Le 04/10/2016 à 16:44, lelama a écrit :
> 
> 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. In my
> opinion, the correct formulation should be that :
> 
> By default:
> - xyz follows the left handed convention
> - rotations with a positive angle with respect to a vector unscrews if the eye
> is in the direction of the vector.
> If we change the camera right vector to change to the right handed convention,
> povray will implicitly change automatically change the second convention and
> will screw positive rotations.
> 

There is only one choice: between left or right handed system.

(As long as we are using perpendicular vectors for up, right and direction)

And whatever the choice, the positive angle rotation along an axis k
will be the shortest rotation which transform axis k+1 into axis k+2
(modulo 3).
(the opposite rotation for the same transformation would be with a
negative angle whose absolute value would be 3 times bigger, modulo the
full rotation)

aka:
* rotation of +90° along +x transform +y into +z
* rotation of +90° along +y transform +z into +x
* rotation of +90° along +z transform +x into +y&

It works for left handed ( +x to the right, +y up, +z forward)
and it works for right-handed ( +x to the left, +y forward, +z up)

(IIRC, right handed is for architect: you do schema of floor on <x,y>, z
is the height of the floors,
while left handed is for mathematician: you draw <x,y> on the
blackboard, and extend the depth with z later)

The shortest rotation is always "screwing" (with a right helix, the
usual screw you encounter 99.9% of the time) along its axis... if that
can make sense to you, but:

the documentation page talks about the aerobic exercise, done with the
relevant hand, thumb along the axis, to fold the straight index finger
onto the already half folded middle finger.
There is no part which talks about screwing.

There is no option to change the fact that
* +y rotated by +quarter of circle +x becomes +z
* +z rotated by +quarter of circle +y becomes +x
* +x rotated by +quarter of circle +z becomes +y

Never. Consider it a part of the definition of the base.




> To produce povray code from an  other software that cooperates with povray,
> sometimes the other software does not follow the same conventions. In this case,
> the documentation is not sufficient and it is necessary to experiment to reelly
> understand what's going on.
> 
> My 2 cents,
> Laurent.
> 
> 
>

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