[x3d-public] Mantis 1392: 5.3.20 SFVec4d and MFVec4d defaultvalue(0 0 0 1) or (0 0 0 0) ?

[Joseph D Williams]

➢ 4D vectors in X3D don't hold quaternions. In X3D rotations are in SF/MFRotation given as axis-angle.


Yes, thank you. Then why isn’t the default same as for rotation default? 


From: Michalis Kamburelis
Sent: Monday, March 14, 2022 9:16 PM
To: Joseph D Williams
Cc: John Carlson; X3D Public Mailing List (x3d-public at web3d.org)
Subject: Re: [x3d-public] Mantis 1392: 5.3.20 SFVec4d and MFVec4d defaultvalue(0 0 0 1) or (0 0 0 0) ?

4D vectors in X3D don't hold quaternions. In X3D rotations are in SF/MFRotation given as axis-angle.

W dniu wt., 15.03.2022 o 00:05 Joseph D Williams <joedwil at earthlink.net> napisał(a):
• three-dimensional (3D) homogeneous vector 
 
For safety, default should be a legal quaternion (xyzw add to 1?) then 0 0 0 0 not good but generally 0 0 0 1 should be harmless as a default. I haven’t found all places this is used
Default rotation is 0 0 1 0 (ready to roll if it gets some angle).  Likewise, 0 0 0 1 is legal and ready to do whatever if its gets some xyz data. 
Thanks,
Joe
 
From: Michalis Kamburelis
Sent: Sunday, March 13, 2022 3:08 PM
To: John Carlson
Cc: X3D Public Mailing List (x3d-public at web3d.org)
Subject: Re: [x3d-public] Mantis 1392: 5.3.20 SFVec4d and MFVec4d defaultvalue (0 0 0 1) or (0 0 0 0) ?
 
The description of 4D vectors states their purpose: "a
three-dimensional (3D) homogeneous vector".
 
The 4D value "0 0 0 1" means "3D position at (0,0,0), when interpreted
as (3D) homogeneous vector".
 
The 4D value "0 0 0 0" would mean "invalid value, when interpreted as
(3D) homogeneous vector".
 
Let's keep the "0 0 0 1" as default. I don't see any reason to change,
"0 0 0 1" makes sense. (One could also argue that "0 0 0 0" makes
sense, but it is a flip a coin, they both can be argued as "better
defaults". So, no reason to break compatibility.)
 
Regards,
Michalis
 
 
niedz., 13 mar 2022 o 19:48 John Carlson <yottzumm at gmail.com> napisał(a):
> 
> My guess is it has to do with a perspective transformation.
> 
> John
> 
> On Sun, Mar 13, 2022 at 1:35 PM Brutzman, Donald (Don) (CIV) <brutzman at nps.edu> wrote:
>> 
>> Clause 5.3.20 SFVec4d and MFVec4d lists default value as (0 0 0 1)
>> 
>> 
>> 
>> Why not (0 0 0 0) similar to other SFVec/MFVec fields?
>> 
>> 
>> 
>> https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-CD1/Part01/fieldTypes.html#SFVec4dAndMFVec4d
>> 
>> 
>> 
>> Similarly for 5.3.21 SFVec4f and MFVec4f
>> 
>> 
>> 
>> https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-CD1/Part01/fieldTypes.html#SFVec4fAndMFVec4f
>> 
>> 
>> 
>> Entered as
>> 
>> Mantis 1392: 5.3.20 SFVec4d and MFVec4d default value (0 0 0 1) or (0 0 0 0) ?
>> https://www.web3d.org/member-only/mantis/view.php?id=1392
>> 
>> 
>> 
>> all the best, Don
>> 
>> --
>> 
>> Don Brutzman  Naval Postgraduate School, Code USW/Br        brutzman at nps.edu
>> 
>> Watkins 270,  MOVES Institute, Monterey CA 93943-5000 USA    +1.831.656.2149
>> 
>> X3D graphics, virtual worlds, Navy robotics https:// faculty.nps.edu/brutzman
>> 
>> 
>> 
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> 
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