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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