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RE: CGZ files

From: Lofton Henderson <lofton@rockynet.com>
Date: Mon, 10 Nov 2008 13:34:36 -0700
Message-Id: <>
To: "WebCGM WG" <public-webcgm-wg@w3.org>

Hi All,

As I'm working on the summary of LC comments, I realize that we haven't 
completely put the CGZ question to rest yet -- there are loose ends.  (See 
my summary below attached.)  Benoit mentioned this also at the end of the 
WG telecon.

About 2.1:  I think we agree that it is a valid content type for 2.1, 
right?  I.e., gzip-compressed binary CGM content is a conforming Class of 
Product for 2.1.  And 2.1 interpreters must handle it.  So we can clarify 
the wording about 2.1 in the 2.1 spec if anyone thinks it is unclear.

About 1.0 & 2.0:  I sense that people want to be able to exchange 
gzip-compressed 1.0 and 2.0 as well.  Correct?

So what are the options:

1.) We could put a non-normative note in 2.1, acknowledging that 
gzip-compressed binary is not conforming 1.0/2.0 content strictly speaking, 
but that it is a useful and used technique, and recommending that 
interpreters should be prepared to handle it.  (And they might encounter it 
in the context of *any* profile of CGM.)

2.) We could process errata for 1.0 & 2.0, making it a formal 
requirement.  (This would be a *substantive* errata.)

3.) Other.  E.g. something along the lines of the "storage/transport 
variant outside of CGM".  My question about this option would be:  how 
exactly would such a perspective be reflected in the 2.1 text?  (Is this 
option essentially the same as #1?)


At 08:59 AM 10/21/2008 -0600, Lofton Henderson wrote:

>At 12:50 PM 10/20/2008 -0400, Weidenbrueck, Dieter wrote:
>>I agree with your analysis.
>>One additional question:
>>Would it make sense to establish cgz as an additional encoding (or
>>something similar) to make it available to other profiles?
>It is an interesting idea, and I hear that people would like to be able to 
>do it.
>"..establish as an additional encoding.." -- That approach would most 
>sensibly involve working a corrigendum to CGM:1999 through ISO.  This 
>would not be hard, process-wise.  But on the other hand, CGZ would be a 
>somewhat different type of encoding than Binary Encoding or Clear Text 
>Encoding.  (It is binary-encoded, then compressed as a whole -- not really 
>the same sort of thing as the other encodings, IMHO.)
>>I'm not
>>thinking about WebCGM 1.0 per se, but other profiles building on ISO
>>8632 might want to use zipping as well.
>Yes, there does seem to be a use case, as I gather from you and from Don.
>>I guess in general I find it a shame to restrict usage of zipping to
>>WebCGM 2.1 only, and I would like to find a way to be able to use
>>compression with any variant of CGM. Probably this could be declared as
>>a kind of a storage/transportation variant outside of CGM?
>We should be clear about the current situation.  It does not prohibit the 
>use of CGZ compression for WebCGM 1.0 instances.  But it isn't supported 
>it as a conformance scenario for 1.0 either, i.e., gzip-compressed CGM is 
>not a conformance "Class of Product", and handling gzip-compressed CGM is 
>not a requirement for the viewer "Class of Product".  (And 1.0 would in 
>fact prohibit it, if it were claimed that the compressed instance is valid 
>WebCGM 1.0).
>What I mean is this... If "BE21" (BE10) is a valid WebCGM 2.1 (1.0) binary 
>metafile, CGZ21 (CGZ10) is a gzip-encoded valid BE21 (BE10) metafile, V21 
>(V10) is a conforming 2.1 (1.0) viewer, and Z is a standalone 
>gzip-decompressor, then...
>Valid WebCGM 2.1 scenario:  ...--> CGZ21 --> [V21]
>Valid WebCGM 1.0 scenario:  ...-->CGZ10 ---> [Z] --> BE10 --> [V10]
>That is, nothing prevents gzip compression of WebCGM 1.0 metafiles.  But 
>it (CGZ10) has no standing as a valid content type for a V10 viewer 
>according to the WebCGM 2.1 spec's conformance section.  (Note that the 
>V21 viewer implicitly or logically integrates the process "Z", because of 
>the conformance specifications of WebCGM 2.1).
>This is kind of like what Dieter says -- that it is handled as part of the 
>transport process, outside of the specifications of the WebCGM 1.0 
>So ... where do we go from here?  Should we put something (informative) 
>into 2.1, pointing out that the conforming 2.1 scenario can essentially be 
>achieved also with 1.0 metafiles, except the compressed content CGZ10 is 
>not a conforming 1.0 "class of product", and therefore must be 
>decompressed into a valid BE10 metafile before it is a conforming input to 
>a V10 viewer process?
>Should we process errata for WebCGM 1.0 and WebCGM 2.0?  (Difficult, 
>because can be argued as a substantive/technical change, and OASIS process 
>prohibits such in Errata.)
>>-----Original Message-----
>>From: public-webcgm-wg-request@w3.org
>>[mailto:public-webcgm-wg-request@w3.org] On Behalf Of Lofton Henderson
>>Sent: Montag, 20. Oktober 2008 12:06
>>To: Bezaire, Benoit; WebCGM WG
>>Subject: RE: CGZ files
>>Oops, my message crossed with yours.
>>In short:  I agree that this is a conformance requirement for WebCGM
>>2.1, specifically that 2.1 viewers must handle gzip-compressed 2.1
>>instances, and that valid 2.1 instances included plain Binary Encoding
>>as well as gzip-compressed instances of binary-encoded 2.1 metafiles.
>>Long analysis:  see my other just-sent message.
>>At 11:23 AM 10/20/2008 -0400, Bezaire, Benoit wrote:
>> >I see your point, however...
>> >
>> >We have customers using WebCGM 1.0 "compliant" tools (IsoDraw/IsoView
>> >v6 for example). Now, these customers could get a WebCGM 1.0 .cgz and
>> >those "compliant" applications would reject them. That's not very
>> >user-friendly.
>> >
>> >Maybe it's better to do this as a WebCGM 2.1 feature.
>> >
>> >Benoit.
>> >
>> >-----Original Message-----
>> >From: Don L. [mailto:dlarson@cgmlarson.com]
>> >Sent: Friday, October 17, 2008 6:59 PM
>> >To: Bezaire, Benoit
>> >Cc: WebCGM WG
>> >Subject: re: CGZ files
>> >
>> >Benoit
>> >
>> > >  Hi  All,
>> > >
>> > >  I find the draft  underspecified about compressed CGM files. More
>> > > specifically, we would like to  know what kind of CGM files may be
>> > > compressed?
>> > >
>> > >  Version1 to  4?
>> > >  Can I compress a  WebCGM 1.0 CGM file for example?
>> > >
>> > >  Is this a WebCGM 2.1  conformance feature for viewer and authoring
>> >tools?
>> > >  Or is this a new WebCGM  2.1 (and only 2.1) 'encoding scheme' ...
>> > > for
>> >
>> > > lack  of a better  word?
>> >
>> >I think 'encoding scheme' is a better characterization. The text for
>> >this feature in the webcgm 2.1 spec was extracted from the SVG spec.
>> >
>> >My thinking is that this is a viewer conformance issue and a WebCGM 2.1
>> >viewer should be able to open a file with a .cgz extension and know
>> >that it needa to decode this file according to the gzip spec. with the
>> >assumption that results will be a file that conforms to the WebCGM
>> >profile (any version e.g.
>> >1.0 , 2.x).
>> >
>> >Don.
>> >
>> > >  Thanks.
>> > >  Benoit.
Received on Monday, 10 November 2008 20:35:26 UTC

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