"Tim Attwood" <tim### [at] anti-spamcomcastnet> wrote:
> POV by default uses a left handed coordinate system... so plus Y is
> up, plus X is right, plus Z is away. It's called left handed because
> you can hold your left hand with the thumb up (Y), the pointer finger
> forward (Z), and the middle finger right (X) to visualize the XYZ axis.

As a side note, I think this is an extremely ugly way of defining a "left
handed" coordinate system, because it appears somewhat arbitrary: If your Y was
away and Z was up, would it still be a left-handed system? After all, you could
point the fingers of your left hand in exactly the same way, just "numbering"
them differently.

The answer is "no", but the description gives no hint why that would be a
right-handed coordinate system instead.

It gets clearer when one introduces a strictly systematic "naming" for the
fingers: Starting from the thumb, call your fingers "X", "Y" and "Z". So point
your thumb to the right (X), your index finger up (Y), and your middle finger
away (Z).


> You can also picture the direction of rotations that way. If you make
> the XYZ axis with your left hand, then if you trace from the palm
> of the left hand over the middle finger, that's positive X, and
> continue tracing over the middle finger, that's positive Z, and then
> tward you around the thumb, that's positive Y.

This, too, appears to be quite arbitrary. There's another, IMHO much more
intuitive way to picture rotation in a left-handed coordinate system:

Make a "thumbs up" sign with your left hand. Hold it so that your thumb points
into the direction you're rotating about. Rotating by a positive value now
corresponds to moving along your curved fingers towards the fingertips.


Same gymnastics work accordingly for a right-handed coordinate system, of
course. Just use your right hand then.


Unfortunately, the POV-Ray documentation is somewhat inconsistent about how to
visualize handedness; while "2.2.1.1  Understanding POV-Ray's Coordinate
System" uses the imagery I just described, "3.3.1.1.7  Handedness" uses the
more contrived variant for depicting the coordinate axes with your hand (but
just refers to 2.2.1.1 for rotation).

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