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From: clipka
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 3 Jun 2016 14:16:49
Message: <5751c991$1@news.povray.org>
Am 26.05.2016 um 18:26 schrieb Stephen:
> On 5/26/2016 4:56 PM, Le_Forgeron wrote:
>> I also want to stress that it is topologically, and logically too,
>> impossible to define left/right with an alien over a radio link:
> 
> This is true but we are not dealing with aliens. (Well there might be
> one or two. ;) )
> 
> PovRay has a default handiness with Y being up. Why not use that to
> define clockwise and anti-clockwise?

I think it's no good to implicitly presume any particular handedness
throughout the docs; while there is a default handedness, there are also
people who override the handedness "by default".


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From: clipka
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 3 Jun 2016 14:18:12
Message: <5751c9e4$1@news.povray.org>
Am 03.06.2016 um 20:16 schrieb clipka:
> Am 26.05.2016 um 18:26 schrieb Stephen:
>> On 5/26/2016 4:56 PM, Le_Forgeron wrote:
>>> I also want to stress that it is topologically, and logically too,
>>> impossible to define left/right with an alien over a radio link:
>>
>> This is true but we are not dealing with aliens. (Well there might be
>> one or two. ;) )
>>
>> PovRay has a default handiness with Y being up. Why not use that to
>> define clockwise and anti-clockwise?
> 
> I think it's no good to implicitly presume any particular handedness
> throughout the docs; while there is a default handedness, there are also
> people who override the handedness "by default".

(Not to mention people using 3rd party tools to generate portions of
their scenes, which might be overriding the handedness as well.)


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From: Stephen
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 3 Jun 2016 14:23:43
Message: <5751cb2f$1@news.povray.org>
On 6/3/2016 7:17 PM, clipka wrote:
> Am 03.06.2016 um 20:16 schrieb clipka:
>> Am 26.05.2016 um 18:26 schrieb Stephen:
>>> On 5/26/2016 4:56 PM, Le_Forgeron wrote:
>>>> I also want to stress that it is topologically, and logically too,
>>>> impossible to define left/right with an alien over a radio link:
>>>
>>> This is true but we are not dealing with aliens. (Well there might be
>>> one or two. ;) )
>>>
>>> PovRay has a default handiness with Y being up. Why not use that to
>>> define clockwise and anti-clockwise?
>>
>> I think it's no good to implicitly presume any particular handedness
>> throughout the docs; while there is a default handedness, there are also
>> people who override the handedness "by default".
>
> (Not to mention people using 3rd party tools to generate portions of
> their scenes, which might be overriding the handedness as well.)
>

Whatever you think yourself.

-- 

Regards
     Stephen


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From: clipka
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 3 Jun 2016 14:24:56
Message: <5751cb78$1@news.povray.org>
Am 27.05.2016 um 02:11 schrieb William F Pokorny:

> POV-Ray today follows the above clockwise vs counter-clockwise 2D
> conventions for determining inside vs outside everywhere except for the
> bezier spline in the prism, where, the inside and outside conventions
> are reversed for the prism sides. Further, all prism ends point outward
> no matter the loop order.

It might be worth actually testing how the clockwise/counter-clockwise
thing works out depending on the handedness of the coordinate system,
rather than just speculating or relying on unconfirmed assumptions.


> So, I guess the question is do we:
> 
> - add some or all the text above about 2d clockwise / counter-clockwise
> conventions?
> 
> - add some figures explicitly showing the two example point lists or
> others showing what would be clockwise and counter-clockwise point lists
> with respect to inside/outside determination?
> 
> - change to more generic wording like 'orientation' and 'reverse
> orientation' without any suggestions about normal, normal directions but
> noting they flip.
> 
> - simply note that point list order matters to surface normal
> determination in lathes and prisms and leave it at that? Starting to
> feel like I have wandered off into the weeds... :-)
> 
> Thoughts?

- Keep the "clockwise" and "counter-clockwise" wording, and add a note
saying that these apply to POV-Ray's left-handed coordinate system,
whereas with a right-handed coordinate system the direction is reverse?

(Of course such a note should only be added if tests show that it is
indeed the case ;))


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From: Bald Eagle
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 3 Jun 2016 15:20:01
Message: <web.5751d7759ebd2b23b488d9aa0@news.povray.org>
clipka <ano### [at] anonymousorg> wrote:

> I think it's no good to implicitly presume any particular handedness
> throughout the docs; while there is a default handedness, there are also
> people who override the handedness "by default".

Uh, wouldn't the WAY they "override" the handedness be directly related to the
default handedness?  I mean, if they're reversing the left-handed default as
part of their custom setup, then they must be presuming that there's a
left-handedness to be reversed in the first place.

Just being an academic PITA    ;)


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From: William F Pokorny
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 3 Jun 2016 17:51:30
Message: <5751fbe2$1@news.povray.org>
On 06/03/2016 02:24 PM, clipka wrote:
> Am 27.05.2016 um 02:11 schrieb William F Pokorny:
>
> It might be worth actually testing how the clockwise/counter-clockwise
> thing works out depending on the handedness of the coordinate system,
> rather than just speculating or relying on unconfirmed assumptions.
>

I assume you mean flipping the handedness of the camera to right and 
looking at all the test cases I created?

It is true I ran only one of the tests posted to povray github this 
way(1). It didn't change result with 3D handedness. It will take me some 
time to crank all the test cases with a right hand camera & review the 
results.

Is there some case you suspect will flip for the 2D curves that I might 
jump to first in an attempt to save some time? You're usually a few 
steps (as in gigantic, flying leaps) ahead of me... :-)

End of a long day, but I'm still thinking 2D is 2D & while given 
rotations and mirroring things might at first look different given a 
viewpoint, there is really only one normal planar 2D representation.

(1)- (bezier, lathe, 12 counter-clockwise loops varying point set 
rotations, straddling y=0 forming rings about y)

>
...
>> So, I guess the question is do we:
>>
...
>
> - Keep the "clockwise" and "counter-clockwise" wording, and add a note
> saying that these apply to POV-Ray's left-handed coordinate system,
> whereas with a right-handed coordinate system the direction is reverse?
>
> (Of course such a note should only be added if tests show that it is
> indeed the case ;))
>

I will check in on this thread tomorrow morning, before running what 
test cases I have with a right hand camera set up.

Bill P.


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From: Le Forgeron
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 3 Jun 2016 22:47:02
Message: <57524126$1@news.povray.org>
Le 03/06/2016 23:51, William F Pokorny a écrit :
> On 06/03/2016 02:24 PM, clipka wrote:
>> Am 27.05.2016 um 02:11 schrieb William F Pokorny:
>>
>> It might be worth actually testing how the clockwise/counter-clockwise
>> thing works out depending on the handedness of the coordinate system,
>> rather than just speculating or relying on unconfirmed assumptions.
>>
> 
> I assume you mean flipping the handedness of the camera to right and looking at all
the test cases I created?
> 
> It is true I ran only one of the tests posted to povray github this way(1). It
didn't change result with 3D handedness. It will take me some time to crank all the
test cases with a right hand camera & review the results.


The result might be the same (actually, it should be so, as the coding of the object
remains identical), but changing the handedness, for the *same* code of the object, is
also changing the notion/visualisation of clockwise/counter-clockwise.

Put coloured spheres at the various points used on object, using H of HSV or HSL
colour coding as the progression along the points.
One handedness would display a clockwise progression, and the other handedness the
counter-clockwise progression.
(you could use text item, but text would need to be oriented to be readable, less
problem with coloured spheres)


> 
> Is there some case you suspect will flip for the 2D curves that I might jump to
first in an attempt to save some time? You're usually a few steps (as in gigantic,
flying leaps) ahead of me... :-)
> 
> End of a long day, but I'm still thinking 2D is 2D & while given rotations and
mirroring things might at first look different given a viewpoint, there is really only
one normal planar 2D representation.
> 
> (1)- (bezier, lathe, 12 counter-clockwise loops varying point set rotations,
straddling y=0 forming rings about y)
> 
>>
> ...
>>> So, I guess the question is do we:
>>>
> ...
>>
>> - Keep the "clockwise" and "counter-clockwise" wording, and add a note
>> saying that these apply to POV-Ray's left-handed coordinate system,
>> whereas with a right-handed coordinate system the direction is reverse?
>>
>> (Of course such a note should only be added if tests show that it is
>> indeed the case ;))
>>
> 
> I will check in on this thread tomorrow morning, before running what test cases I
have with a right hand camera set up.
> 
> Bill P.


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From: William F Pokorny
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 4 Jun 2016 10:10:05
Message: <5752e13d$1@news.povray.org>
On 06/03/2016 10:46 PM, Le_Forgeron wrote:
> Le 03/06/2016 23:51, William F Pokorny a écrit :
>>
>> I assume you mean flipping the handedness of the camera to right and looking at all
the test cases I created?
>>
>> It is true I ran only one of the tests posted to povray github this way(1). It
didn't change result with 3D handedness. It will take me some time to crank all the
test cases with a right hand camera & review the results.
>
> The result might be the same (actually, it should be so, as the coding of the object
remains identical), but changing the handedness, for the *same* code of the object, is
also changing the notion/visualisation of clockwise/counter-clockwise.
>

Indeed there was no change in inside vs outside determination for any 
test case using a right handed camera set up.

> Put coloured spheres at the various points used on object, using H of HSV or HSL
colour coding as the progression along the points.
> One handedness would display a clockwise progression, and the other handedness the
counter-clockwise progression.
> (you could use text item, but text would need to be oriented to be readable, less
problem with coloured spheres)
>
>

See:

http://news.povray.org/povray.binaries.images/thread/%3C5752dc4c%40news.povray.org%3E/

The viewer's perception due left versus right handed camera set ups does 
of course changes for the prism due the point set being described in the 
x,z plane, but not for the lathe due in being describe in the x,y plane.

One could say for the case of the prism, in moving to the right handed 
camera - while not changing any other scene set up - we have decided to 
look at the 2d clock face upside down. ;-)

Anyway, I understand the concern. Guess it comes to whether the wording 
Jim added in :

Note: The use of clockwise and counter-clockwise in the following 
examples should not be taken imply any notion of handedness.

enough to address it or do we need to add something more?


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From: William F Pokorny
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 6 Jun 2016 07:32:17
Message: <57555f41$1@news.povray.org>
On 05/26/2016 08:03 PM, Jim Holsenback wrote:
> On 5/25/2016 6:53 PM, Jim Holsenback wrote:
>> On 5/24/2016 9:32 AM, William F Pokorny wrote:
>>> Would someone who knows what they are doing wiki-wise - or who has the
>>> "create" authority for the reference->object->lathe/prism sections can
>>> create the talk/discussion area. Or, I suppose, if the update looks good
>>> to those reviewing, just get it into the actual reference.
>>
>> lathe additions have been added:
>> http://wiki.povray.org/content/Reference:Lathe
>>
>> didn't have time to do prism yet
>
> prism additions have been added:
> http://wiki.povray.org/content/Reference:Prism ... thanks for the
> contribution
>
Jim,

Another version of both lathe and prism documentation is available at:

http://wiki.povray.org/content/User:Wfpokorny

Thanks.

Bill P.


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From: Jim Holsenback
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 6 Jun 2016 14:06:03
Message: <5755bb8b$1@news.povray.org>
On 6/6/2016 7:32 AM, William F Pokorny wrote:
> Another version of both lathe and prism documentation is available at:
>
> http://wiki.povray.org/content/User:Wfpokorny

LOL semantics ... updated.


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