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Defect Report Number: 8632-1/039

 

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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 1993

 

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Deadline on response from editor: : 1 May 1993

 

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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):

Annex C, p.272

 

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

 

We have spotted a small problem with the use of SDRs for data records of ESCAPE, GDP, etc.

 

Recall that one of the uses of SDR was so that the information could be decoded without the application having to know the exact GDP/ESCAPE (and therefore without having to know the particular sequence of parameters).

 

The problem is with enumeratives within SDR. In binary and character, these are encoded as integer: 0, 1, 2, 3, ... In clear text, they are words: off, on, ...

 

There is one circumstance where it doesn’t work well: "blind" intertranslation of encodings. A tool which translates, for example, binary to clear text must know the exact definition of an ESCAPE whose SDR contains an enumerative. ESCAPE 22 is one example. This is unfortunate, as it is contrary to the spirit of the SDR method.

 

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Solution proposed by the submitter (optional):

    1. We should not alter the standard or the SDR coding.
    2. We should (take note, registration), avoid the use of Enumeratives in registered SDRs.
    3. We should add an informative note to Amd.1 final text, strongly recommending against the use of E in SDRs.
    4. We should make a defect resolution to add a similar note to the CGM:1992 text.

 

 

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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):

 

Add to Annex C.2.2 as Note 3:

The binary and character encodings of the enumerated (E) data type in SDR use integers, (e.g. 0,1,2), whereas the clear text encoding of the enumerated data type traditionally uses words (e.g. ‘on’, ‘off’). This difference may prevent blind intertranslation between encodings. It is recommended that the use of enumerated (E) data type in SDR coding for ESCAPE, APPLICATION DATA and GDP elements be avoided. Type index (I) should be used instead. Although a recommended practice, there are some existing registered items which use the word-encoded method for E within SDR.