POV-Ray : Newsgroups : povray.documentation.inbuilt : Orientation and left handed frame : Re: Orientation and left handed frame Server Time
16 Jan 2022 16:37:06 EST (-0500)
  Re: Orientation and left handed frame  
From: lelama
Date: 4 Oct 2016 17:10:01
Message: <web.57f41999844df0737a63dc9f0@news.povray.org>
clipka <ano### [at] anonymousorg> wrote:

> Actually this is not a matter of choice of the coordinate system -- it
> is a matter of choice of how to represent rotations.

Agree with that.

> As Jerome already mentioned, there is a very strong consensus across
> disciplines of theoretical and applied science of what constitutes
> positive rotation

OK, didn't know that. Thanks for the remark.

> 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.

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