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From: mckie@sky.arc.nasa.gov (Bill McKie)
Newsgroups: comp.graphics
Subject: CGM HLHSR in SunPHIGS
Keywords: CGM, SunPHIGS, HLHSR
Message-ID: <1991Jun3.004506.10500@news.arc.nasa.gov>
Date: 3 Jun 91 00:45:06 GMT
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Organization: NASA Ames Atmospheric Science Group
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Originator: mckie@sky.arc.nasa.gov


Thanks to all who have contributed to the discussion of hidden line &
hidden surface removal in CGM output using SunPHIGS, which I mentioned
in article 14261 of this newsgroup.

As there has been some variation in what people thought I was talking
about in 14261, and because it still appears to me that something is
lacking in the SunPHIGS handling of CGM output, I'll try to make the
statement of the problem (& why someone might be concerned about it)
clearer here.

As at least one other person has confirmed, there is currently no way
to generate SunPHIGS CGM output files whose images have the hidden
lines and surfaces removed, and there is apparently no clear reason
why this cannot be done.

The desired thing is to get SunPHIGS CGM output that contains images
which look like those we get on the console screen, when we print or
otherwise look at the CGM images.  Without this capability, CGM is not
a useful output type when dealing with 3-D images in which it is
intended to have hidden lines and surfaces removed by SunPHIGS, mainly
because the images one sees when printing or otherwise looking at the
CGM files are not similar (with respect to HLHSR) to those one sees on
the console output.

In short, the expected and useful behavior of a SunPHIGS program is
to get a final display which looks very similar whether image output
is directed to the console, or to a CGM which is subsequently displayed
by some means.

Although CGM is a 2-D standard, a CGM should be logically very similar
to a console canvas, which is also 2-D and which displays images from
SunPHIGS with hidden lines and hidden surfaces removed, even when there
is no z-buffer hardware on the console.

E.g. with our Sun GX h/w (which has no z-buffer), I believe the
SunPHIGS HLHSR phase of the graphics pipeline for console output occurs
entirely within the SunPHIGS software.  It would seem that this same
kind of thing could be done in the software which handles CGM output,
because HLHSR is in some sense, a part of the final transformations to
the 2-D image.  [Ultimately, even 3-D images with hidden lines &
surfaces removed end up as sets of 2-D polyline and 2-D polygon
primitives, both of which CGMs handle.]

CGMs are a nice standard way to store & transmit images for future
viewing & printing, and using CGMs is apparently the only way to
generate high resolution hard copy from SunPHIGS.  To get hard copy,
we first run a SunPHIGS program which outputs images onto a CGM, optionally
viewing the images simultaneously on the console workstation.  We then
use the gplot program to convert the image(s) in the CGM to color or
B&W PostScript (and sometimes to a variety of other devices and
formats).  The PostScript is then printed.

Screen dumps are an alternative route to moving images to a printer,
but they don't carry the same high resolution as do the CGMs, and are
inadequate for many complex images.

One reason we purchased SunPHIGS (it's not cheap) is that it handles
for us the complicated pipeline of graphics transformations that
ultimately end up in a 2-D projection, including HLHSR.  It is a
disappointment that the HLHSR part of this pipeline is apparently
available for some SunPHIGS output and not available for others,
especially since the other features of SunPHIGS work so well. (We're
currently using version 1.3)

Can anyone from Sun comment on this problem and how soon we could
expect relief?  We've lived with the problem for over a year using
SunPHIGS 1.1.  When we upgraded to 1.3, I found that the problem had
not gone away.

Thanks,
-Bill McKie
NASA Ames Research Center
mckie@sky.arc.nasa.gov