Am 04.10.2016 um 16:44 schrieb lelama:
> 
> The documentation of povray seems to me inacurate on the oritentation
> considerations if I look at http://www.povray.org/documentation/view/3.7.0/15/
> 
> To fix the orientattion conventions, there are 2 choices to be done :
> - does xyz follow the left or right handed convention ?
> - do we screw or unscrew the rotations with a positive angle ?
> 
> These two choices may be done independently. The documentation suggests that
> these two choices are dependent, making the doc difficult to read.

I don't see the documentation implying anything of that sort. It simply
describes how POV-Ray does things, and neither asserts nor denies a
connection between the two features.

> In my
> opinion, the correct formulation should be that :
> 
> By default:
> - xyz follows the left handed convention

- What /is/ the "left handed convention"? The current wording easily
explains POV-Ray's coordinate system by means of physiological
properties shared by well over 99.99% of all human beings, rather than
relying on the reader being familiar with arbitrary conventions
established in mathematics or computer graphics. Also, there are even
conventions out there using the terms "left handed" and "right handed"
with the reverse meaning, assigning the x, y and z axis to the middle
finger, index finger and thumb instead. By explicitly stating the finger
assignment, the current wording avoids this problem.

- The "left handed convention" does not say anything about the
orientation of the axes in space (such as which axis is up), just their
relation to each other. POV-Ray has a clearly defined default
orientation in space, and the current wording mentions that.

> - rotations with a positive angle with respect to a vector unscrews if the eye
> is in the direction of the vector.

- The current wording again explains POV-Ray's handling of rotations by
means of easily accessible physiological properties, rather than relying
on the arbitrary engineering conventions of what rotation direction will
tighten or loosen a (conventional) screw, or even whether the screw's
head would be facing towards or away from you when you do that feat.
"Clockwise" and "anti-clockwise" would probably be better references
than screws, but I think human physiological properties win hands down
(pun somewhat intented).


I see no fault in the current wording, except maybe for the coordinate
system diagram that is ambiguous, since we don't really know whether the
Z axis points into or out of the image. We should probably add some
3-dimensional shape near the center to resolve the ambiguity.

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