POV-Ray : Newsgroups : povray.documentation.inbuilt : Orientation and left handed frame : Re: Orientation and left handed frame Server Time
28 Jan 2022 11:44:40 EST (-0500)
  Re: Orientation and left handed frame  
From: clipka
Date: 4 Oct 2016 17:34:39
Message: <57f4206f$1@news.povray.org>
Am 04.10.2016 um 23:05 schrieb lelama:

>> POV-Ray's documentation [...] just
>> documents how POV-Ray does it.
>> [...] other people might experience it as unnecessarily confusing [...]
> 
> The point is not a question of taste, it is about ambiguity. When the camera is
> given a negative right vector, it is not clear what is a positive rotation in
> this context. Only the expert users that are aware that there is a quite uniform
> convention among different disciplines may guess. The other users have to check
> that what they have in mind correspond (or not ) to povray's behaviour. Maybe
> there is a line in the doc that I missed, but I have not seen where I could find
> the solution without trial and error for a camera with a negative right vector.
> 
> Some people consider a  rotation as a geometric concept (screwing/unscrewing),
> like physicists using the corkscrew rule. For instance in france, the corkscrew
> rule is quite common, probably because of the wine ;-) These people will
> probably guess that povray keeps the same screwing/unscrewing convention for a
> negative right vector and for a positive right vector. And they will be wrong.
> When a negative right vector is used, povray keeps the same algebraic formulas
> for the rotations and toggles the corkscrew rule. The other choice (toggling the
> formulas and keeping the corkscrew rule ) woud have been possible too.

"The left hand can also be used to determine rotation directions. To do
this we must perform the famous 'Computer Graphics Aerobics' exercise.
[...]
In a right-handed system we use our right hand for the 'Aerobics'."

I think that's clear enough.

If for some reason you ignore the hand-based description in favour of
some other culture-specific rule, and then get confused because your
culture-specific rule doesn't cover questions that are answered quite
clearly in the hand-based description, I wouldn't call that a fault of
the POV-Ray documentation.


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