be that the intent is to follow the logic in CAD
programs and the like.
Check out the *left-handed* Cartesian system I found:
http://www.xmlcreate.com/NCGuide/Basics101/basics101.html
--
text{ttf"arial","bob h",.1,0pigment{rgb 9}translate<-1,-.2,3>}
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"bob h" <omn### [at] charternet> wrote in message
news:3c0cf1c5@news.povray.org...
> "Jaime Vives Piqueres" <jai### [at] ignoranciaorg> wrote in message
> news:3C0### [at] ignoranciaorg...
>
> Hopefully I cancelled my reply to Ron's message before too many people saw
> it.
> Don't know what I was thinking, saying that image_map projected onto the
xy
> plane. It of course projects onto the xz plane by default. Thought
POV-Ray
> had some logic there before I realized my mistake.
What mistake, Bob?! Ha ha ha
I give up, can't win :-( I was right in the first place. Through
stumbling over this topic.
Bye now :-)
--
text{ttf"arial","bob h",.1,0pigment{rgb 9}translate<-1,-.2,3>}
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"Warp" <war### [at] tagpovrayorg> wrote in message news:3c0cf281@news.povray.org...
> That's not odd at all. What is odd is the strange principle that support for
> left-handedness will never be added.
>
Well, it is if it's principle intent is to be a POV modeller - or have i
missed/misunderstood something?
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bob h <omn### [at] charternet> wrote:
: Check out the *left-handed* Cartesian system I found:
: http://www.xmlcreate.com/NCGuide/Basics101/basics101.html
Funny, it describes the *right-handed* system using the *left-hand-metaphor*.
--
#macro N(D,I)#if(I<6)cylinder{M()#local D[I]=div(D[I],104);M().5,2pigment{
rgb M()}}N(D,(D[I]>99?I:I+1))#end#end#macro M()<mod(D[I],13)-6,mod(div(D[I
],13),8)-3,10>#end blob{N(array[6]{11117333955,
7382340,3358,3900569407,970,4254934330},0)}// - Warp -
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Warp <war### [at] tagpovrayorg> wrote:
: Funny, it describes the *right-handed* system using the *left-hand-metaphor*.
The problem with this hybrid system is that it doesn't work if you try
to visualize the direction of rotations with your left hand.
--
#macro N(D,I)#if(I<6)cylinder{M()#local D[I]=div(D[I],104);M().5,2pigment{
rgb M()}}N(D,(D[I]>99?I:I+1))#end#end#macro M()<mod(D[I],13)-6,mod(div(D[I
],13),8)-3,10>#end blob{N(array[6]{11117333955,
7382340,3358,3900569407,970,4254934330},0)}// - Warp -
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I have always thought of the x/y plane as being the sheet of graphpaper,
but that graph paper pasted up on the monitor. Therefore x is across the
monitor, y is up and down the monitor and z is out of the monitor, just like
it would be on that piece of graphpaper. To have it make sense to me, I use
right <-1.333,0,0> so that positive x is on the right, positive y is up and
positive z is towards me. I start of with my camera at +z, as if I were
looking down on the peice of graphpaper.
Probably a really screwy way of doing it, but it makes sense to me.
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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.)
--
#local R=rgb 99;#local P=R-R;#local F=pigment{gradient x}box{0,1pigment{gradient
y pigment_map{[.5F pigment_map{[.3R][.3F color_map{[.15red 99][.15P]}rotate z*45
translate x]}]#local H=pigment{gradient y color_map{[.5P][.5R]}scale 1/3}[.5F
pigment_map{[.3R][.3H][.7H][.7R]}]}}}camera{location.5-3*z}//only my opinions
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"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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