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12 Dec 2024 18:56:56 EST (-0500)
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From: kurtz le pirate
Subject: Splines
Date: 12 Jul 2024 04:53:38
Message: <6690ef12$1@news.povray.org>
Hello,

I'm now looking at splines and trying to understand how POVRay treats
this object.

I made a trace in Inkscape and exported it to POV.
Export looks like this :

#declare path35 = prism {
 linear_sweep
 bezier_spline
 1.0, //top
 0.0, //bottom
 12 //nr points
 /*   0*/ <249.99999874, 886.42857827>, <316.34878488, 854.35384819>,
          <403.57141417, 756.42857575>, <312.85712126, 640.71428031>,
 /*   1*/ <312.85712126, 640.71428031>, <222.14285480, 525.00000000>,
          <128.57142803, 566.42857323>, <109.28570835, 735.00000000>,
 /*   2*/ <109.28570835, 735.00000000>, <90.00000000, 903.57142677>,
          <183.65121260, 918.50330457>, <249.99999874, 886.42857827>
 }


If I've understood correctly, each line has 4 vectors :
- the point,
- control point 1,
- control point 2,
- next point.

So i thought the two control points were the handle points.
So I tried to draw it all. This is the attached image.
Left: Inkscape screenshot, Right : POV

And it doesn't quite work as expected :
- square for principal points are (seems) OK
- first handle seems OK (with fine lines)
- second handles are wrongs (with heavy lines) :(


* Doesn't the third vector represent the second control point ?
* It also seems strange to me that the fourth vector is equal to the
  next main point.


There are a few things I didn't understand.
Is there a doc, a web page, ... that explains POV splines
in more detail ?



Thanks

-- 
Kurtz le pirate
Compagnie de la Banquise


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Attachments:
Download 'spline.jpg' (20 KB)

Preview of image 'spline.jpg'
spline.jpg


 

From: ingo
Subject: Re: Splines
Date: 12 Jul 2024 06:20:00
Message: <web.66910256aeceb74717bac71e8ffb8ce3@news.povray.org>
In the POV-Image the fat handle does seem to be flipped around the normal of the
other handle attached to the same point.


kurtz le pirate <kur### [at] gmailcom> wrote:
> Hello,
>
> I'm now looking at splines and trying to understand how POVRay treats
> this object.


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From: William F Pokorny
Subject: Re: Splines
Date: 12 Jul 2024 09:03:59
Message: <669129bf$1@news.povray.org>
On 7/12/24 04:53, kurtz le pirate wrote:
> If I've understood correctly, each line has 4 vectors :
> - the point,
> - control point 1,
> - control point 2,
> - next point.

I think the confusion is Inkscape is grouping by the tangents and 
POV-Ray by the Bezier spline segments as above.

It should be that each POV-Ray segment has a starting, on curve, tangent 
point and an ending, on curve, tangent point(*) - between which are two 
control points coming from two different Inkscape tangents(**).

(*) - These being <249.99999874, 886.42857827>,  <312.85712126, 
640.71428031> and <109.28570835, 735.00000000>

(**) The pairs being pulled from the six inkscape control points as the 
two between each of 3 pairs of starting and ending on curve points.

Bill P.


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From: kurtz le pirate
Subject: Re: Splines
Date: 12 Jul 2024 12:04:56
Message: <66915428$1@news.povray.org>
Thank you for your comments.
It helps, but it's still not very clear to me.

I've added a few elements to the drawing to help me.
Image and code (a bit messy) attached.





-- 
Kurtz le pirate
Compagnie de la Banquise


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Attachments:
Download 'code.tgz.gz' (3 KB) Download 'drawing_bezier_spline.png' (54 KB)

Preview of image 'drawing_bezier_spline.png'
drawing_bezier_spline.png


 

From: yesbird
Subject: Re: Splines
Date: 12 Jul 2024 12:46:48
Message: <66915df8$1@news.povray.org>
On 12/07/2024 11:53, kurtz le pirate wrote:
> There are a few things I didn't understand.
> Is there a doc, a web page, ... that explains POV splines
> in more detail ?

There was a discussion about strange POV splines behaviour, somehow I've
forced it to work, but the control and implementation are still mystery
to me:
news://news.povray.org:119/64105423$1@news.povray.org
-- 
YB


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From: William F Pokorny
Subject: Re: Splines
Date: 12 Jul 2024 15:09:55
Message: <66917f83$1@news.povray.org>
On 7/12/24 12:46, yesbird wrote:
> On 12/07/2024 11:53, kurtz le pirate wrote:
>> There are a few things I didn't understand.
>> Is there a doc, a web page, ... that explains POV splines
>> in more detail ?
> 
> There was a discussion about strange POV splines behaviour, somehow I've
> forced it to work, but the control and implementation are still mystery
> to me:
> news://news.povray.org:119/64105423$1@news.povray.org

Another form of the link which might be easier to use:

https://news.povray.org/povray.advanced-users/thread/%3C85b87be3-7ea0-95d9-62fa-0aa3cad0a315%40gmail.com%3E/

Bill P.


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From: yesbird
Subject: Re: Splines
Date: 12 Jul 2024 16:46:14
Message: <66919616$1@news.povray.org>
On 12/07/2024 22:09, William F Pokorny wrote:
> Another form of the link which might be easier to use: ...

Thanks, I will take it into account, but how you got it ?
I was using: RightClick -> "Copy Message Location" in Thunderbird.
--
YB


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From: Bald Eagle
Subject: Re: Splines
Date: 12 Jul 2024 16:55:00
Message: <web.669197ccaeceb7471f9dae3025979125@news.povray.org>
yesbird <sya### [at] gmailcom> wrote:
> On 12/07/2024 22:09, William F Pokorny wrote:
> > Another form of the link which might be easier to use: ...
>
> Thanks, I will take it into account, but how you got it ?
> I was using: RightClick -> "Copy Message Location" in Thunderbird.
> --
> YB

Use the URL in the browser/web interface
http://news.povray.org/


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From: Josh English
Subject: Re: Splines
Date: 12 Jul 2024 23:20:14
Message: <6691f26e$1@news.povray.org>
On 7/12/2024 1:53 AM, kurtz le pirate wrote:
> 
> Hello,
> 
> I'm now looking at splines and trying to understand how POVRay treats
> this object.
> 

Your understanding is correct. Here's an example with:
a) simpler coordinates
b) an array for tracking the indices.

It ain't pretty, but it works.

The points 0, 3, 6, and 9 are points on the curve.

Point 0 has control points 11 and 1
Point 3 has control points 2 and 4
Point 6 has control points 5 and 7
Point 9 has control points 8 and 10

Josh


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Attachments:
Download 'spline_question_answer.pov.txt' (2 KB) Download 'spline_question_answer.png' (8 KB)

Preview of image 'spline_question_answer.png'
spline_question_answer.png


 

From: yesbird
Subject: Re: Splines
Date: 13 Jul 2024 02:59:52
Message: <669225e8$1@news.povray.org>
On 12/07/2024 23:53, Bald Eagle wrote:
> Use the URL in the browser/web interface
> http://news.povray.org/

Thanks.
--
YB.


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