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

[Joseph D Williams]

➢ 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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