[x3d-public] Extrusions, take 3: initial proposed spec changes

Wed Feb 3 16:06:36 PST 2016

I'm *importing* X3Ds into Blender, not exporting them. Extrusion is a part of the X3D standard. Whether a scene was written by hand or generated by a tool, Blender shouldn't choke on it, or produce meshes that are inconsistent with other X3D implementations. Right now it does both, because the standard is incomplete.

________________________________
From: Leonard Daly [Leonard.Daly at realism.com]
Sent: Wednesday, February 03, 2016 5:38 PM
To: Alekseyev, Vsevolod (NIH/NIAID) [E]; x3d-public at web3d.org
Subject: Re: [x3d-public] Extrusions, take 3: initial proposed spec changes

Seva,

Clarifying the details for V3.x is good and important for historical purposes.

What code produces Extrusion nodes? I checked the Blender file out (for .blend at http://archive.blender.org/development/architecture/blender-file-format/ and http://www.atmind.nl/blender/blender-sdna.html) and did not find any reference to an extrusion node. I don't think the X3D exporter does either.

If you have a very limited set of extrusion generators, it would be helpful to know. There are edge cases in all simple extrusions and it may be possible to write the specification to match they way those applications generated it.

Leonard Daly

I am putting together an X3D importer for Blender, and, potentially, for another open source project. As a part of that effort, I have to convert X3D geometries, all 15 of them, into vertex/face arrays. As long as there are X3D 3.3 scenes out there, Extrusion support is not going away.

I’m not *using* extrusions in any meaningful sense. All I want is a standard that covers the entire parameter space. Is that too much to ask for?

From: Leonard Daly [mailto:Leonard.Daly at realism.com]
Sent: Wednesday, February 03, 2016 11:04 AM
To: x3d-public at web3d.org<mailto:x3d-public at web3d.org>
Subject: Re: [x3d-public] Extrusions, take 3: initial proposed spec changes

Seva,

What do you use Extrusion for?

I am thinking that the node may not be needed anymore. There are a couple of use cases that I haven't figured out yet and I wish to make sure your uses are understood and a functional solution exists in V4.

Leonard Daly

On 2/3/2016 5:36 AM, Alekseyev, Vsevolod (NIH/NIAID) [E] wrote:

"If only 2 non-coincident spine points are provided, the corresponding SCP planes for each are perpendicular to the vector defined by these two points."

That doesn't help clarify the issue at all. First, this language adds nothing to the "coincident spine -> rotate the XZ" rule, second, axes in the plane must be defined, not just the plane itself. And, as we all can see from the misplaced wings on http://www.jishop.com/temp/x3d/#AV8bHarrier , the choice of axes does matter.

________________________________________

From: Don Brutzman [brutzman at nps.edu<mailto:brutzman at nps.edu>]

Sent: Wednesday, January 27, 2016 12:44 PM

To: Alekseyev, Vsevolod (NIH/NIAID) [E]; x3d public mailing list

Subject: Re: [x3d-public] Extrusions, take 3: initial proposed spec changes

Thanks again Seva for the excellent test cases.

Dick, Leonard, Roy and I worked on today's weekly X3D Working Group teleconference to craft improved specification prose.  Details follow.

Next week we would like to continue with the prior wiki page examples, plus the long series of emails on this and related threads.

http://www.web3d.org/wiki/index.php/Extrusion_Edge_Cases

Together we created new Mantis issue 923 from the mail, with responses as follows.  Review and comment welcome.

Access note:  Web3D members have access to all issues.

Issue 0000923: 13.3.5 Extrusion - Edge cases

http://www.web3d.org/member-only/mantis/view.php?id=923

==========================================================================

==========================================================================

==========================================================================

Description:

Hi X3D community,

http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/geometry3D.html#Extrusion [^]

I've tried and failed twice to explain the problems with the definition of the Extrusion node. So here's a tl;dr version:

Dear authors of the X3D standard, please take a careful look at the standard and answer me:

1) When the spine goes (0,0,0)-(0,-1,0), what is the SCP?

Note: all answers but one will render one of the reference models in the Savage archive invalid.

2) When the spine goes (0,0,0)-(0,1,0)-(0,0,0), what is the SCP?

3) When the spine goes (0,0,0)-(1,1,0)-(1,1,0)-(1,1,0)-(2,0,0), what is the SCP for the three middle points?

==========================================================================

==========================================================================

==========================================================================

Case (1). Need to clearly define SCP when only 2 spine points are defined.

13.3.5.3 Algorithmic description

http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/geometry3D.html#Algorithmicdescription [^]

Suggested prose additions:

"If only 2 non-coincident spine points are provided, the corresponding SCP planes for each are perpendicular to the vector defined by these two points."

"If fewer than 2 non-coincident spine points are provided, the extrusion is not well defined and no results are rendered."

======================================

Case (2), first consideration. The prose for orientation is inserted in the middle of the SCP definition. This muddles the definition for SCP.

Suggested move for existing sentences:

     "The SCP is then rotated by the corresponding orientation value. This rotation is performed relative to the SCP. For example, to impart twist in the cross-section, a rotation about the Y-axis (0 1 0) would be used. Other orientations are valid and rotate the cross-section out of the SCP."

Place these as a new paragraph at the end of the section, after the complete definition of SCP. With slight modification:

     "Each SCP is then rotated by the corresponding orientation value. This rotation is performed relative to the SCP itself. For example, to impart twist in the cross-section, a rotation about the local Y-axis (0 1 0) would be used. Other orientation values are valid and may rotate the cross-section out of the plane of the original SCP."

======================================

Case (2), second consideration. Does the existing algorithm handle the example in question, in that three sequential spine points are defined where the first and third are coincident? This is a kind of reflection upon itself.

We would expect to have 3 SCPs, each parallel, where the first and third are coincident. We further need to be careful with plane orientation (direction of normal) for each of these SCPs.

When 2 vectors are coincident head-to-tail (and vice versa) the cross product does not appear to be well defined, since many perpendiculars are possible. A similiar situation may occur if they two vectors are parallel. These appear to be a special case.

These circumstances appear to be addressed in the following section.

13.3.5.4 Special cases

http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/geometry3D.html#Specialcases [^]

======================================

Case (3). Suggested specification prose revision.

     "If two points are coincident, they both have the same SCP."

is better expressed by

     "If two or more sequential points in a spine array are coincident, they are each treated as a single point when computing the corresponding SCP, and each will have an identical SCP.

NOTE. This case is useful when animating the spine array without needed to simultaneously modify the corresponding orientation and scale arrays."

======================================

Additional change A.

Figure 13.5 title is "Spine-aligned cross-section plane at a spine point."<http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/geometry3D.html#f-Spine-alignedcross-section[%5E]Needtoadd>

http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/geometry3D.html#f-Spine-alignedcross-section [^]<http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/geometry3D.html#f-Spine-alignedcross-section[%5E]Needtoadd>

 <http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/geometry3D.html#f-Spine-alignedcross-section[%5E]Needtoadd>

Need to add "<http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/geometry3D.html#f-Spine-alignedcross-section[%5E]Needtoadd>(SCP)" in the figure title above.

======================================

Additional change B.

13.3.5.4 Special cases

http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/geometry3D.html#Specialcases [^]

Change

     "If two points are coincident, they both have the same SCP."

to

     "If two sequential spine points are coincident, they both have the same SCP."

==========================================================================

==========================================================================

==========================================================================

all the best, Don

--

Don Brutzman  Naval Postgraduate School, Code USW/Br       brutzman at nps.edu<mailto:brutzman at nps.edu>

Watkins 270,  MOVES Institute, Monterey CA 93943-5000 USA   +1.831.656.2149

X3D graphics, virtual worlds, navy robotics http://faculty.nps.edu/brutzman

_______________________________________________

x3d-public mailing list

x3d-public at web3d.org<mailto:x3d-public at web3d.org>

http://web3d.org/mailman/listinfo/x3d-public_web3d.org

--
Leonard Daly
3D Systems & Cloud Consultant
X3D Co-Chair on Sabbatical
LA ACM SIGGRAPH Chair
President, Daly Realism - Creating the Future

--
Leonard Daly
3D Systems & Cloud Consultant
X3D Co-Chair on Sabbatical
LA ACM SIGGRAPH Chair
President, Daly Realism - Creating the Future
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://web3d.org/pipermail/x3d-public_web3d.org/attachments/20160204/7d99ee8a/attachment-0001.html>