[x3d-public] Typo in spec ?

[Roy Walmsley]

Andreas,

You make a good point below. There is no reason why the SFVec4x fields could not be used as a general 4D vector. The text could be modified to read, for example:

"The SFVec4x field or event specifies a three-dimensional (3D) homogeneous vector or a four-dimensional (4D) vector. An MFVec4x field or event specifies zero or more 3D homogeneous or 4D vectors.", with the 'x' replaced by 'f' or 'd' as appropriate.

Roy

-----Original Message-----
From: Andreas Plesch [mailto:andreasplesch at gmail.com] 
Sent: 29 September 2015 13:09
To: Roy Walmsley; Richard F. Puk
Cc: X3D Graphics public mailing list
Subject: Re: [x3d-public] Typo in spec ?

Apologies, I had overlooked the significance of the word homogeneous as used in projective geometry. As long as the SFVec4f field is used only in this way, the spec. Is clear (clipplane uses it consistent with that definition). In this case, saying "homogenous coordinate" rather than "vector" could be more specific. But why restrict the definition ? Could it become a general 4d vector ? This way, the fourth coordinate could be used as time or for volumetric attributes.

Andreas

On September 29, 2015, at 5:24 AM, Roy Walmsley <roy.walmsley at ntlworld.com> wrote:

Hi Andreas and Dick,

I can see that it could be confusing referring to an SFVec4d as a 3D homogeneous vector. However, technically, this is the correct description, the term 'homogeneous' implying the extra dimension to the standard three dimensional vector. And the remainder of the paragraph points out that "
SFVec4d's and MFVec4d's are represented as a 4-tuple of double-precision floating point values ...", thus clarifying the definition. The final paragraph gives the default value with the 4-tuple.

The text quoted above could be expanded to incorporate the components, thus reading, " SFVec4d's and MFVec4d's are represented as a 4-tuple (components x, y, z, w) of double-precision floating point values ...". The danger in doing this is that it might imply a definition of the component names. And it is inconsistent with  other definitions. So I am inclined not to support such a change. 

Comments are welcomed. I just offered the idea for consensus building.

Regards,

Roy

-----Original Message-----
From: x3d-public [mailto:x3d-public-bounces at web3d.org] On Behalf Of Richard F. Puk
Sent: 29 September 2015 05:16
To: 'Andreas Plesch'; 'X3D Graphics public mailing list'
Subject: Re: [x3d-public] Typo in spec ?

Hi, Andreas --

A 3D homogeneous vector consists of four components: x, y, z, w.

  -- Dick

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-----Original Message-----
From: x3d-public [mailto:x3d-public-bounces at web3d.org] On Behalf Of Andreas Plesch
Sent: Monday, September 28, 2015 5:45 PM
To: X3D Graphics public mailing list
Subject: [x3d-public] Typo in spec ?

http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/fieldsDef.
html#SFVec4dAndMFVec4d
has
" The SFVec4d field or event specifies a three-dimensional (3D) homogeneous vector."
as a first sentence. 

Should this instead say " The SFVec4d field or event specifies a four-dimensional (4D) homogeneous vector." ?

Andreas
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