> Am 01.03.2013 22:42, schrieb Alain:

>>> Am 01.03.2013 04:14, schrieb Alain:

>>>>
>>>> In my math classes when looking at geometry, we only used the
>>>> left-handed coordinate and rotation system. Also, Y was always UP and Z
>>>> forward, NEVER relative to the paper's surface.
>>>>
>>>> It was the same in my physics courses.
>>>
>>> What? For us it was /always/ right-handed, for both maths and physics. I
>>> can't believe it's different across the world in such disciplines.
>>>
>>> ... unless of course you use a different scheme to assign axes to the
>>> fingers. We used thumb=X, index=Y, and middle=Z.
>>
>> Using the right hand, it require you to twist your arm to have the thumb
>> point toward +X, then the index point to +Z
>>
>> In my courses, with the left hand, it was: thumb = +Y, index= +Z,
>> middle= +X with the index pointing forward and the thumb up.
>> Very easy to hold, and you can still have a pen in your right hand...
>
> That's a left-handed system indeed, but I doubt it's the standard used
> in professional mathematics and physics. (Then again, in professional
> mathematics the handedness of the world is just abstracted away anyway.
> That leaves physics though.)
>
>
>> Maybe just a coincidence, but it's the same thing that is used to get
>> the direction of the force exerced on a conductor in a magnetic field.
>> The index is in the direction of the current (positive), the thumb in
>> the direction of the magnetic field (north pole), and the middle finger
>> in the direction of the force.
>
> If you swap any of the two components, you can do the same stunt with
> the right hand.
>
> But here are two stunts you can't do left-handed:
>
> Make a "thumbs-up" sign with your right hand. Point your thumb in the
> direction of current in a wire. The fingers will curl in the direction
> of the magnetic field around the wire.
>
> Make a "thumbs-up" sign with your right hand. Hold your hand so that the
> fingers curl in the direction of current flowing in a coil. The thumb
> will point in the direction of the magnetic field inside the coil.
>
>
>> For the rotations, you point the thumb toward the + side of an axis, and
>> the fingers curl in the direction of the positive rotation.
>
> That's true in both left- and right-handed coordinate systems, simply
> due to the way positive rotation is defined. (In other words, in a
> left-handed coordinate system positive rotation is defined just the
> other way round as in a right-handed one.)
>
>
>> That's why, for me, ther was no learning curve to use the POV-Ray
>> coordinate system. It's exactly the same I always used in school.
>
> It might be a UK-schools special, which would explain why POV-Ray uses
> it. (Then again, you are not from the UK, are you?)
No. Canada.
>
>
>>> If you put a proper math-style 2D coordinate system (X axis right, Y
>>> axis "top") onto paper, and then add +Z as height above ground, that's a
>>> left-handed system to me.
>>>
>> That's exactly how architecs work. And that's right handed.
>
> Doing the gymnastics again, yes - it is indeed.
>

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