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From: rthomson@mesa.dsd.es.com (Rich Thomson)
Subject: Re: CGM HLHSR in SunPHIGS
Message-ID: <1991May28.194621.13640@dsd.es.com>
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Reply-To: rthomson@dsd.es.com (Rich Thomson)
Organization: Design Systems Division, Evans & Sutherland, SLC, UT
References: <1991May27.185153.29565@dsd.es.com> <1529@cvbnetPrime.COM>
Distribution: usa
Date: Tue, 28 May 91 19:46:21 GMT


In article <1991May27.185153.29565@dsd.es.com>,
    rthomson@mesa.dsd.es.com (Rich Thomson) wrote:
>> I don't use SunPHIGS, but from what I read in Appendix H of the PHIGS
>> standard, CGM is 2D and not 3D.  I would suspect that this makes it
>> impossible for the workstation to support HLHSR, which is a 3D
>> concept.

In article <1529@cvbnetPrime.COM>
	jdonsbac@reverie.Prime.COM (Jeff Donsbach) writes:
>Not at all. One of the final transformation in the graphics pipeline
>is from 3D space to 2D space (ie the screen). The CGM output workstation
>is just another 2D workstation. It should be possible, although using
>a Z buffering technique probably wouldn't be.

I went back and read the PHIGS standard, and I don't think this is
correct.  Workstations of type MO (metafile output) are not just
another workstation.  Section 4.6.1, page 68 of ISO/IEC 9592-1:1988(E)
says (my comments in []'s):

    The categories MO [metafile output] and MI [metafile input]
    are special facilities for filing graphical information for
    external storage and exchange.  They are treated as workstations
    for the purposes of control, but otherwise have quite different
    characteristics (see section 4.9).

The mentioned section 4.9 says:

    [...] For the capture of structure definitions, PHIGS provides an
    archive file format which enables the contents of the centralized
    structure store to be captured.  Pictures in PHIGS are created by
    traversal of the centralized structure store.  PHIGS provides the
    MO workstation category to capture pictures generated by the
    traversal process.  The precise way in which this is realized is
    implementation dependent.  Annex H of this part of ISO/IEC 9592
    describes a mechanism which enables pictures to be captured in a
    sensible manner as a CGM metafile.  In this case the MO
    workstation uses display update mode WAIT-NIVE and a picture
    description is recorded in the metafile when the UPDATE
    WORKSTATION or REDRAW ALL STRUCTURES functions are invoked by the
    application program.

    [...]

    It is implementation dependent whether some workstation control
    and inquiry functions are applicable or meaningful for MO and MI
    workstations. [...]

This tells me that although the MO workstation is not just another
workstation, it does follow the mechanism of traversal and records 2D
pictures.  This is why HLHSR has no meaning here -- after traversal,
all hidden lines and surfaces are already removed.

If you want to maintain HLHSR information in some archived format of
the structure, use archive files.

						-- Rich
-- 
  ``Read my MIPS -- no new VAXes!!'' -- George Bush after sniffing freon
	    Disclaimer: I speak for myself, except as noted.
UUCP: ...!uunet!dsd.es.com!rthomson		Rich Thomson
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