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Subject: RE: [cgmo-webcgm] problems with NUBS
Proposal: I will send Defect Report Number 8632:1999-1/001 to Dick Puk and ask him to expedite it through SC24. This should close the topic of NUBS/NURBS defect processing. According to Dick, best case will be 1 month for SC24 completion, worst case will be 3 months. I suggest that tomorrow (Wed.) telecon is the drop-dead for objections to this proposal to finish the defect correction. Regards, -Lofton. At 01:14 PM 9/6/2005 -0600, Fuhr, Richard D wrote: >[...Forwarding for Richard, who can't write to the TC list yet...] > >To answer your questions below. > >1. The proposed "better description" that I sent out last week (copied >below) was intended to merely be informatative and non-technical for the >purpose of communicating to the people on this mailing list, and would >not need to be incorporated into future versions of the CGM >specification itself. If someone had implemented B-splines per "CGM:1999 >plus def1999_1-001", the proposed "better description" would not require >changes to implementation (viewer, authoring tool, etc)? > >2. The Defect Report Number 8632:1999-1/001 (a.k.a def1999_1-001) does >need to be incorporated in future versions of the CGM specification. > >Regards, > >Richard Fuhr > >-----Original Message----- >From: Lofton Henderson [mailto:lofton@rockynet.com] >Sent: Sunday, September 04, 2005 11:33 AM >To: Fuhr, Richard D; cgmo-webcgm@lists.oasis-open.org >Subject: RE: [cgmo-webcgm] problems with NUBS > >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. > >
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