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Defect Report Number: 8632-1/054

 

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Submitter: Henderson

 

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Addressed to: JTC1/SC 24/WG 6 Rapporteur Group on ISO/IEC 8632, CGM

 

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WG secretariat: NNI

 

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Date Circulated by WG secretariat: 1 February 1995

 

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Deadline on response from editor: : 1 May 1995

 

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Defect Report concerning IS 8632:1992 Computer Graphics: Metafile for the storage and transfer of picture description information (CGM) Part 1, Functional

specification.

 

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Qualifier (e.g. error, omission, clarification required): Clarification.

 

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References in document (e.g. page, clause, figure and/or table numbers):

 

 

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Nature of defect (complete, concise explanation of the perceived problem):

Question: What is one supposed to do with a degenerate ellipse? The computation won't work.

 

If one considers the parametric equations

x=a*cos(t) + b*sin(t) and

y=c*cos(t) + d*sin(t)),

where a, b, c, d are derived from the three points, the computation works fine even with coincident points — you get a locus consisting of a single point or a straight line through the colinear points, and this is the degenerate ellipse. The X/Y implicit equations of p.315, D.4.5.10, probably don’t work so well — I imagine there is the risk of zero denominators, etc.

 

If CGM:1992 had a recommendation (annex D) for degenerate ellipse which is similar to that for degenerate circular arc (D.4.5.4), the locus defined by the parametric equations corresponds to what one would probably define for an ellipse equivalent of D.4.5.4 (p.314).

 

If there is a defect report in all of this, it is probably that Ellipse needs to have a recommendation in Annex D that is similar to D.4.5.4, and the Model Profile needs to endorse the specification of degeneracy as it does for the circular elements.

It is an anomoly amongst V1 primitives, but note that there is no treatment in Annex D of degenerate primitives for V2 and V3.

 

When I looked up in Model Profile I discovered minor defects there --

    1. T.19.16, T.19.17, etc all refer to 7.5.4.3, 7.5.4.4 for the meaning of the degeneracy. They ought to refer to T.14.2 and T.14.3 (and maybe to T.24.9 and T.25.10 as well). This could be considered an editorial flaw.

2) The MP violates the requirements of 7.5.4.3 and 7.5.4.4. For the V2 Closed Figure primitive there is no specification of what to do with degeneracy.

 

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Solution proposed by the submitter (optional):

 

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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):

 

No change with regard to degeneracy of ellipses.

 

In Table 19 change all instances of "7.5.4.3" and "7.5.2.3" to "T.14.2". Three instances in each of T.19.1, T.19.2, T.19.13, T.19.15, T.19.18, T.19.20, T.19.21, T.19.22, T.19.23, T.19.24, T.19.25, T.19.26.

 

In Table 19 change all instances of "7.5.4.4" to "T.14.3" Three instances in each of T.19.7, T.19.8, T.19.11, T.19.12, T.19.14, T.19.16, T.19.17, T.19.19.