Is there a specific reason why POV-ray uses a left-handed coordinate
system?

-- 
Kaveh

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Kaveh <kav### [at] delete_thisfocalimagecom> wrote:
> Is there a specific reason why POV-ray uses a left-handed coordinate
> system?

  For 90% of people it's easier to write with the right hand while they
do axis visualizations with their left hand.

-- 
#macro M(A,N,D,L)plane{-z,-9pigment{mandel L*9translate N color_map{[0rgb x]
[1rgb 9]}scale<D,D*3D>*1e3}rotate y*A*8}#end M(-3<1.206434.28623>70,7)M(
-1<.7438.1795>1,20)M(1<.77595.13699>30,20)M(3<.75923.07145>80,99)// - Warp -

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Warp <war### [at] tagpovrayorg> wrote:

> Kaveh <kav### [at] delete_thisfocalimagecom> wrote:
> > Is there a specific reason why POV-ray uses a left-handed coordinate
> > system?
> 
>   For 90% of people it's easier to write with the right hand while they
> do axis visualizations with their left hand.

Is this really the reason? So has everyone else got it wrong, e.g.
maths, physics, etc?

-- 
Kaveh

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Kaveh <kav### [at] delete_thisfocalimagecom> wrote:
> Is this really the reason?

  No, it was humor.

-- 
#macro M(A,N,D,L)plane{-z,-9pigment{mandel L*9translate N color_map{[0rgb x]
[1rgb 9]}scale<D,D*3D>*1e3}rotate y*A*8}#end M(-3<1.206434.28623>70,7)M(
-1<.7438.1795>1,20)M(1<.77595.13699>30,20)M(3<.75923.07145>80,99)// - Warp -

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On Fri, 15 Nov 2002 12:41:45 +0530, kav### [at] delete_thisfocalimagecom
(Kaveh) wrote:

>Is there a specific reason why POV-ray uses a left-handed coordinate
>system?

Left-handed people have, on average, an IQ about 5 points higher than
the average for right-handed people. That's the reason that the
POV-Team has in its long history *only* accepted left-handed people in
its ranks. The only right-handed person in there is Lutz Kretzschmar
(author of Moray) and they've been trying to get rid of him for years,
but after the hit squad took down the wrong guy for the 11th time
because Chris Cason *again* misspelled Lutz's name, they gave it up
and let him go on with his heresy for the sake of the POV community
(the last one of false victims was, by mishap, a volunteer who taught
POV in a community school in Zaire).

How do I know? Well, being a TAG member, I have certain privileges.
Though after what I just disclosed, you might not hear from me
anymore...





Tongue *so* deep in the cheek it almost comes out of the ear :)


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

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In article <1flp1lf.b3ye8bmkt22oN%kaveh@delete_this.focalimage.com>,
 kav### [at] delete_thisfocalimagecom (Kaveh) wrote:

> Is this really the reason? So has everyone else got it wrong, e.g.
> maths, physics, etc?

In screen coordinates, x is usually from left to right, and y from 
bottom to top. The left handed coordinate system is simply an extension 
of this: z going into the screen. It is not something unique to POV, it 
is used in many other places as well.
Even in right handed coordinate systems, the y axis is often "up" 
(OpenGL for example). The difference is that the z axis points out of 
the monitor instead of into it. A depth buffer is often called a 
"z-buffer", but never a "y-buffer". I think the main source of the "z is 
up" system is actually architectural drawings, it makes sense when x and 
y are already used for a horizontal plane on paper. In math, I often see 
the z coordinate visualized as depth into a graph instead of height.

Neither way is "wrong", they are just different conventions. An analogy 
would be row-major vs. column-major matrices.

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/

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If this is too inconvenient, you can always modify your camera
statement, as referenced in:
http://www.povray.org/documentation/view/28/  
--LibraryMan

Kaveh wrote:
> 
> Is there a specific reason why POV-ray uses a left-handed coordinate
> system?
> 
> --
> Kaveh

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More specifically,
http://www.povray.org/documentation/view/339/

--MMW

LibraryMan wrote:
> 
> If this is too inconvenient, you can always modify your camera
> statement, as referenced in:
> http://www.povray.org/documentation/view/28/
> --LibraryMan
> 
> Kaveh wrote:
> >
> > Is there a specific reason why POV-ray uses a left-handed coordinate
> > system?
> >
> > --
> > Kaveh

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LibraryMan <mrm### [at] attnet> wrote:

> If this is too inconvenient, you can always modify your camera
> statement, as referenced in:
> http://www.povray.org/documentation/view/28/  
> --LibraryMan

No, doesn't worry me at all. Just that I studied physics, and was used
to right handed systems. Thanks for link anyway.

> 
> Kaveh wrote:
> > 
> > Is there a specific reason why POV-ray uses a left-handed coordinate
> > system?
> > 
> > --
> > Kaveh


-- 
Kaveh

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By the way, when calculating handedness-related things literally
using your hand as an aid, it's very important to remember that
the thum is *always* the x-axis (and the next fingers are the y and
the z axes).
  I once saw a site which explained the handedness in the wrong way.
IIRC it had the middle finger as x and the thumb as z. This completely
reverses the whole concept. If you do it this way, all rotations will
be reversed (you can't use your hand to visualize the direction of the
rotation in a given coordinate system because it will be reversed).

  (If you didn't know, you can easily visualize which direction does
a positive angle rotate an object by pointing your thumb (of the left
hand if you are using the left-handed coordinates) towards the positive
axis around which you are rotating and bend your fingers.)

-- 
#macro M(A,N,D,L)plane{-z,-9pigment{mandel L*9translate N color_map{[0rgb x]
[1rgb 9]}scale<D,D*3D>*1e3}rotate y*A*8}#end M(-3<1.206434.28623>70,7)M(
-1<.7438.1795>1,20)M(1<.77595.13699>30,20)M(3<.75923.07145>80,99)// - Warp -

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Warp <war### [at] tagpovrayorg> wrote:

>   By the way, when calculating handedness-related things literally
> using your hand as an aid, it's very important to remember that
> the thum is *always* the x-axis (and the next fingers are the y and
> the z axes).

I find it easier just to use the curled fingers method, not the index
and third fingers. To get the sense of the z axis, I position the left
hand such that the four fingers curl from the x to the y axis. Then the
thumb shows the direction of the z axis.

[...]

> 
>   (If you didn't know, you can easily visualize which direction does
> a positive angle rotate an object by pointing your thumb (of the left
> hand if you are using the left-handed coordinates) towards the positive
> axis around which you are rotating and bend your fingers.)

Yes, this is the way I use the curled fingers.

-- 
Kaveh

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Kaveh <kav### [at] delete_thisfocalimagecom> wrote:
> I find it easier just to use the curled fingers method, not the index
> and third fingers. To get the sense of the z axis, I position the left
> hand such that the four fingers curl from the x to the y axis. Then the
> thumb shows the direction of the z axis.

  That description was very unclear. I did not visualize at all which
direction the thumb is pointing at...

-- 
#macro M(A,N,D,L)plane{-z,-9pigment{mandel L*9translate N color_map{[0rgb x]
[1rgb 9]}scale<D,D*3D>*1e3}rotate y*A*8}#end M(-3<1.206434.28623>70,7)M(
-1<.7438.1795>1,20)M(1<.77595.13699>30,20)M(3<.75923.07145>80,99)// - Warp -

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On 18 Nov 2002 15:34:03 -0500
Warp <war### [at] tagpovrayorg> wrote:

>   (If you didn't know, you can easily visualize which direction does
> a positive angle rotate an object by pointing your thumb (of the left
> hand if you are using the left-handed coordinates) towards the positive
> axis around which you are rotating and bend your fingers.)

  I use only this rule for all my "visualizations" when I'm writing scenes, and I
agree with you: when you have the right hand already on the numeric pad, the
lef-handed system is very useful. Not joking!

--
Jaime Vives Piqueres
http://www.ignorancia.org

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Warp <war### [at] tagpovrayorg> wrote:

> Kaveh <kav### [at] delete_thisfocalimagecom> wrote:
> > I find it easier just to use the curled fingers method, not the index
> > and third fingers. To get the sense of the z axis, I position the left
> > hand such that the four fingers curl from the x to the y axis. Then the
> > thumb shows the direction of the z axis.
> 
>   That description was very unclear. I did not visualize at all which
> direction the thumb is pointing at...

It was not clear, I agree. I will put it another way. Put your left hand
in the 'aerobics' position, with fingers curled. Now, point your thumb
in the +z direction. Now if you were to rotate the x axis to coincide
with the y, it your fingers are giving the correct direction.

In a right handed system, sometimes a screw is used. If the screw has
its tip in the +z direction, turning the screw from x to y pushes the
screw in the z direction. But this is of course opposite to the
left-handed system.

All easier to see of course than to describe. 

I just find it easier to have something real like a screw, than to
remember a mnemonic.

-- 
Kaveh

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Jaime Vives Piqueres <jai### [at] ignoranciaorg> wrote:

> On 18 Nov 2002 15:34:03 -0500 Warp <war### [at] tagpovrayorg> wrote:
> 
> >   (If you didn't know, you can easily visualize which direction does a
> > positive angle rotate an object by pointing your thumb (of the left hand
> > if you are using the left-handed coordinates) towards the positive axis
> > around which you are rotating and bend your fingers.)
> 
>   I use only this rule for all my "visualizations" when I'm writing
> scenes, and I agree with you: when you have the right hand already on the
> numeric pad, the lef-handed system is very useful. Not joking!

I think the reason I brought this up in the first place was that when
studying physics, I found it easiest to imagine a normal wood screw,
turning from x to y axis. In a right handed system, the screw moves in
the +z direction. Now I have to use my hand, as there are no left-handed
screws!

-- Kaveh

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Kaveh wrote:

> Now I have to use my hand, as there are no left-handed
> screws!


There is such thing as 'left-handed screws', but you do not see them 
very often. Most of the time, there is a 'right-handed screw' at the 
other end, thus a turn of the screw either separate or bring closer the 
parts with the bolts (symmetrically!).
Typical application: pairs of compass for technical drawing.

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Let me try (A degree in physics must be useful somewhere)

For a LEFT handed Coord. sys.

1)  Place the 4 fingers of you LEFT hand so they point in the +X direction.
In the default configuration of POVRay this is heading off the Right side of
your moniter.

2)  "Curl" your fingers so they move from +X to +Y.  Again Default POVRay:
"curl" from right to top of your moniter

3) Your thumb now points in the direction of the +Z axis.  In the example
above "into" the screen.



"Kaveh" <kav### [at] delete_thisfocalimagecom> wrote in message
news:1flwt0q.1eoksy3ek3y68N%kaveh@delete_this.focalimage.com...
> Warp <war### [at] tagpovrayorg> wrote:
>
> > Kaveh <kav### [at] delete_thisfocalimagecom> wrote:
> > > I find it easier just to use the curled fingers method, not the index
> > > and third fingers. To get the sense of the z axis, I position the left
> > > hand such that the four fingers curl from the x to the y axis. Then
the
> > > thumb shows the direction of the z axis.
> >
> >   That description was very unclear. I did not visualize at all which
> > direction the thumb is pointing at...
>
> It was not clear, I agree. I will put it another way. Put your left hand
> in the 'aerobics' position, with fingers curled. Now, point your thumb
> in the +z direction. Now if you were to rotate the x axis to coincide
> with the y, it your fingers are giving the correct direction.
>
> In a right handed system, sometimes a screw is used. If the screw has
> its tip in the +z direction, turning the screw from x to y pushes the
> screw in the z direction. But this is of course opposite to the
> left-handed system.
>
> All easier to see of course than to describe.
>
> I just find it easier to have something real like a screw, than to
> remember a mnemonic.
>
> --
> Kaveh

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jfmiller <jfm### [at] hotmailcom> wrote:

> Let me try (A degree in physics must be useful somewhere)
> 
> For a LEFT handed Coord. sys.
> 
> 1)  Place the 4 fingers of you LEFT hand so they point in the +X direction.
> In the default configuration of POVRay this is heading off the Right side of
> your moniter.
> 
> 2)  "Curl" your fingers so they move from +X to +Y.  Again Default POVRay:
> "curl" from right to top of your moniter
> 
> 3) Your thumb now points in the direction of the +Z axis.  In the example
> above "into" the screen.

Correct. Thank you for the clarification. :-)

Having thought about it, for me at least, a right-handed system would be
easier to use. I remember I never liked mnemonics. The 3-finger system
is really a mnemonic, whereas using the screw analogy (at least in my
simple mind) seems to have some physical basis to it.

Also, all vector calculus I learned was right-handed. If I remember
rightly, the vector A x B, gives a vector C whose direction is given by
using the right handed rule or screw rule, going from A to B. 

In physics, When a current goes through a wire, you get a magnetic
field, going around it in a right handed way, although I think this is
just a matter of convention.

So, in short, everything I have come across has used the right-handed
screw analogy, except POVray. As I am not comfortable with a left-handed
screw, I now have to use my left hand.

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Kaveh <kav### [at] delete_thisfocalimagecom> wrote:

> As I am not comfortable with a left-handed
> screw, I now have to use my left hand.

Oh boy. I *knew* I would get myself into trouble at last. :-(

Just a little anecdote I remember, in our physics class. A friend had
terrible trouble using the right hand finger rule. He said it always
came out the wrong way. When I asked him to demonstrate it to me, he was
holding his index and third fingers in the wrong directions, with the
third pointing fingers forwards, the index bent!

-- 
Kaveh

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jfmiller <jfm### [at] hotmailcom> wrote:
> 1)  Place the 4 fingers of you LEFT hand so they point in the +X direction.
> In the default configuration of POVRay this is heading off the Right side of
> your moniter.

> 2)  "Curl" your fingers so they move from +X to +Y.  Again Default POVRay:
> "curl" from right to top of your moniter

> 3) Your thumb now points in the direction of the +Z axis.  In the example
> above "into" the screen.

  And someone thinks this is easier than simply pointing your thumb to
+X, index finger to +Y and middle finger to +Z?

-- 
#macro M(A,N,D,L)plane{-z,-9pigment{mandel L*9translate N color_map{[0rgb x]
[1rgb 9]}scale<D,D*3D>*1e3}rotate y*A*8}#end M(-3<1.206434.28623>70,7)M(
-1<.7438.1795>1,20)M(1<.77595.13699>30,20)M(3<.75923.07145>80,99)// - Warp -

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>   And someone thinks this is easier than simply pointing your thumb to
> +X, index finger to +Y and middle finger to +Z?

No not really, It's all a matter of what you've learned.  By using the
"curl" method it is then easier to introduce the consept of rotation as
"curling" ones finger in the direction of rotation and having the thumb pint
along the correct axis.  It also helps in sphereical Coords to remember
which way theta rotates.  But really it's just a matter of preference.

JFMILLER

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