From: grieggs@jpl-devvax.jpl.nasa.gov (John T. Grieggs)
Subject: (31aug92) comp.graphics Frequently Asked Questions (FAQ)
Followup-To: poster
Reply-To: grieggs <grieggs@jpl-devvax.jpl.nasa.gov>
Organization: Jet Propulsion Laboratory, Pasadena, CA
Date: Mon, 31 Aug 1992 20:31:16 GMT
Expires: Mon, 14 Sep 1992 20:31:17 GMT

Archive-name: graphics/faq

This message is automatically posted once a week in an effort to cut
down on the repetitive junk in comp.graphics.  It was last changed on
31aug92.  If you have answers to other frequently asked questions that
you would like included in this posting, please send me mail.  If you
don't want to see this posting every week, please add the subject line
to your kill file.  Thank you.
---
_john

	John Grieggs grieggs@jpl-devvax.jpl.nasa.gov JohnG@portal.com
---
Last update: 08/31/92

The FAQ is doing well.  Here's what's new:

POV mailing list (erich@eye.com).

New IP address for gondwana.ecr.mu.oz.au (bernie@ecr.mu.oz.au).

Errata for "Digital Image Warping" (erich@eye.com).

IP address for weedeater.math.yale.edu (erich@eye.com).

IP addresses for marsh.cs.curtin.edu.au, ftp.cosy.sbg.ac.at (me).

General re-formatting and tweaking (me).

		grieggs@jpl-devvax.jpl.nasa.gov



Contents:

    1) General references for graphics questions.
    2) Drawing three-dimensional objects on a two-dimensional screen.
    3) Quantizing 24 bit images down to 8 bits.
    4) Converting color into grayscale.
    5) Quantizing grayscale to black&white.
    6) Rotating a raster image by an arbitrary angle.
    7) Free image manipulation software.
    8) Format documents for TIFF, IFF, BIFF, NFF, OFF, FITS, etc.
    9) Converting between vector formats.
    10) How to get Pixar films.
    11) How do I draw a circle as a Bezier (or B-spline) curve?
    12) How to order standards documents.
    13) How to FTP by email.
    14) How to tell whether a point is within a planar polygon.
    15) How to tessellate a sphere.
    16) Specific references on ray-tracing and global illumination.
    17) SIGGRAPH information online
    18) SIGGRAPH Panels Proceedings available
    19) Graphics mailing lists
    20) Specific references on file formats
    21) What about GIF?
    22) What is morphing?
    23) How to ray-trace height fields
    24) How to find the area of a 3D polygon


1) General references for graphics questions:

    Computer Graphics: Principles and Practice (2nd Ed.), J.D. Foley,
	A. van Dam, S.K. Feiner, J.F. Hughes, Addison-Wesley 1990, ISBN
	0-201-12110-7
    Procedural Elements for Computer Graphics, David F. Rogers, McGraw
	Hill 1985, ISBN 0-07-053534-5
    Mathematical Elements for Computer Graphics 2nd Ed., David F. Rogers
	and J. Alan Adams, McGraw Hill 1990, ISBN 0-07-053530-2
    Three Dimensional Computer Graphics, Alan Watt, Addison-Wesley 1990, ISBN
	0-201-15442-0
    An Introduction to Ray Tracing, Andrew Glassner (ed.), Academic Press
	1989, ISBN 0-12-286160-4
    Graphics Gems, Andrew Glassner (ed.), Academic Press 1990, ISBN
	0-12-286165-5
    Graphics Gems II, James Arvo (ed.), Academic Press 1991, ISBN
	0-12-64480-0
    Graphics Gems III, David Kirk (ed.), Academic Press 1992, ISBN
	0-12-409670-0 (with IBM disk) or 0-12-409671-9 (with Mac disk)
    Digital Image Warping, George Wolberg, IEEE Computer Society Press
	Monograph 1990, ISBN 0-8186-8944-7
    Digital Image Processing (2nd Ed.), Rafael C. Gonzalez, Paul Wintz,
	Addison-Wesley 1987, ISBN 0-201-11026-1
    A Programmer's Geometry, Adrian Bowyer, John Woodwark, Butterworths 1983,
	ISBN 0-408-01242-0 Pbk

An automatic mail handler at Brown University allows users of "Computer
Graphics: Principles and Practice," by Foley, van Dam, Feiner, and
Hughes, to obtain text errata and information on distribution of the
software packages described in the book.  Also, users can send the
authors feedback, to report text errors and software bugs, make
suggestions, and submit exercises.  To receive information describing
how you can use the mail handler, simply mail graphtext@cs.brown.edu
and put the word "Help" in the Subject line.  Use the Subject line
"Software-Distribution" to receive information specifically concerning
the software packages SRGP and SPHIGS.

Errata for "An Introduction to Ray Tracing" is available on
wuarchive.wustl.edu in graphics/graphics/books/IntroToRT.errata.

Errata for "Digital Image Warping" is in the same directory as
"Digital-Image-Warping.errata".

All C code from the "Graphics Gems" series is available via anonymous ftp
from princeton.edu.  Look in the directory pub/Graphics/GraphicsGems for
the various volumes (Gems, GemsII, GemsIII), and get the README file first.


2) Drawing three-dimensional objects on a two-dimensional screen.

The simple answer is, you divide by the depth.  For a more verbose
explanation, see any of the above references, starting with:

The Foley & Van Dam & Feiner & Hughes "Computer Graphics" book is certainly
a good start.  Chapter 6 is "Viewing in 3D", then read chapter 15,
"Visible-Surface Determination".  For more information go to chapter 16 for
shading, chapter 19 for clipping, and branch out from there.


3) Quantizing 24 bit images down to 8 bits.

Find a copy of "Color Image Quantization for Frame Buffer Display" by
Paul Heckbert, SIGGRAPH '82 Proceedings, page 297.  There are other
algorithms, but this one works well and is fairly simple.  Implementations
are included in most raster toolkits (see item 7 below).

A variant method is described in "Graphics Gems", p. 287-293.  Note that
the code from the "Graphics Gems" series is all available from an FTP site,
as described above.

Check out John Bradley's "Diversity Algorithm", which is incorporated into
the xv package and described in the back of the manual.


4) Converting color into grayscale.

The NTSC formula is:

    luminosity = .299 red + .587 green + .114 blue


5) Quantizing grayscale to black&white.

The only reference you need for this stuff is:

    Digital Halftoning, Robert Ulichney, MIT Press 1987, ISBN 0-262-21009-6

But before you go off and start coding, check out the image manipulation
software mentioned in item 7 below.  All of the packages mentioned can do
some form of gray to b&w conversion.


6) Rotating a raster image by an arbitrary angle.

The obvious but wrong method is to loop over the pixels in the source
image, transform each coordinate, and copy the pixel to the destination.
This is wrong because it leaves holes in the destination.  Instead,
loop over the pixels in the destination image, apply the *reverse*
transformation to the coordinates, and copy that pixel from the source.
This method is quite general, and can be used for any one-to-one
2-D mapping, not just rotation.  You can add anti-aliasing by doing
sub-pixel sampling.

However, there is a much faster method, with antialising included,
which involves doing three shear operations.  The method was originally
created for the IM Raster Toolkit (see below); an implementation is
also present in PBMPLUS.  Reference: "A Fast Algorithm for Raster
Rotation", by Alan Paeth (awpaeth@watcgl.waterloo.edu) Graphics
Interface '86 (Vancouver).  An article on the IM toolkit appears in
the same journal.  An updated version of the rotation paper appears
in "Graphics Gems" (see section [1]) under the original title.


7) Free image manipulation software.

There are a number of toolkits for converting from one image format to
another, doing simple image manipulations such as size scaling, plus
the above-mentioned 24 -> 8, color -> gray, gray -> b&w conversions.
Here are pointers to some of them:

    PBMPLUS, by Jef Poskanzer.  Comprehensive format conversion and image
    manipulation package.  The latest version is always available via
    anonymous FTP as ftp.ee.lbl.gov:pbmplus*.tar.Z,
    wuarchive.wustl.edu:graphics/graphics/packages/pbmplus/pbmplus*.tar.Z,
    and export.lcs.mit.edu:contrib/pbmplus*.tar.Z.

    IM Raster Toolkit, by Alan Paeth (awpaeth@watcgl.uwaterloo.ca).
    Provides a portable and efficient format and related toolkit.  The
    format is versatile in supporting pixels of arbitrary channels,
    components, and bit precisions while allowing compression and machine
    byte-order independence.  The kit contains more than 50 tools with
    extensive support of image manipulation, digital halftoning and format
    conversion.  Previously distributed on tape c/o the University of
    Waterloo, an FTP version will appear someday.

    Utah RLE Toolkit.  Conversion and manipulation package, similar to
    PBMPLUS.  Available via FTP as cs.utah.edu:pub/urt-*,
    princeton.edu:pub/Graphics/urt-*, and freebie.engin.umich.edu:pub/urt-*.

    Fuzzy Pixmap Manipulation, by Michael Mauldin <mlm@nl.cs.cmu.edu>.
    Conversion and manipulation package, similar to PBMPLUS.  Version 1.0
    available via FTP as nl.cs.cmu.edu:/usr/mlm/ftp/fbm.tar.Z,
    ftp.uu.net:pub/fbm.tar.Z, and ucsd.edu:graphics/fbm.tar.Z.

    Img Software Set, by Paul Raveling <raveling@venera.isi.edu>.  Reads and
    writes its own image format, displays on an X11 screen, and does some
    image manipulations.  Version 1.3 is available via FTP as
    export.lcs.mit.edu:contrib/img_1.3.tar.Z, and
    venera.isi.edu:pub/img_1.3.tar.Z along with a large collection of color
    images.

    Xim, by Philip R. Thompson.  Reads and writes its own image format,
    displays on an X11 screen, and does some image manipulations.
    Available in your nearest X11R4 source tree as contrib/clients/xim.
    A more recent version is available via ftp from video.mit.edu.  It uses
    x11r4 and the OSF/Motif toolkit to provide basic interactive image
    manipulation and reads/writes GIF, xwd, xbm, tiff, rle, xim, and other
    formats.

    xloadimage, by Jim Frost <madd@std.com>.  Reads in images in various
    formats and displays them on an X11 screen.  Available via FTP as
    export.lcs.mit.edu:contrib/xloadimage*, and in your nearest comp.sources.x
    archive.

    TIFF Software, by Sam Leffler <sam@okeeffe.berkeley.edu>.  Nice
    portable library for reading and writing TIFF files, plus a few tools
    for manipulating them and reading other formats.  Available via FTP as
    ucbvax.berkeley.edu:pub/tiff/*.tar.Z or ftp.uu.net:graphics/tiff.tar.Z

    xtiff, an X11 tool for viewing a TIFF file.  It was written to handle
    as many different kinds of TIFF files as possible while remaining
    simple, portable and efficient.  xtiff illustrates some common problems
    with building pixmaps and using different visual classes.  It is
    distributed as part of Sam Leffler's libtiff package and it is also
    available on export.lcs.mit.edu, ftp.uu.net and comp.sources.x.
    xtiff 2.0 was announced in 4/91; it includes Xlib and Xt versions.

    ALV, a Sun-specific image toolkit.  Version 2.0.6 posted to
    comp.sources.sun on 11dec89.  Also available via email to
    alv-users-request@cs.bris.ac.uk.

    popi, an image manipulation language.  Version 2.1 posted to
    comp.sources.misc on 12dec89.

    ImageMagick, an X11 package for display and interactive manipulation
    of images.  Uses its own format (MIFF), and includes some converters.
    Available via FTP as export.lcs.mit.edu:contrib/ImageMagick.tar.Z

    Khoros, a huge (~100 meg) graphical development environment based on
    X11R4.  Khoros components include a visual programming language, code
    generators for extending the visual language and adding new application
    packages to the system, an interactive user interface editor, an
    interactive image display package, an extensive library of image and
    signal processing routines, and 2D/3D plotting packages.  Available via
    FTP as pprg.eece.unm.edu:pub/khoros/*.

    LaboImage, a SunView-based image processing and analysis package.  It
    includes more than 200 image manipulation, processing and measurement
    routines, on-line help, plus tools such as an image editor, a color
    table editor and several biomedical utilities.  Available via anonymous
    FTP as ads.com:pub/VISION-LIST-ARCHIVE/SHAREWARE/LaboImage_3.1.tar.Z

    The San Diego Supercomputer Center Image Tools, software tools for
    reading, writing, and manipulating raster images.  Binaries for some
    machines available via anonymous FTP in sdsc.edu:sdscpub.

    The Independent JPEG Group has written a package for reading and
    writing JPEG files.  FTP to ftp.uu.net:graphics/jpeg/jpegsrc.v?.tar.Z

Don't forget to set binary mode when you FTP tar files.  For you MILNET
folks who still don't have name servers, the IP addresses are:

    ads.com			128.229.30.16
    cs.utah.edu			128.110.4.21
    coral.cs.jcu.edu.au		137.219.17.4
    export.lcs.mit.edu		18.24.0.12
    freebie.engin.umich.edu	141.212.103.21
    ftp.ee.lbl.gov		128.3.112.20
    ftp.uu.net			137.39.1.9 or 192.48.96.9
    gondwana.ecr.mu.oz.au	128.250.70.62
    karazm.math.uh.edu		129.7.7.6
    marsh.cs.curtin.edu.au	134.7.1.1
    nic.funet.fir		128.214.6.100
    ftp.cosy.sbg.ac.at		141.201.2.15
    ftp.ncsa.uiuc.edu		141.142.20.50
    nl.cs.cmu.edu		128.2.222.56
    pprg.eece.unm.edu		129.24.24.10
    princeton.edu		128.112.128.1
    sdsc.edu			132.249.20.22
    ucbvax.berkeley.edu		128.32.133.1
    venera.isi.edu		128.9.0.32
    weedeater.math.yale.edu	128.36.23.17
    wuarchive.wustl.edu		128.252.135.4
    zamenhof.cs.rice.edu	128.42.1.75

Please do *not* post or mail messages saying "I can't FTP, could someone
mail this to me?"  There are a number of automated mail servers that will
send you things like this in response to a message.  See item 13 below for
details on some.

Also, the newsgroup alt.graphics.pixutils is specifically for discussion
of software like this.  You may find useful information there.


8) Format documents for TIFF, IFF, BIFF, NFF, OFF, FITS, etc.

You almost certainly don't need these.  Read the above item 7 on free
image manipulation software.  Get one or more of these packages and
look through them.  Chances are excellent that the image converter you
were going to write is already there.  But if you still want one of the
format documents, many such files are available by anonymous ftp from
zamenhof.cs.rice.edu in directory pub/graphics.formats.  It is also
mirrored at ftp.cosy.sbg.ac.at in dir /pub/mirror/file-formats/graphic.

These files were collected off the net and are believed to be correct.
This archive includes pixel formats, and two- and three-dimensional object
formats.  The future of this archive is uncertain at the moment, as Mark
Hall <foo@cs.rice.edu> will apparently no longer be maintaining it.

A second graphics file format archive is now being actively maintained
by Quincey Koziol (koziol@ncsa.uiuc.edu).  The latest version exists at
ftp.ncsa.uiuc.edu in /misc/file.formats/graphics.formats.  Apparently,
neither of these is complete, you might want to check both.

FITS stands for Flexible Image Transport System, it's an image file
format most often used in astronomy.  Refer to the sci.space FAQ for
more information if needed.


9) Converting between vector formats.

A lot of people ask about converting from HPGL to PostScript, or MacDraw
to CGM, or whatever.  It is important to understand that this is a very
different problem from the image format conversions in item 7.  Converting
one image format to another is a fairly easy problem, since once you
get past all the file header junk, a pixel is a pixel -- the basic objects
are the same for all image formats.  This is not so for vector formats.
The basic objects -- circles, ellipses, drop-shadowed pattern-filled
round-cornered rectangles, etc. -- vary from one format to another.
Except in extremely restricted cases, it is simply not possible to do
a one-to-one conversion between vector formats.

On the other hand, it is quite possible to do a close approximation,
rendering an image from one format using the primitives from another.
As far as I know, no one has put together a general toolkit of such
converters, but two different HPGL to PostScript converters have been
posted to comp.sources.misc.  Check the index on your nearest archive
site.

A related frequent question is how to convert from some vector format
to a bitmapped image - from PostScript to Sun raster format, or HPGL to
X11 bitmap.  For example, some of the commercial PostScript clones for
PC's allow you to render to a disk file as well as a printer.  Also,
the PostScript interpreters in the NeXT box and in Sun's X11/NeWs can
be used to render to a file if you're clever.  But in general, the
answer is no.  However, if someone were to put together a vector to
vector conversion toolkit, adding a vector to raster converter would be
trivial.


10) How to get Pixar films.

The various John Lasseter / Pixar computer animated shorts are available
on video tape.  You can order them from Direct Cinema Limited:

    Film                           Individual Price      Institutional Price
    Luxo, Jr.				$14.95			$50.00
    Red's Dream				$19.95			$75.00
    Tin Toy				$24.95			$75.00
    Knickknack				$24.95			$75.00
    Luxo, Jr./Red's Dream/Tin Toy	$39.95			$100.00

All tapes are on 1/2" VHS NTSC.  Add $10/tape for PAL format.  Also
available:

    Tin Toy T-shirt			$15.00
    Knickknack 3D T-shirt		$15.00 (includes glasses)

For individual orders, add $5 S&H for the first tape or shirt, $2 for
each additional tape or shirt.  For institutional orders, add $5 S&H
for the first tape, $3 for each additional tape.  Foreign shipping, add
$3/tape or shirt.  Call 800-525-0000 (213-396-4774 international,
213-396-3233 FAX) to charge to your credit card.  Call first to verify
prices and availability.  Or, just write to:

    Direct Cinema Limited
    1749 14th Street
    Santa Monica, CA 90404-4342

Allan Braunsdorf has this to say:

At SIGGRAPH they were selling a tape with all four shorts
for $25.  That was a sale price.  You can get it for slightly
more than that normally.  ($35 maybe.)  I believe it's
available from RenderMan Retail (at Pixar's address).

Does anyone know this address and the real price?


11) How do I draw a circle as a Bezier (or B-spline) curve?

The short answer is, "You can't."  Unless you use a rational spline you
can only approximate a circle.  The approximation may look acceptable,
but it is sensitive to scale.  Magnify the scale and the error of
approximation magnifies.  Deviations from circularity that were not
visible in the small can become glaring in the large.  If you want to
do the job right, consult the article:

  "A Menagerie of Rational B-Spline Circles"
  by Leslie Piegl and Wayne Tiller
  in IEEE Computer Graphics and Applications, volume 9, number 9,
  September, 1989, pages 48-56.

For rough, non-rational approximations, consult the book:

  Computational Geometry for Design and Manufacture
  by I. D. Faux and M. J. Pratt,
  Ellis Horwood Publishers, Halsted Press, John Wiley 1980.

For the best known non-rational approximations, consult the article:

  "Good Approximation of Circles by Curvature-continuous Bezier Curves"
  by Tor Dokken, Morten Daehlen, Tom Lyche, and Knut Morken
  in Computer Aided Geometric Design, volume 7, numbers 1-4 (combined),
  June, 1990, pages 33-41 [Elsevier Science Publishers (North-Holland)]


12) How to order standards documents.

The American National Standards Institute sells ANSI standards, and also
ISO (international) standards.  Their sales office is at 1-212-642-4900,
mailing address is 1430 Broadway, NY NY 10018.  It helps if you have the
complete name and number.

Some useful numbers to know:

CGM (Computer Graphics Metafile) is ISO 8632-4 (1987).  GKS (Graphical
Kernel System) is ANSI X3.124-1985.  PHIGS (Programmer's Hierarchical
Interactive Graphics System) is ANSI X3.144-1988.  IGES is ASME/ANSI
Y14.26M-1987.  Language bindings are often separate but related numbers;
for example, the GKS FORTRAN binding is X3.124.1-1985.

Standards-in-progress are made available at key milestones to solicit
comments from the graphical public (this includes you!).  ANSI can let
you know where to order them; most are available from Global Engineering
at 1-800-854-7179.


13) How to FTP by email.

There are a number of sites that archive the Usenet sources newsgroups
and make them available via an email query system.  You send a message
to an automated server saying something like "send comp.sources.unix/fbm",
and a few hours or days later you get the file in the mail.

In addition, there is at least one FTP-by-mail server.  Send mail to
ftpmail@decwrl.dec.com saying "help" and it will tell you how to use
it.  Note that this service has at times been turned off due to abuse.


14) How to tell whether a point is within a planar polygon.

Consider a ray originating at the point of interest and continuing to
infinity.  If it crosses an odd number of polygon edges along the way,
the point is within the polygon.  If the ray crosses an even number of
edges, the point is either outside the polygon, or within an interior
hole formed from intersecting polygon edges.  This idea is known in
the trade as the Jordan curve theorem; see Eric Haines' article in
Glassner's ray tracing book (above) for more information, including
treatment of special cases.

Another method is to sum the absolute angles from the point to all
the vertices on the polygon.  If the sum is 2 pi, the point is inside,
if the sum is 0 the point is outside.  However, this method is about an
order of magnitude slower than the previous method because evaluating the
trigonometric functions is usually quite costly.

Code for both methods can be had from Eric Haines (erich@eye.com), and will
appear in the Ray Tracing News, Vol. 5, No. 2, available from princeton.edu:
pub/Graphics/RTNews/RTNv5n2.Z.


15) How to tessellate a sphere.

One simple way is to do recursive subdivision into triangles.  The
base of the recursion is an octahedron, and then each level divides
each triangle into four smaller ones.  Jon Leech <leech@cs.unc.edu>
has posted a nice routine called sphere.c that generates the coordinates.
It's available for FTP on ftp.ee.lbl.gov and princeton.edu.

16) Specific references on ray-tracing and global illumination.

Rick Speer maintains a cross-indexed ray-tracing bibliography:

Highlights of this edition-

    i) more than 500 citations spanning the period from 1968 through
       November '91;
    ii) papers from all Siggraph, Graphics Interface, Eurographics, CG
        International and Ausgraph proceedings through December, '91;
    iii) all citations keyworded for easy lookup;
    iv) cross-indices by keyword and author;
    v)  glossary of the 119 keywords used.

The bib is in the form of a PostScript file.  The printout is 41 pages long.
Below is a list of ftp sites and the dirs that contain the file. It's named
"speer.raytrace.bib.ps.Z" and is compressed at most sites-

             Site                             Dir
	wuarchive.wustl.edu	graphics/graphics/bib/RT.BIB.Speer/
	karazm.math.uh.edu	pub/Graphics/
	gondwana.ecr.mu.oz.au	pub/papers/
	nic.funet.fi		pub/sci/papers/graphics
	coral.cs.jcu.edu.au     graphics/papers/

Eric Haines (erich@eye.com) maintains ray tracing and radiosity/global
illumination bibliographies.  These are in "refer" format, and so can be
searched electronically (a simple awk script to search for keywords is
included with each).  The bibliographies are available at most of the
sites listed above, and the most current versions are maintained at
princeton.edu: pub/Graphics/Papers as "RayBib.*" and "RadBib.*".

Tom Wilson (wilson@cs.ucf.edu) has collected over 300 abstracts from ray
tracing related research papers and books.  The information is essentially
in plaintext, and Latex and troff formatting programs are included.  This
collection is available at most of the sites above as "rtabs.*".

17) SIGGRAPH information online

[from Steve Cunningham and Ralph Orlick]

ACM-SIGGRAPH announces its online information site at  siggraph.org
(128.248.245.250).  This site now provides SIGGRAPH information via both
anonymous ftp and an electronic mail archive server.

The anonymous ftp service is very standard, and the ftp directory includes
both conference and publications subdirectories.

To retrieve information by electronic mail, send mail to
     archive-server@siggraph.org
and in the subject or the body of the message include the message  send
followed by the topic and subtopic you wish.  A good place to start is with
the command
     send index
which will give you an up-to-date list of available information.


18) SIGGRAPH Panels Proceedings available

[from Steve Cunningham and Bob Judd]

ACM SIGGRAPH announces the availability of the SIGGRAPH '91 Panels Proceedings
at the  siggraph.org  site (128.248.245.250).  The proceedings are available
in three formats:
     text   (ASCII)
     rtf    (rich text format, suitable for many word processors)
     word   (MS Word for the Macintosh)
They may be retrieved from siggraph.org in two ways:

(1) by anonymous ftp
    change to one of the directories
       publications/s91/panels_proceedings/[text|rtf|word]
    The text and rtf files may be downloaded in ASCII mode, while the word
    files are stored in MacBinary format and must be downloaded in binary 
    mode.

    Each directory contains a Table of Contents file (TOC) that describes the
    contents of each panel file.

(2) by electronic mail
    send mail to
       archive-server@siggraph.org
    You can retrieve either the  text  or  rtf  files.  We suggest that you
    first retrieve the index files by putting one of the messages
       send panel91-txt index
       send panel91-rtf index
    in the subject or body of the message.  You will get the necessary
    information to retrieve the actual transcript files.


19) Graphics mailing lists

There are a variety of graphics-related mailing list out there, each
covering either a single product or a single topic.  I have been an
active participant in one of these for some time now, and find the
focus and expertise which can be brought to bear on an isolated topic
to be nothing short of amazing.

Please send me the appropriate information if you have any others you
would like to see added.

Name:		Imagine mailing list
Description:	Discussion forum for users of the Imagine 3D Rendering and
		Animation package by Impulse, Inc.
Platforms:	Amiga
Subscription:	imagine-request@email.sp.unisys.com
Posting:	imagine@email.sp.unisys.com

Name:		DCTV mailing list
Description:	Discussion forum for users of the Digital Creations DCTV
		box, software, and file formats
Platforms:	Amiga
Subscription:	DCTV-request@nova.cc.purdue.edu
Posting:	DCTV@nova.cc.purdue.edu

Name:		Rayshade Users mailing list
Description:	Discussion forum for users of the Rayshade raytracer
Platforms:	Most UNIX boxes, Amiga, Mac, IBM
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20) Specific references on file formats

    Graphics File Formats, David Kay and John Levine, Windcrest/McGraw-Hill
      1992, ISBN 0-8306-3059-7 paper, ISBN 0-8306-3060-0 $36.95 hardcover.
      Comments - 26 formats, no software (this is good, IMHO - I prefer
      books which are non-platform-dependent).


21) What about GIF?

GIF stands for Graphics Interchange Format.  It is portable and usable upon
a wide variety of platforms.  It is quite limited in some ways (yes, the
keeper of the FAQ has some opinions after all), and in fact, I don't like
it much.  However, it looks to me like the most-Frequently Asked Question
which was not previously covered in this list.  The following is a list
of newsgroups and the like where one could go to find out about GIF.

Subject: alt.binaries.pictures FAQ - General info
Subject: alt.binaries.pictures FAQ - OS specific info
Newsgroups: alt.binaries.pictures.d,alt.binaries.pictures.misc,
	alt.binaries.pictures.utilities,alt.binaries.pictures.fractals,
	alt.binaries.pictures.fine-art.d,news.answers

Available in the indicated USENET newsgroup(s), or via anonymous ftp from
pit-manager.mit.edu (18.172.1.27) in the files:

/pub/usenet/news.answers/pictures-faq/part1
/pub/usenet/news.answers/pictures-faq/part2

Also available from mail-server@pit-manager.mit.edu by sending a mail
message containing any or all of:

send usenet/news.answers/pictures-faq/part1
send usenet/news.answers/pictures-faq/part2

Send a message containing "help" to get general information about the
mail server.

Also, you could check out the resources described in sections 7, 8, and
20 above for more information.


22) What is morphing?

Warping is the deformation of an image by mapping each pixel to a new
location. Morphing is blending from one image or object to another one.
Valerie Hall has written an excellent introduction to warping and
morphing. This is available for anonymous ftp from marsh.cs.curtin.edu.au
in the directory pub/graphics/bibliography/Morph. There are three files:

        morph_intro.ps.Z     (PostScript version, many pictures - 1.5M)
        morph_intro.txt.Z    (text version)
        m_responses.Z        (Responses to morphing questions)

The files are compressed, so you must use binary transfer and
uncompress them afterwards.


23) How to ray-trace height fields

Height fields are a special case in ray-tracing.  They have a number of uses,
such as terrain rendering, and some optimization is possible.  Thus, they
get their own FAQ section.  Note that further references can no doubt be
located via the ray-tracing bibs in section 16 above.

Unless someone else sends me information to the contrary, I assume that
the following paper is the definitive reference:

F. Kenton Musgrave
Grid Tracing: Fast Ray Tracing For Height Fields
July, 1988
<musg88.ps.Z>

This is available as "Research Report YALEU/DCS/RR-639" from Yale University,
it's also in the SIGGRAPH '91 Fractal Modeling in 3D Computer Graphics and
Imaging course notes, and (best of all) it's available on the net:

    nic.funet.fi		pub/sci/papers/musg88.ps.Z
    weedeater.math.yale.edu	pub/Papers/musg88.ms.Z
    princeton.edu		pub/Graphics/Papers/musg88.ms.Z
    coral.cs.jcu.edu.au		graphics/papers/musg88.ps.Z
    gondwana.ecr.mu.OZ.AU	pub/papers/musg88.ms.Z and musg88.ps.Z

An implementation of this paper may be found in Rayshade.

Another paper exists:

%A David W. Paglieroni
%A Sidney M. Petersen
%T Parametric Height Field Ray Tracing
%J Proceedings of Graphics Interface '92
%I Canadian Information Processing Society
%C Toronto, Ontario
%D May 1992
%P 192-200


24) How to find the area of a 3D polygon

The area of a triangle is given by (in C notation),

     area = 0.5 * ( ( x[0] * y[1] ) + ( x[1] * y[2] ) + ( x[2] * y[0] ) -
	            ( x[1] * y[0] ) - ( x[2] * y[1] ) - ( x[0] * y[2] ) );

and the area of a planar polygon is given by

     area = 0.0;

     for ( i = 0; i < n - 1; i++ )
         area += ( x[i] * y[i + 1] ) - ( x[i + 1] * y[i] );
     area += ( x[n - 1] * y[0] ) - ( x[0] * y[n - 1] );
     area /= 2.0;

If the area is a negative number, the polygon or triangle is
clockwise, if positive, it is counterclockwise.

>From Ronald Golman's Gem (in Graphics Gems II - see section 1 above), "Area
 of Planar Polygons and Volume of Polyhedra:"

The area of a polygon P0, P1, P2, ... Pn, not in the x-y plane, is
given by

     Area(Polygon) = 1/2 * | N . Sigma { Pk x Pk+1 } |

where N is the unit vector normal to the plane and P is a polygonal
vertex.  The . represents the dot product operator and the x
represents the cross product operator.  Sigma represents the summation
operator.  | | represents the absolute value operator.  Pn+1 is equal
to P0.

-- 
John T. Grieggs (Telos @ Jet Propulsion Laboratory)
4800 Oak Grove Drive, Pasadena, Ca. 91109 M/S 525-3660    (818) 306-6506
Uucp: {cit-vax,elroy,chas2}!jpl-devvax!grieggs
Arpa: ...jpl-devvax!grieggs@cit-vax.ARPA