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From: Jim Holsenback
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 26 May 2016 21:22:39
Message: <5747a15f$1@news.povray.org>
On 5/26/2016 8:11 PM, William F Pokorny wrote:
> Thoughts?

well ... i think i'll indulge my sweet tooth and make myself a root beer 
float (substitute dr pepper) ... i'll leave the haggling up to you all 
and will monitor this thread as well as your user page then make 
whatever changes are needed once the chatter has settled down.


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From: Stephen
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 27 May 2016 03:37:19
Message: <5747f92f$1@news.povray.org>
On 5/27/2016 12:07 AM, Bald Eagle wrote:
> Stephen <mca### [at] aolcom> wrote:
>
>> The default value is: right<1.33,0,0>. This means that the +x-direction
>> is to the right. It is called a left-handed system
>
> I would say that there is a definite handedness to POV-Ray, given the sign of
> different directions on the axes, and the direction of rotation around the axes
> in the (+) direction, I'd have to say that there's a default and inherent
> chirality.
>

I would say so too.
I know that if I use an image map and RH handiness in Bishop3D. The 
image is reversed. Likewise PoseRay, on its PovRay output tab. States:
Right handed (PoseRay default) Y up
Left handed (POV-Ray default) Y up

I also used Moray where Z is up and am quite aware of all the kerfuffle 
around this topic.



> Left-handed coordinate system, and "left-hand rule" for determining things like
> direction of rotation or direction of a normal.
>
> But that's my take as an organic chemist, casual and incidental student of
> physics and POV-Ray (l)user.
>

Personally I prefer the Y up because when I started work as an 
Electronic Engineer, in the early 70's. The Z axis*, on an oscilloscope, 
could be used to control the brightness of the waveform simulating depth.

<Aside> I seem to remember from that distant past. I was told that 
engineers use the Y up, from the oscilloscope example. And that 
mathematicians prefer the Z up coordinate system because they visualise 
a XY graph on paper with the Z axis coming up from the desk. It doesn't 
hold but it helps me visualise converting from one system to another.


* It was called the Z buffer.

# If you think I am using too many commas. It is to keep Dr. John amused. :)


-- 

Regards
     Stephen


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From: Jim Holsenback
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 28 May 2016 10:03:43
Message: <5749a53f$1@news.povray.org>
On 5/26/2016 8:11 PM, William F Pokorny wrote:
> Thoughts?

i added a note just before the passages in question for both the lathe 
and prism objects ... hopefully that will suffice


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From: William F Pokorny
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 30 May 2016 08:54:47
Message: <574c3817$1@news.povray.org>
On 05/28/2016 10:03 AM, Jim Holsenback wrote:
> On 5/26/2016 8:11 PM, William F Pokorny wrote:
>> Thoughts?
>
> i added a note just before the passages in question for both the lathe
> and prism objects ... hopefully that will suffice
>

Yes, I think is good. Certainly MUCH simpler than any convoluted 
explanation I had in the works for Jérôme's request :-). Thanks!

Bill P.


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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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