textfiles/internet/FAQ/compgfx.faq

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---
Jef

      Jef Poskanzer  jef@well.sf.ca.us  {ucbvax, apple, hplabs}!well!jef
                        "...Is this a trick question?"

- - - - - - - - - -

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 GIF, TIFF, IFF, BIFF, NFF, OFF, 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 tesselate a sphere.


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
    Principles of Interactive Computer Graphics (2nd Ed.), Newman and
      Sproull, McGraw Hill, ISBN 0-07-046338-7
    Procedural Elements for Computer Graphics, David F. Rogers, McGraw
      Hill, ISBN 0-07-053534-5
    Mathematical Elements for Computer Graphics 2nd Ed., David F. Rogers
      and J. Alan Adams, McGraw Hill, ISBN 0-07-053530-2
    Applied Concepts in Microcomputer Graphics, Bruce Artwick, Prentice-Hall,
      ISBN 0-13-039322-3
    Digital Picture Processing, vols. 1&2, Azriel Rosenfeld and Avi Kak,
      Academic Press 1976
    Three Dimensional Computer Graphics, Alan Watt, Addison-Wesley, 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.), Acedemic Press 1990, ISBN
      0-12-286165-5

More specific technical references can be obtained from an ACM/SIGGRAPH
mail daemon.  Send a message to

    graf-bib-server@decwrl.dec.com

Just place important keywords in the Subject: field of your mail message.
E.g.,

    Subject: ray traced musical spheres

Please be as specific as possible.  "Graphics" is too vague and would
only flood the mailer.  Additional keywords help.  Some of the complete
reference files are obtainable via anonymous ftp thru gatekeeper.dec.com
(pub/misc/graf-bib) [megabytes].  Additional years references will be
added over time.

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.

Finally, all C code from "Graphics Gems" is available via anonymous ftp
from weedeater.math.yale.edu.  Look in the directory pub/GraphicsGems/src,
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.


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).


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, 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 export.lcs.mit.edu:contrib/pbmplus.tar.Z and
    ftp.ee.lbl.gov:pbmplus.tar.Z.  The version of 22nov89 (which currently
    is still the latest version, except for the one official patch so far)
    was posted to comp.sources.misc, and is therefore accessible via mail
    to one of the archive servers.  This version is also available in the
    X11R4 release tape.

    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-*,
    weedeater.math.yale.edu:pub/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,
    uunet.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 uunet.uu.net:graphics/tiff.tar.Z

    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.unm.edu:pub/khoros/*.

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:

    export.lcs.mit.edu          18.30.0.238
    ftp.ee.lbl.gov              128.3.254.68
    cs.utah.edu                 128.110.4.21
    nl.cs.cmu.edu               128.2.222.56
    venera.isi.edu              128.9.0.32
    ucbvax.berkeley.edu         128.32.133.1
    weedeater.math.yale.edu     130.132.23.17
    freebie.engin.umich.edu     141.212.68.23
    pprg.unm.edu                129.24.13.10

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 GIF, TIFF, IFF, BIFF, NFF, OFF, 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
titan.rice.edu (128.42.1.30) in directory public/graphics.formats.
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.  Other file format descriptions are welcome, send to
Mark Hall <foo@rice.edu>.


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 primatives 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                                Price
    Luxo, Jr.                           $14.95
    Red's Dream                         $19.95
    Tin Toy                             $24.95
    Knickknack                          $24.95
    Luxo, Jr./Red's Dream/Tin Toy       $49.95

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)

Add $5 S&H for the first tape or shirt, $2 for each additional tape or
shirt.  Foreign shipping, add $3/tape or shirt.  Call 800/525-0000
(213/396-4774 international) 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


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.

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 (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 ANSI X3.122-1986.  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 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.

There are also a couple of sites that will perform general FTP retrievals
for you in response to a similar mail query.  For information on using
one of them, send a message like this:

    To: info-server@cs.net

    request: info
    topic: help-ftp
    request: end

(NOTE: this server is currently "down for repairs".  No estimate on when
or if it will return.)

And for info on another one, send this:

    To: bitftp@pucc.bitnet

    help

Please be considerate, and don't over-use these services.  If people
start using them to retrieve hundreds of megabytes of GIF files, they
will probably disappear.


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 intersectiong 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.


15) How to tesselate 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 weedeater.math.yale.edu.




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