RE: [cgmo-webcgm] problems with NUBS

[Lofton Henderson <lofton@rockynet.com>]

We're close on nailing the defect resolution(s).  One final question for 
Richard.

At 09:55 AM 9/1/2005 -0700, Fuhr, Richard D wrote:
>Here are my thoughts on the issues below.
>
>*.  While NUBS and NURBS are certainly not required for WebCGM 2.0, they 
>offer the following benefits. [...cut...]

I'm limiting this message to Defect correction.  We can handle keep-remove 
decision separately.

>[...]
>1.  The ATA and WebCGM profiles should require clamped splines, since the 
>Model Profile does.

Everyone seems to agree here.

Although there is a transcription error in the MP column of the PPF, the 
ATA profile (which allows nurbs) normatively specifies that the MP values 
are per CGM:1999 Annex I (and the MP column of the ATA profile is 
informative).  Same with WebCGM 1.0 (which didn't allow NURBS), and WebCGM 
2.0 2nd CD draft (which presently does allow them).

Plus, everyone *wants* clamped.


>2.  We did indeed submit a defect report but it somehow did not get 
>incorporated into the spec.

Right.  I have attached a copy.  Dick Puk (SC24/WG6) says it will take 1-3 
months to expedite this through SC24.

>3.  In section 6.6.10.1.12 of ISO/IEC 8632-1 Second Edition 1999-12-15, 
>the recursive definition of B-splines uses the half-open interval T[i] <= 
>t < T[i+k] and I believe this is correct.

Okay, so considering only Rob's original comment about the divergence 
between this current CGM:1999 text and the old defect report (1-065, 
against the previous CGM:1992 text) ... strictly speaking, nothing needs 
changing in CGM:1999 text, for accuracy and correctness.

Is that right?

>     Regarding the interval over which the basis functions evaluate to 
> non-zero, it would be better to describe the property a bit differently.

To clarify, is the proposed better description (see below):

1.) informative and non-technical?  [Editorial clarification]

2.) normative and technical?  [Technical defect fix.]

I.e., if someone had implemented B-splines per "CGM:1999 plus 
def1999_1-001", would the proposed "better description" require changes to 
implementation (viewer, authoring tool, etc)?


>     Let J be the CLOSED interval = {t : T[i] <= t <= T[i+k]}.  Then the 
> B-spline basis function B[i,k] is zero OUTSIDE this interval, it is 
> positive in the OPEN interval K = {t : T[i] < t < T[i+k]} and it may be 
> either 0 or positive at the end points of the closed interval J (this 
> depends upon the particular basis function and the particular knot sequence).

Can you phrase this as exact wording for a defect correction?  I.e., 
looking at CGM:1999, 6.6.10.1.2, it looks to me like you're addressing the 
4 equations that recursively define the basis functions B[i,k] (bottom of 
p.49 in my ISO edition of CGM:1999).

What, exactly, would change in those current basis function definitions?

>     Informally speaking, for those B-spline basis functions whose graphs 
> look like bell-shaped curves, the values of the basis functions are zero 
> at each end of the closed interval J.  However, the first and last basis 
> functions defined over a clamped knot sequence (i.e., one having knot 
> multiplicity equal to degree+1 at the start and end of the sequence) 
> attain values of 1.0 at, respectively, the first and last point of the 
> interval J.


Regards,
-Lofton.

Title: Reflectance

1	Defect Report Number: 8632:1999-1/001
2	Submitter: Henderson
3	Addressed to: JTC1/SC 24/WG 6 Rapporteur Group on ISO/IEC 8632, CGM
4	WG secretariat: ???
5	Date Circulated by WG secretariat: DD MMM 2001
6	Deadline on response from editor: :DD MMM 2001
7	Defect Report concerning IS 8632:1999 Computer Graphics: Metafile for the storage and transfer of picture description information (CGM) Part 1, Functional specification.
8	Qualifier (e.g. error, omission, clarification required): Correction
9	References in document (e.g. page, clause, figure and/or table numbers): clause 7.6.24, page 177; clause 7.6.25, page 178.
10	Nature of defect (complete, concise explanation of the perceived problem): There is a mistake in each of the Description sections, for NUBS and NURBS. In the Description section of clause 7.6.24, the last sentence of the 2nd paragraph says: "The end value shall be less than the n-th knot value (where n is the number of control points). In the Description section of clause 7.6.25, the last sentence of the 3rd paragraph says: "The end value shall be less than or equal to the n-th knot value (where n is the number of control points). The two statements are inconsistent, and both are incorrect. The correct statement in both cases is: "The end value shall be less than or equal to the (n+1)st knot value (where n is the number of control points)." Note: this latter sentence is as agreed by a technical committee examining problems with the NUBS & NURBS specifications prior to the 1999 republication of ISO/IEC 8632. However the agreed correction was not accurately reflected in the defect report, 8632:1992-1/065, nor in the final text of ISO/IEC 8632:1999.
11	Solution proposed by the submitter (optional): Replace the last sentence of the 2nd paragraph of the Description section of 7.6.24, and the last sentence the 3rd paragraph of the Description section of 7.6.25, with the sentence: "The end value shall be less than or equal to the (n+1)st knot value (where n is the number of control points)."
12	Editor's response (any material proposed for processing as a technical corrigendum to, an amendment to, or a commentary on the International Standard or DIS final text is attached separately to this completed report): Amend the standard as proposed above.