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From: William F Pokorny
Subject: Lathe and Prism updates requested by Christoph.
Date: 24 May 2016 13:32:34
Message: <574457f2$1@news.povray.org>
Jim et al,

I have not been able to figure out how to initiate discussion/talk 
editable text from the wiki where there is not already a discussion 
active for the wiki section.

So! I put my initial draft of Lathe and Prism updates Christoph asked I 
do under my user account at :

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

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.

Thanks.
Bill P.


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From: Jim Holsenback
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 24 May 2016 16:02:34
Message: <57447b1a$1@news.povray.org>
On 5/24/2016 9:32 AM, William F Pokorny wrote:
> Jim et al,
>
> I have not been able to figure out how to initiate discussion/talk
> editable text from the wiki where there is not already a discussion
> active for the wiki section.
>
> So! I put my initial draft of Lathe and Prism updates Christoph asked I
> do under my user account at :
>
> http://wiki.povray.org/content/User:Wfpokorny
>
> 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.
>
> Thanks.
> Bill P.

looks good so far (just a quick look for now) ... i'll look more closely 
and get your content into the docs proper when i have a bit more time. 
you can do: 
http://wiki.povray.org/content?title=Special:RecentChanges&hidebots=0 to 
keep an eye on what i've/i'm done/doing ... in case i forget to give a 
heads up when completed.


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From: Jim Holsenback
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 25 May 2016 22:53:35
Message: <57462cef$1@news.povray.org>
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


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From: Le Forgeron
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 26 May 2016 15:57:10
Message: <57471cd6$1@news.povray.org>
Le 26/05/2016 00:53, Jim Holsenback a écrit :
> 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
> 

Thanks for your time :-)

I'm going to be painful, hopefully not to much for Jim, but other(s):

what is "clockwise" and "counterClockwise" ?

And before someone jumps in to state the obvious, think again about it.




Usually (Danger Will Robinson, Danger), +X is to the right and +Y is up.

And the direction of Z when going 3D is a holy war between left-hand and right-hand
system.


But it is not unusual either that +X is to the right and +Y is down (many pictures
format have such convention).
(origin at top left, and in fact, western-text has that same convention: Nth lines,
Mth character on that line)


And some tenants of right-hand system like +X to the left, +Y up and +Z away (and most
3D formats are rather right-handed... at least STL is strongly thought as such)



I also want to stress that it is topologically, and logically too, impossible to
define left/right with an alien over a radio link: even using physical laws to have
the same experiment. You can define down and up (gravity is helpful), you can define
away and nearer, but,
as the alien's environment might be matter or anti-matter, you cannot use anything to
provide the same definition of left & right.
And if you cannot define left/right, clockwise and counterclockwise are lost too.


All this, to ask for more explanation, or illustration, of "clockwise" and
"counterClockwise".


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From: Stephen
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 26 May 2016 16:26:23
Message: <574723af$1@news.povray.org>
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?


-- 

Regards
     Stephen


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From: Le Forgeron
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 26 May 2016 19:27:13
Message: <57474e11$1@news.povray.org>
Le 26/05/2016 18:26, Stephen a écrit :
> 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?
> 

Povray has no default handiness.
There is a bit of usage and samples, but no definite handiness.


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From: Stephen
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 26 May 2016 19:55:34
Message: <574754b6$1@news.povray.org>
On 5/26/2016 8:27 PM, Le_Forgeron wrote:
> Le 26/05/2016 18:26, Stephen a écrit :
>> 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?
>>
>
> Povray has no default handiness.
> There is a bit of usage and samples, but no definite handiness.
>


Just taking it from:

3.4.2.1.7 Handedness

The right vector also describes the direction to the right of the 
camera. It tells POV-Ray where the right side of your screen is. The 
sign of the right vector can be used to determine the handedness of the 
coordinate system in use.

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

Implied default handiness?

Anyway, if there are problems with nomenclature. Why not define 
clockwise when looking from a positive Y-axis position, or whatever? 
There is no need (IMO) to make the naming more complicated than the 
development. :)

-- 

K.I.S.S. ;)

     Stephen


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From: Bald Eagle
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 26 May 2016 23:10:01
Message: <web.574781c09ebd2b235e7df57c0@news.povray.org>
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.

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.


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From: Jim Holsenback
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 27 May 2016 00:04:08
Message: <57478ef8$1@news.povray.org>
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


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From: William F Pokorny
Subject: Re: Lathe and Prism updates requested by Christoph.
Date: 27 May 2016 00:11:30
Message: <574790b2$1@news.povray.org>
On 05/26/2016 11:56 AM, Le_Forgeron wrote:
> Le 26/05/2016 00:53, Jim Holsenback a écrit :
>
> Thanks for your time :-)
>
> I'm going to be painful, hopefully not to much for Jim, but other(s):
>
> what is "clockwise" and "counterClockwise" ?
>
> And before someone jumps in to state the obvious, think again about it.
>
> Usually (Danger Will Robinson, Danger), +X is to the right and +Y is up.
>
> And the direction of Z when going 3D is a holy war between left-hand and right-hand
system.
>
> But it is not unusual either that +X is to the right and +Y is down (many pictures
format have such convention).
> (origin at top left, and in fact, western-text has that same convention: Nth lines,
Mth character on that line)
>
> And some tenants of right-hand system like +X to the left, +Y up and +Z away (and
most 3D formats are rather right-handed... at least STL is strongly thought as such)
>
> I also want to stress that it is topologically, and logically too, impossible to
define left/right with an alien over a radio link: even using physical laws to have
the same experiment. You can define down and up (gravity is helpful), you can define
away and nearer, but,
> as the alien's environment might be matter or anti-matter, you cannot use anything
to provide the same definition of left & right.
> And if you cannot define left/right, clockwise and counterclockwise are lost too.
>
> All this, to ask for more explanation, or illustration, of "clockwise" and
"counterClockwise".
>

I'm not opposed to making things clearer. I was aiming for wording I 
thought most would easily understand, while conforming somewhat to 
conventions.

The clockwise vs counter-clockwise concept is used commonly for 2D 
geometric algorithms and the lathe and prism spline point lists are 2D. 
In the case of the lathe we are working in a normally defined x,y plane. 
In the case of prisms we are working in a normally defined x,z plane.

There are different ways to make the determination, but for splines 
which form closed loop(s), a counter-clockwise loop would be one such 
that the sum over all the loop edges of (x2-x1)*(y2+y1) is negative. 
Points inside a counter-clockwise loop of edges are considered to be 
inside the shape. A clockwise loop reverses the points to shape 
relationship.

In the case of lathes we can have edge(s) which do not form a closed 
loop. There we chose a point (x3,y3) to the left of an edge and look at 
the determinant (cross product) :

(x2-x1)*(y3-y1)-(y2-y1)*(x3-x1).

If the value returned is >0, the 'edge-by-direction' is 
counter-clockwise with respect to the test point and the test point is 
considered to be inside. If <0 the test point is considered outside. If 
exactly 0, the test point is collinear with the edge.

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.

Asides: There are degenerate, fringe point lists which will also fall 
outside these 2D conventions, but I think users will not often deal with 
them. The 2d clockwise vs counter-clockwise conventions are used mostly 
with what is our linear spline. Of course for prisms replace y with z in 
the equations above.

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?

Bill P.


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