Ray Tracing News

"Light Makes Right"

July 13, 1990

Volume 3, Number 3

Compiled by Eric Haines erich@acm.org . Opinions expressed are mine.

All contents are copyright (c) 1990, all rights reserved by the individual authors

Archive locations: anonymous FTP at ftp://ftp-graphics.stanford.edu/pub/Graphics/RTNews/,
wuarchive.wustl.edu:/graphics/graphics/RTNews, and many others.

You may also want to check out the Ray Tracing News issue guide and the ray tracing FAQ.


========Net News Cullings========


Well, I've been absorbed in quality assurance for the updated version of the radiosity/ray tracing software we've done with HP. One thing we've added is texture mapping, and I definitely feel a sense of deja vu, since my second graphics project at Cornell was to integrate Rikk Carey's texture mapping software into the modeller and ray tracer. Here it is six years later and we're finally using texture mapping in a commercial product.

I should mention some resources for all of you who have scanners. Phil Brodatz's "Textures" book (Dover) is definitely recommended - it's mostly nicely photographed images of sand, gravel, fur, wood, paper, straw matting, etc etc, perfect for scanning. Another Dover book, "The Grammar of Ornament" by Owen Jones, has many colorful designs from a large variety of cultures. There are also many other Dover collections of designs which are worth looking over.

A related book that just came out is "Computers, Pattern, Chaos, and Beauty" by Clifford Pickover (St. Martin's Press). At first I thought this was just another fractal book, but there are many other functions and algorithms explored here. This book is something like a collection of "Computer Recreations" columns, and is worth checking out. One topic mentioned in the book is creating sea shells by a series of spheres. This was also covered in the IEEE CG&A November 1989 article, pages 8-11. Here's a code fragment that outputs a series of spheres in NFF (I leave a good view & lights & colors to you). Cut the number of steps way down for a rough idea where the surface is located, and have fun playing with the various numbers and formulae.


int argc;  char *argv[];
static	double	gamma = 1.0 ;	/* 0.01 to 3 */
static	double	alpha = 1.1 ;	/* > 1 */
static	double	beta = -2.0 ;	/* ~ -2 */
static	int	steps = 600 ;	/* ~number of spheres generated */
static	double	a = 0.15 ;	/* exponent constant */
static	double	k = 1.0 ;	/* relative size */
double	r,x,y,z,rad,angle ;
int	i ;

    for ( i = -steps*2/3; i <= steps/3 ; ++i ) {
	angle = 3.0 * 6.0 * M_PI * (double)i / (double)steps ;
	r = k * exp( a * angle ) ;
	x = r * sin( angle ) ;
	y = r * cos( angle ) ;
	/* alternate formula: z = alpha * angle */
	z = beta * r ;
	rad = r / gamma ;
	printf( "s %g %g %g %g\n", x, y, z, rad ) ;

The interesting thing about the database generated is that it's pretty slow for most ray tracers (mine included). There are a lot of spheres in a tiny cluster at the start of the shell, and the final spheres are large enough that this tiny cluster is a small part of the environment. Because the spheres overlap, each octree node, grid cell, or 5D list is going to be pretty long - there's a huge amount of depth complexity in this database. Definitely a pathological case for most ray tracers, I suspect.

Another book which just came out (as in, I just received my copy minutes ago) is the new Foley & van Dam (& Feiner & Hughes) "Computer Graphics: Principles and Practice" from Addison Wesley. 1174 pages, with a fair number of color plates. Definitely a good place to start in on any topic. A must have; enough said.

"Graphics Gems", edited by Andrew Glassner, should also be out soon. Mine's on order, but hasn't arrived in time to review this issue. Look for it - it should be of interest.

One more thing. I recently created a radiosity bibliography which was posted to USENET. If you missed it on comp.graphics, it should be available on freedom.graphics.cornell.edu (see the RT News header) soon.

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Ray Tracing Roundtable Announcement

There will be a "birds of a feather" room for ray tracers to meet and get to match names to faces at SIGGRAPH. We'll meet 5:30 to 6:30 pm on Thursday of SIGGRAPH at some room (see the room listings at registration). Note that this shouldn't conflict with much, as it's after the sessions and before the reception.

As befits ray tracers, we're aiming for a minimal, parallel approach for this year's meeting. There won't be any roundtable, since last year's meeting had near a hundred people - just going around the room getting names and interests took half an hour. So, there is absolutely no agenda for this meeting; it's simply a place and time to come and meet others in the field. Hope to see you there.

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New People, Address Changes, etc

Haakan 'ZAP' Andersson - efficiency, texturing, speed, etc.
Box 40
178 00 Ekeroe
+(46)16 - 964 60
[no e-mail address for now...]

Working with CAD technology, developing add-ons to the popular AutoCAD, doing RayTracing as a personal midnight oil project, and it might even be released realsoonow by EMT. Since my computational powers are limited (PC style) I enjoy things that make things pretty without bending the clock. I love anything that says 3D, but is also interested in other simulations of real life (AI, sound and speech synthesis etc.).

P.S. I showed "RayTracker" ( My Program ) at an exhibition and some people thought it was darnfast...even though they were using some renderman stuff earlier... my suspicion was that my VERY low resolution led them astray speed-wise, but I do another nice thing: I trace every 8ighth pixel (Well, really every 64th (8x8)) first, so you get a quick look of what's happening very quickly. Actually, it's a good timer! When the scan has done the screen the FIRST time, a 64th of the image is done, i.e. the amount of seconds (minutes) this scan takes, this amount of minutes (hours) will the full image take, approximately. That's nice. Well, that IS all for now.


Mark A. Cartwright - 3D Modeling, 3D Human Interface, Ray Tracing, Macintosh. University of Texas Computation Center COM 1 Austin, Texas 78712-1110 Phone: (512)-471-3241 ext 306 email: markc@emx.utexas.edu

My background is mostly graphics work - with a little Numeric Control thrown in for good measure. Prior to my position here at the University I worked at Byte By Byte here on the Macintosh version of Sculpt 3D. Modeling and all that relates to object creation and editing -- and of course ray tracing are my main areas of interest.


Pierre Poulin
Dept. of Computer Science
University of British Columbia
Vancouver, B.C., Canada
V6T 1W5

Just in case you might be interested to change it, my new email address is now:


I followed my supervisor at the University of British Columbia. The DGP group at Toronto kindly kept me on their system, but I would prefer being contacted directly rather than each time through Toronto...


Drew Whitehouse
E-mail:  drw900@csc2.anu.edu.au
Visualization Programmer,               Fax   : (06) 2473425
Australian National University,         Phone : (06) 2495985
Supercomputer Facility.
GPO Box 4, Canberra ACT Australia 2601.

I work at the Australian National University Supercomputer Facility as a visualization programmer. My interests are scientific visualization, volume rendering and raytracing. Currently I'm keeping myself busy inserting bits and pieces into the MTV raytracer. The main thing being some sort of volume rendering, both for voxel data and Perlin style hypertexture (what I've really got in mind is a tribble teapot..... )


Michael A. Kelly
3105 Gateway St. #191
Springfield, Oregon  97477
(503) 726-1859 (home)
(503) 342-2277 (work)

Interests: accurate duplication of real-world objects, color, efficiency

Current Project:
I am attempting to write a rendering program for the Macintosh II series.
'Render3D' will be, at least at first, a shareware program.  It is a simple
ray tracer at this point, but by the end of summer it will be considerably
more.  Some of the features I plan to include are:

	Accurate surface color definitions using spectral energy distributions
		- color calculated on a wavelength-by-wavelength basis
		(now implemented)
	Radiosity method for diffuse reflections
	Phong and Gouraud shading for polygons
	Support for parametrically defined objects
		(including bicubic surface patches)
	Treat light sources as objects
		any normal object can be treated as a light source
	Texture mapping
	Import and Export several file formats, including RenderMan

Of course, I probably won't get to all of these before the end of summer, but I hope to finish the first four or five of them by the beginning of fall term. (I am currently an undergraduate computer science major at the University of Oregon, with one year left to go.)


# Erik Jansen
# Dept of Industrial Design
# Delft University of Technology
# Jaffalaan 9
# 2628 BX Delft
# The Netherlands

Erik informs me that the site for a few people has changed:

alias	wim_bronsvoort	wim@duticg.tudelft.nl
alias	erik_jansen	fwj@duticg.tudelft.nl
alias	frits_post	frits@duticg.tudelft.nl

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Jarke van Wijk Thesis Availability, by Erik Jansen (fwj@duticg.tudelft.nl)

I saw your question about the book of Jarke van Wijk in the [January] issue. Jack wrote a thesis book which is nicely printed by the Delft University Press.

The title is: "On New Types Of Solid Models and Their Visualization with Ray Tracing".

Subjects that are covered:
- translational, rotational sweep; also in TOG 3(3) 223-237
- sphere sweep, also in EG'84 73-82; and in Computers and Graphics 9(3) 283-290
- blending surfaces; comparable with the work of Middletich and Sears, Sigg'85
- bicubic patches for non-rectangular meshes; also in CAGD 3(1) 1-13
- SML, solid modeling language; also in CAD 18(9).

There is no discussion in the thesis about efficiency methods (only bounding boxes).

The book can be ordered for Dfl. 36.30 (ca. $20 including send costs) by: Delft University Press Stevinweg 1 Delft The Netherlands fax: +31-15-781661

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New Name For "Distributed Ray Tracing", by Paul Heckbert et al

[If you have more comments, please also send them for publication here. -- EAH]

A few weeks ago, I asked the following question of the people on the Ray Tracing News mailing list:

    From ph@spline.Berkeley.EDU Wed Apr 11 18:48:05 1990

    This is a survey.  Assuming we could rename "Distributed Ray Tracing",
    what should we call it:

    1) Stochastic Ray Tracing
    2) Probabilistic Ray Tracing
    3) Distributed Ray Tracing (old name ok)
    4) other


   14  stochastic ray tracing
    3  monte carlo ray tracing
    2  distributed ray tracing
    1  probabilistic ray tracing
    1  rob cook ray tracing

"Stochastic Ray Tracing" is the clear favorite of those who responded. Many commented that they've always found the term "distributed ray tracing" to be a confusing misnomer. Brian Corrie put it well:

    I have a serious problem with the name distributed ray tracing. The
    main reason is the name directly conflicts with the notion of using
    a distributed computer system for ray tracing. Trying to describe
    both of them is a hassle. Because parallel ray tracing is becoming
    quite prominent, this name conflict can be a major pain.

The plot thickens, however. Michael Cohen and Joe Cychosz observed that one could distribute rays regularly throughout a distribution, or by some other deterministic algorithm, and most people would still call it "distributed ray tracing". Wouldn't the proposed new name "stochastic ray tracing" then be misleading?

I agree. That's why I've started using the name "distribution ray tracing". To me it captures the central idea that you're modeling the surface properties as numerical parameters each with a probability distribution instead of a single, specific value; analogous to the distinction between a random variable and a standard variable. There are various numerical integration techniques for simulating this distribution: uniform sampling or stochastic sampling, non-adaptive or adaptive, etc, but these are implementation details relative to the main concept, which is that there's a variable in the shading formula with a probability distribution that necessitates integration. Another advantage of the name is that it's similar to the old one.

Andrew Glassner pointed out that there is another concept in Cook's and Kajiya's papers that deserves a name, and that concept is that the stochastic sampling can be done independently in each parameter. Andrew suggested "simultaneous multidimensional sampling". Another possibility that occurs to me is "independent sampling".

What do y'all think?

What I'd like to know is: why didn't the reviewers of Cook's 1984 paper scream when he proposed that title?

Paul Heckbert

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NFF Files from Thierry Leconte

[I found some wonderful NFF files at the site irisa.irisa.fr [] in the NFF directory. Particularly good is the Amiga computer model, but all of them are pretty worthwhile.

BowlingPin.nff - guess
amiga.nff - an Amiga 2000 keyboard and computer (no monitor) - very nice
balls.nff - just a scene generated by SPD ball program
crypt.nff - a mysterious crypt (with columns and whatnot)
expresso.nff [sic] - T. Todd Elvins' Utah espresso maker in NFF format
hangglider.nff - a hang glider
jaws.nff - don't be afraid, it's just a nff scene
matchbox.nff - a box of matches and some matches
room.nff - an office with the desk and others things
spirale.nff - a spiral spline thingie
spring.nff - a spring thing
sps7.nff - a bull sps7 computer box
teapot.nff - the mythical one
temple.nff - pseudo-roman pagan temple something
tpotbis.nff - ye olde teapot, with lid ajar
watch.nff - a wristwatch
x29.nff - fighter plane

Unfortunately, there are some problems with some databases. Problems included:

    - polygonal patches with normals given as [0,0,0].
    - polygons with no area (usually a triangle with a doubled vertex).
    - inconsistently defined polygons.  The NFF format specifies that you
      should output the polygon vertices in a particular order (counter-
      clockwise when viewed from above in a right-handed coordinate
      system).  I suspect you use double sided polygons so that it does not
    - other minor problems.

The files I had problems with:

amiga.nff - there are a few polygons with no area (doubled vertices).  The
	first is around line 1518 of the file.
jaws.nff - there are tons of polygonal patches with normals of [0,0,0]
room.nff - polygons with [0,0,0] normals, and some no area polygons.  The first
	two polygons in this file are HUGE.  I cannot get the normals per
	vertex to display properly on my system, because some of the vertex
	normals differ inconsistently from the polygon normal (I haven't quite
	figured this out).
spring.nff - the "Shine" and "Transmission" values are switched here, I
	suspect.  Shine is 0, while Kt is 30!  You should definitely fix the
	material setup line here.
temple.nff - no area polygon around line 433.  I can't get the normals to line
	up properly, similar problem to room.nff.
watch.nff - lots of [0,0,0] normals here.
x29.nff - on line 3641 there is an extra vertex - the polygon says it has
	3 vertices, but 4 vertices appear (possibly my file is corrupt).

Some comments I pasted together from Thierry Leconte (Thierry.Leconte@irisa.fr), who is the caretaker of the files:

In fact I'm only a novice in ray-tracing area. I work on distributed systems and parallelism. But ray-tracers are good applications to test such systems. Now I work on a modified version of VM_pRay (the parallel ray-tracer of Didier.Badouel@irisa.fr) which run on our own distributed system (called GOTHIC). We are writing a motif based window interface for it and I am trying to collect as many nff files as I can in order to run nice demos on the Gothic system. I have made available most of these files and some utilities (among others yours) via anonymous ftp from irisa.irisa.fr. Most of the non classicals scenes I have come from a scene designer Xavier Bouquin who works on a amiga with the Scult4D program. and Philippe.Joubert@irisa.fr has written a sculpttonff converter. But if someone knows other converters or interesting nff files I will be happy to add them to my collection!

VM_pRAY (ray tracer) of Didier Badouel is available at the same site.

Feel free to use, copy ,modify and distribute these files provided that they are not use for commercial purpose and that the name of the author is mentioned.

Most of these scenes was made on an amiga with Scult4D (a truly great modeler) then they have been converted to nff file with sc2nff (a PD converter available on the same site, same directory in the utils.tar archive).

The author of crypt,jaws,matchbox,room,temple,watch is Xavier Bouquin (email to pjoubert@irisa.fr).

teapot, x29 were ftp'ed from cs.uoregon.edu.

amiga, hangglider, teapotbis were PD Scult4D files available on a amiga site archive (will try in the future to collect any PD Scult4D file an convert them to nff).

sps7 was made by hand.

balls and spirale - generated by program.

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RADIANCE Software Available, by Greg Ward (greg@hobbes.lbl.gov)

For a little over a year now I have been distributing a ray tracing lighting simulation called RADIANCE through the Sun Academic Software Portfolio. Since the software runs on many machines besides the Sun, and not everyone gets the portfolio as it is, I thought you might be willing to publicize the software through your rtnews mailing list.

RADIANCE was written as a lighting simulation more than a renderer, so it does not allow some of the tricks that are permitted in other ray tracing programs. (For example, all light sources fall off as 1/r^2.) It has some of the nicer features of advanced renderers, such as textures and bump maps (I've always argued for calling them patterns and textures, respectively), octree spatial subdivision, several surface primitives and hierarchical instancing. It's main distinction, however, is its ability to calculate diffuse interreflection like a radiosity calculation. (See the article by Ward, Rubinstein and Clear in the 1988 Siggraph proceedings.)

The software is free, and comes with all the C source code, but is not available through anonymous ftp. We want to keep track of people who have a copy of the package so that we can send out update announcements, bug fixes and so forth. For this reason we also ask that people do not redistribute the package without written permission (e-mail is fine).

Since I am just a mellow Californian who can't handle answering a 24 hour support hotline, I want to discourage the idly curious, those who just want to check out another ray tracer, from acquiring this software. If you are interested primarily in computer graphics, there are plenty of other ray tracing programs that do a great job producing attractive imagery. If, on the other hand, you are really serious about lighting simulation, this is the program for you.

Summary Description of RADIANCE:

	Lighting calculation and image synthesis using advanced ray tracing.
	Scenes are built from polygons, cones, and spheres made of plastic,
	metal, and glass with optional patterns and textures.

Detailed Description:

	RADIANCE was developed as a research tool for predicting the
	distribution of visible radiation in illuminated spaces.  It takes as
	input a three-dimensional geometric model of the physical environment,
	and produces a map of spectral radiance values in a color image.  The
	technique of ray tracing follows light backwards from the image plane
	to the source(s).  Because it can produce realistic images from a
	simple description, RADIANCE has a wide range of applications in
	graphic arts, lighting design, engineering and architecture.

	The reflectance model accurately predicts both diffuse and specular
	interactions, making it a complete simulation applicable to the design
	of unusual electric and day lighting systems.  Scenes are described by
	boundary representations with polygons, spheres and cones.  Materials
	include plastic, metal, glass, and light.  Textures and patterns can
	be described as functions or data.  Additional programs (generators)
	produce descriptions of compound objects, and allow regular
	transformations and hierarchical scene construction.  A 3D editor is
	being developed.

	The software package includes some image processing software and
	display programs for X, SunView, and the AED512, and comes with
	converters to Sun rasterfile and Targa formats.  Code is provided for
	writing other drivers, and the list is expected to grow.

Interface Description:

	The software is well integrated with the UNIX environment.  Many of
	the programs function as filters, with a reasonable degree of
	modularity.  An interactive program provides quick views of the scene,
	useful for debugging and view determination.  Scenes are described in
	a simple ascii format that is easy to edit and program.  Generators
	are provided for boxes, worms, surfaces of revolution, prisms, and
	functional surfaces (eg.  bicubic patches).  A small library of
	patterns and textures is included.  In general, the software is
	sensible but not mouse-based.

Overall Goals of Developer:

	The primary goal of the program is the accurate simulation of light in
	architectural spaces.  Secondary goals are image synthesis and
	geometric modeling.  Efficiency is an important concern in any ray
	tracing method.

Obtaining RADIANCE:

	Send a 30+ Mbyte tape cartridge with return envelope and postage to:

		Greg Ward
		Lawrence Berkeley Laboratory
		1 Cyclotron Rd., 90-3111
		Berkeley, CA  94720

If you have any questions regarding the applicability of this software to your needs, feel free to call or (preferably) send e-mail to me directly.


Greg Ward

Lighting Systems Research Group
(415) 486-4757

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Rayshade Updates & SPD Bug, by Craig Kolb

[from various notes to me]

The Rayshade 3.0 ray tracer is now up to patch level 5. Rayshade 3.1 is coming along nicely. It fixes some of the major problems in rayshade, and adds a bunch of new features.

By the way, while playing with the SPD, I ran across a couple of very minor problems. In lib.c, you make mention of OUTPUT_POLYGONS, when you really mean OUTPUT_PATCHES.

[I have sent Craig release 2.7 of the SPD package, and it's now available for FTP from weedeater.math.yale.edu ( It has minor typo fixes (like the above) and clarifications of the NFF, but no code fixes. - EAH]

Craig also notes that Przemek Prusinkiewicz's book, "The Algorithmic Beauty of Plants", will be released at SIGGRAPH this year, complete with lots of pretty raytraced pictures using Rayshade. Some of the images generated were in the March 1990 issue of IEEE CG&A (including the front cover).

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New Version of Vort Ray Tracer, by David Hook (uunet.UU.NET!munnari!dgh)

[the following was pieced together from two notes - EAH]

I have just installed a new version of the ray tracer in
pub/graphics/vort.tar.Z on munnari.OZ.AU.  It's a bit of an improvement on the
last one, and has some stuff for displaying images as animations on sun's,
PC's, and X11.

One thing that may be of interest, one of the guys I work with, one Bernie Kirby, implemented some marble and wood texturing using in the ray tracer. He had the following to say:

	It is an implementation of Ken Perlin's noise function as described in
	"Hypertexture, Computer Graphics, Vol 23, No.3, July 1989, pp
	255-256".  Most of the function names are as given in the paper.
	Also, I think that there may be an error in the paper as the given
	formulae of OMEGA would always produce zero if the actual point falls
	on a lattice point.  (ie. GAMMA(i, j, k) .  (u, v, w) = 0).  Anyway,
	I added in a pseudo random value for the noise at each lattice point
	as well as the gradient values (as described in his '85 paper and it
	seems to work.)

	The original version (ie.  the one that had zeros at the lattice
	points) produced almost exactly the same effects as displayed in Fig 2
	of J. P. Lewis' paper in SIGRAPH '89 "Algorithms for solid noise
	synthesis".  The changed algorithm still displays some of these
	artifacts (if you really look for them) but no where near as badly as
	in Lewis's paper.

The only other things of note are that Vort no longer requires primitives to be axis-aligned and that most things can have tile patterns mapped on to them (this includes toruses, although flat pixel files mapped onto a donut can tend to look a little weird). Some people may find the mapping functions of some use (although having just got a copy of "Introduction to Ray-Tracing" I notice most of them are in there, the book shop slugged me $100 bucks for it, sometimes I don't believe this place...)


PS: we ran your standard rings benchmark at four rays per pixel and it took two and a half days on a Sun 4 3/90. Is that a record? ( :-) ) Next project is to speed the mother up...


[Also, a package called Vogel is also available at munnari - EAH]

From Tom Friedel, tpf@jdyx.UUCP:

With Vogel you get 3-d transformation, 3-d and 2-d clipping, perspective, orthogonal views, some patch functions, and back face removal. I've wanted to build a bigger package on these routines, some sort of .nff previewer, but haven't had the time. Good package because you get device independence with X11, postscript, others.

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Graphics Interface '90, by Eric Haines

An interesting conference/proceedings which often is overlooked is "Graphics Interface". This year's conference, held in Halifax, Nova Scotia, Canada, had quite a few papers on ray tracing. I particularly liked Don Mitchell's "Robust Ray Intersection with Interval Arithmetic" paper. I'm sure we'll be seeing interval analysis applied to more areas of graphics in the years ahead, and this is an interesting application of the technique. Graphics Interface proceedings can be ordered from: Morgan Kaufmann Publishers, (415)-965-4081. What follows is a list of papers that may be of interest:

	Image and Intervisibility Coherence in Rendering
		Joe Marks, Robert Walsh, Jon Christensen and Mark
		Friedell, Harvard U.

	Robust Ray Intersection with Interval Arithmetic
		Don P. Mitchell, AT&T Bell Laboratories, Murray Hill

	Approximate Ray Tracing
		David E. Dauenhauer and Sudhanshu K. Semwal, U. Colorado
		at Colorado Springs

	Octant Priority for Radiosity Image Rendering
		Yigong Wang and Wayne A. Davis, U. Alberta

	Exploiting Temporal Coherence in Ray Tracing
		J. Chapman, T. W. Calvert and J. Dill, Simon Fraser U.

	A Ray Tracing Method for Illumination Calculation in
	Diffuse-Specular Scenes
		Peter Shirley, U. Illinois at Urbana-Champaign

	Voxel Occlusion Testing: A Shadow Determination Accelerator for
	Ray Tracing
		Andrew Woo and John Amanatides, U. Toronto

	Some Regularization Problems in Ray Tracing
		John Amanatides and Don Mitchell, AT&T Bell Laboratories,
		Murray Hill

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Real3d Review, Haakan "ZAP" Andersson

I was attending a computer exhibition in Stockholm the other day, and among all the printers, PC computers and other boring stuff, there were some bright spots. In one stand I saw a screen with a checkerboard pattern and a mirrored sphere on it.... I began to Yawn loudly, but as I was going to leave a mirrored hand plunged up from the ground and grabbed the sphere, and the camera made a spin around the whole scene. It made me interested enough to enter and look.

REAL-3D is apparently a new (so new it's a RealSoonNow product) from the icy Finland. It's a raytracing program for the Amiga, and it's darn fast (for being an Amiga, that is). I talked to the program author and he said he did NOT use the math coprocessor, and it was all in assembler...(Good Grief) When discussing efficiency he proudly declared that he had NOT looked at others for algorithms, because he did not want to be influenced, so he had invented it all from scratch. He had been programming full-time for four years, and from what I saw, the result was satisfactory, to say the least.

A beautiful, OSF/Motif-like user interface and a great interactive object editor that would make Sculpt-Animate 4d go home and hide under a rug. All objects were arranged in a hierarchy for animating, and materials and textures could be assigned to whole hierarchy tree branches. He could even do solid modelling (I saw a cylinder cut out from another before my very eyes) with different textures in the 'hole' and the piece the hole was cut out of. It was also one of the few Amiga tracers to support texture mapping, i.e. you could map any IFF image onto any surface via some different kinds of projective mapping (Parallel, Ball, Cylinder)

It had a good hybrid between analytic objects (spheres, cylinders, cones and even hyperbolics (!)) and polygon surfaces. A nice entity was the 'sphere-line' he used, you simply drew a series of connected lines, and all vertices got a sphere, and all connecting lines became a cylinder. [AKA "worms" or "cheese logs" in other parts of the world - EAH]

Animation support looked quite straight forward, and he made a simple animation of the Real3d logo being flown through before my very eyes (Though as a wire frame, but I got the hang of animating).


Even though the program ran on an Amiga 2000 (14 Mhz clock) it's speed was rather fast. Comparing with my own tracer, running on a 386 machine with full math coprocessor support, his program looked faster..(!) The wire frame representations, used for placing cameras an so forth was real-time, and he traced a hyperbolic with wood texture with a cylindric hole in about a minute. He also claimed that finding the colors (for the HAM mode in the Amiga) was half the work. He said that an early version used Black&White and it was almost twice as fast... (bragging? :-) Upon being asked how it could be that fast, he said, "Only me and my brother knows how it works..." then he smiled. Heaven knows why...?


REAL3d looks like a speedy hack for the Amiga, and the editor alone makes it a joy to watch. I hope this little blond guy from Finland will sell some of these programs, so he will be able to continue to develop it into something really nice.

[Sorry, I don't have an address for where to get it. Anyone? - EAH]

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Bits From a Letter by David Jevans

...As for the Arnaldi paper on "mailboxes" (keeping a ray_id for each object to avoid multiple intersections), I though that was common knowledge! Cleary's analysis of ray tracing paper (Visual Computer '88), MacDonald's paper and my paper (Graphics Interface '89) all mention the technique and reference the paper. It's a good thing that you put in the RTN if people really don't know about it!

Anyway, my recent work has some pretty important results for the ray tracing community. Essentially I show that further work in reducing ray-object intersections is pretty much pointless (we are close to the fastest possible time anyway). I'm just writing a section that illustrates that hierarchies of 3D voxel grids are superior to bounding volume hierarchies (as if anyone will believe me! :-)

David Jevans @ University of Calgary, Department of Computer Science Calgary, Alberta, Canada T2N 1N4 uucp: jevans@cpsc.ucalgary.ca vox: 403-220-7685

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On Antialiasing, & Light and Such, by Haakan "ZAP" Andersson (no email)

(Disclamier: Since I havn't read all theses about anti-aliasing, this might be known already, so all I claim to be my ideas may be not, so the stupidity is solely on my part :-)

When raytracing, instead of having one wiz_bang function trace(ray) that traces through the entire object list (traversing AHBV's as necessary) and pinning down the poor bastard of an object blocking our way of light and shining in our face, you might do one little thingy:

Use TWO functions, ONE that traverses the bounding volumes for a ray, building an object_list of primitive object this ray (might) intersect, and ONE that does the real object intersection. i.e. this:

	current_list = traverse_abvh(ray);
	pixel = trace_this_list(ray,current_list);
	clear_list(current_list); /* Get rid of it somehow */

Why da hek do this? Well, when anti-aliasing, you will virtually hit the same object(s) inside each pixel, and the 'slack' around the object vs. bbox will allow the 'current_list' to contain all objects this ray hits, and any ray being very close to this very ray, i.e. all rays within a single pixel. So, when anti-aliasing, simply call 'trace_this_list()' with the same list all over again, only SLIGHTLY different 'ray's. Ok, there CAN be wrongies some places, but since you do this for eye-rays, and _I_say_ that for eye-rays you should use 2d bounding boxes on screen INSTEAD of real bboxes, you simply let the 2d bboxes be one pixel bigger than they should be, and viola, each pixel will always yield AT LEAST the object_list containing all objects to be hit in this pixel and the neighbouring ones. Got that?

Any flaws? Well, since current_list() need to be alloced/malloced in some way, there might be a speed problem. Another solution is using a static object list, and KEEPING the bbox traversal code in 'trace_this_list()' but ALSO having a 'traverse_abvh()' function, used only upon eye-rays when anti-aliasing is in effect. The fact that the list 'trace_this_list()' gets does NOT contain any bboxes once in a while (i.e. when we supply a list made by 'traverse_abvh()' instead of the 'full' object list) is not a problem from that functions point of view.

Any flaws NOW? Well, you might always run out of static storage space. But you can always 'realloc()' :-)

[Comments: this has a certain sense to it. By making a "likely candidate list" for a pixel you can stop wasting time traversing the darn hierarchy. You could even sort it by the box hit distance, so that when you get do get a hit from the candidate list you can then simply test this hit against the remaining candidate distances. As soon as you reach a candidate distance that is beyond your hit distance, you stop intersecting. This candidate list idea is similar in feel to my Light Buffer lists and Jim Arvo's 5D lists.

The trick in all this is to make sure your bounding boxes do not fit too tightly that a pixel makes a difference: this is easy enough to calculate for eye rays. - EAH]


Light and Such

"Something's REALLY wrong,
with Bui-Thui Phong"

(I think that's the guys name, and since Bui-Thui is his last name,
Bui-Thui shading is more appropriate than Phong shading, but...)

By Haakan

I have been dissatisfied with Phong's shading model for quite some time, since it does not accurately enough apply to reality. There is at least two effects in 'real life' that i think is important enough to be mad att ol' mr Phong. I have called these 'The Rough bug' and 'The nonlinearity bug'

* The Rough bug

There is many many ways to see The Rough bug in action in real life, and many poems have been written about the biggest proof about The Rough bug's, many lovers have been sitting under it, many songs written about it, and many a astronomer hase gazed upon it: The Moon.

Let's get out late at night, and trying to look straight up. You will see the moon, or half of it, or a third, or nothing ("Half moon tonight. At least it's better than no moon at all" - Fortune program) But does this moon look like a Phong shaded sphere? Were is the ambient, diffuse, and above all, specular light? What you (and I) see, is darkness, darkness, more darkness, and smack, white moon surface, more white moon surface, swack, Darkness, darkness... Who stole Kd and Ka?

The answer lies in the texture of the moon's surface. It's really a VERY large amount of small objects, stones, rocks, sand, mountains, e.t.c. So, any given spot upon the moon surface, will have normal vectors pointing in every possible direction, and (using Phong in a small scale) will be shaded (almost) equally, until, of course, it's in shadow! This is The Rough Bug, Rough sur- faces are subject to this effect, and the Phong diffuse component gets more and more out-of-sync with the actual surface as the surface get's rougher.

Some questions emerge:

Q: Is there any way to let a phong-type shading model take this into account, without having to model the surface exactly, either in actual geometry or as a bump map?

Another example of this bug you may find outside, at sunset (or almost sunset) if you stand on a large, flat surface covered with sand, dirt, concrete or something similar. Looking towards the setting sun, the surface you are stan- ding on is darker than when you look away from the sun, by the same reason, the surface's Roughness. Small rocks and stuff reflect the sun as you look away from it (by 'it' i refer to the sun, not the rocks. You will not see anything if you look away from the rocks :-) and does not as you look into it (same 'it'). So much for Specular reflection. Perhaps some kind of inverted specular reflection....??

* The Non Linearity Bug

Without having ANY physicists backing me up, I dare claim that the (very linear) equation of the standard Ray Tracing reflection model is goddamnwrong! I was wearing a ring the other day, sitting in front of a window, looking out into the sun. Since my mind is constantly on RayTracing, i saw the reflection in the window of the sun reflecting in my ring. I didn't see myself, nor the ring, not even the room I was in, reflecting in the window glass, only the (very bright) reflection of the ring (Perhaps the window used adaptive tree pruning, filtering away all reflections below 0.1? Nah...don't think so :-)

Another example was when a friend of mine was standing in front of the very same window, but it was very very late at night, and the window was black. But he was backlit by the light from the room, and as I observed his reflection in the glass, I saw it had somehow higher contrast then my direct vision of him standing there. The dark parts of his face (where the shadow of his nose fell, f'rinstance) was pitch black in the reflection, but the tip of the very same nose, being lit from behind, appeared very bright in the reflection. There were no such differences in luminosity between nose-tip and nose shadow in the true image of him, not even from the windows viewpoint, something I verified by crawling up between the window and him, observing him accurately (with him thinking I was a complete fool -- which I am).

A question emerges:

Q: Can this 'non linearity' of reflected images somehow be imported into the raytracing algorithm, or is this a local effect in the human eye? (Perhaps in MY eyes only... ?)

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======== USENET cullings follow ===============

Summary: Uniform Random Distribution of Points on Sphere's Surface, Marshall Cline, (cline@cheetah.ece.clarkson.edu)

Organization: ECE Dept, Clarkson Univ, Potsdam, NY

The original problem was:
> I need to uniformly(!) spray a sphere's surface with randomly located dots.
> We can assume the sphere has radius R and is centered at the origin.

SOLUTION #1 (by far the most popular):
> Choose 3 Uniform random values: (rand(-R,R), rand(-R,R), rand(-R,R)).
> If this is inside the sphere, project that vector onto the sphere's surface.
(Sorry I have no references; many many people suggested this)

> Choose 3 Gaussian randoms:
> Project this vector onto the sphere's surface.
bill@stat.washington.edu (Bill Dunlap, Statistics, U. Washington)
jd@shamu.wv.tek.com (JD)

(Projecting vector onto the sphere's surface is done by dividing each component by sqrt of sum of squares (ie: the vector's length).)

> Pick a random latitude by the inverse sine of a number uniformly
> distributed over [-1,1]. Pi times another such random number gives
> you a random longitude, and you're done.
dougm@rice.edu (Doug Moore)

Several other solutions suggested dividing the sphere's surface into small patches and projecting uniformly into a randomly chosen patch.

Thanks to all who answered.
Marshall Cline

PS: I'm implementing this on the Connection Machine. The SIMD nature of the CM makes the first soln difficult, since each processor will have to wait until that last straggler finds a point inside the sphere. There are ways around this, like a list of cached 3-space points in each processor, but there's always a chance that one processor's list will be very short. Thus I'm going to try the Normal(0,1) solution first.

PPS: paul@hpldola.HP.COM (Paul Bame) suggested a method (simulated annealing) which would "evenly distribute" points on the sphere's surface. Although my app requires a uniform random distribution, I'm posting this as it may be appealing (though slow) for someone who wants evenly distributed points.

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Ray Tracing & Radiosity, by Frank Vance (fvance@airgun.wg.waii.com)

Organization: Western Geophysical, Houston

The SIGGRAPH '87 proceedings contain three articles (see biblio. below) which, taken as a whole, seem to imply that image synthesis will have to combine both ray tracing and radiosity in order to be able to accurately render images that contain many "real-world" phenomena. Two of the papers point out the difficulty of using ray tracing to render such things as atmospheric scattering and "participating media".

I have not seen any further discussion of this view (although I have not yet seen the SIGGRAPH '89 Proceedings [go easy on me, OK?]), and am wondering what other researchers, particularly die-hard ray tracers, thought of this. Can ray tracing correctly render such things without resorting to radiosity tricks? Or is the distinction between ray tracing and radiosity essentially artificial? What's your opinion?

 All below from SIGGRAPH '87 Proceedings a.k.a. Computer Graphics, July 1987,
   v.21 n.4

	Wallace, John R.; Michael F. Cohen; Donald P. Greenberg
	"A Two-Pass Solution to the Rendering Equation: A Synthesis
	 of Ray Tracing and Radiosity Methods", pp 311-320

	Rushmeier, Holly E.; Kenneth E. Torrance
	"The Zonal Method for Calculating Light Intensities in the
	 Presence of a Participating Medium", pp 293-302

	Nishita, Tomoyuki; Yasuhiro Miyawaki; Eihachiro Nakamae
	"A Shading Model for Atmospheric Scattering Considering
	Luminous Intensity Distribution of Light Sources", pp 303-310


Re: Ray Tracing & Radiosity, by Mark VandeWettering (markv@gauss.Princeton.EDU)

What I think SHOULD be implied is that normal raytracing techniques are inadequate to solve a wide variety of lighting situations, particularly those which deal with solutions to the "ambient" light contribution, diffuse interreflection, participating media, or color bleeding.

This doesn't mean that raytracing can't be used to solve problems like this. As a matter of fact, radiosity can be implemented quite simply using a raytracer rather than a zbuffer-er for the hemicube calculations. Raytracing was a part of Holly Rushmeier's participating media radiosity solution, where rays were used to perform spatial line integrals of the lighting equation. If you examine the 88 and 89 Siggraph proceedings, you will see that many researchers have shifted to raytracing-like approaches to implement radiosity solutions.

>Can ray tracing correctly render such things without resorting to radiosity
>tricks? Or is the distinction between ray tracing and radiosity
>essentially artificial? What's your opinion?

The distinctions aren't artificial, but they are subtle. For a while, radiosity meant using matrix equations to solve energy transfer between Lambertian reflectors. Later, the n^2 memory requirements were relaxed by using progressive radiosity, and the algorithm became practical and competitive with other methods. Now, integrations of raytracing and radiosity are beginning to show further improvements in both speed and the kinds of situations they cover (specular reflection). And you can be sure that there will continue to be radiosity papers in THIS years Siggraph too. (I can hardly wait!)

Raytracing is generally conceived to offer solutions to precisely the situations where early radiosity solutions failed: environments with highly specular environments. It used to be thought that raytracing was too expensive, but improvements in hardware and in algorithms have made raytracing tractable and attractive.

Now, I believe that most algorithms "of the future" will have some sort of a raytracing core to them, if not for modelling light interactions then probably just for checking visibility of points.

How 'bout anyone else? Any more ideas?

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Ray-Tracing the Torus, by Prem Subrahmanyam (prem@geomag.gly.fsu.edu)

Ok, I've contributed my quadric ray-tracing code. Now, if someone could tell me how to do the above, I would greatly appreciate it. I know it is a 4th order equation, but I have not even succeeded in locating the equation for the torus in my math textbooks (except for a spherical coordinate version--and I don't want to try to convert). Any help would be appreciated.

[this has been answered a few times already in the RT News, but I found Bob Weber's reference of interest. He also gives a taste of Pat's explanation -- EAH]


Reply from Bob Webber (webber@fender.rutgers.edu):
Organization: Rutgers Univ., New Brunswick, N.J.

For planar curves we have J. Dennis Lawrence's A Catalog of Special Plane Curves (Dover 1972) to satisfy those times when one wakes up in the middle of the night, racking one's mind trying to remember the equation for the hippopede. However, for 3-d, the best I have seen is Pat Hanrahan's A Survey of Ray-Surface Intersection Algorithms that appears in Andrew Glassner's An Introduction to Ray Tracing (Academic Press 1989). There we find, among other things, the equation for a torus as:

   (x**2 + y**2 + z**2 - (a**2 + b**2))**2 = 4 a**2 (b**2 - z**2)

This describes a torus centered at the origin defined by a circle of radius b being swept along a center defined by a circle of radius a. It is derived from considering the intersection of a plane with a torus that yields the two circles:

   ((x - a)**2 + z**2 - b**2)((x + a)**2 + z**2 - b**2) = 0

[if you are unfamiliar with this construction, it is worthwhile pausing here and savouring how this equation actually works -- sometimes the equations are prettier than the pictures] and then spinning this intersection by replacing x**2 with x**2 + y**2 (after some algebraic simplification, which converted the above to:

     (x**2 + z**2 - (a**2 + b**2))**2 = 4 a**2 (b**2 - z**2)

). The section includes a reference to an unpublished 1982 note by Hanrahan entitled: Tori: algebraic definitions and display algorithms. The general scheme for a number of variations on the torus is to start with a quartic curve of the form f(x**2,y)=0 and then substitute x**2+y**2 for x**2 and z for y.

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Need Help on Superquadrics, by Wayne McCormick (wayne@cpsc.ucalgary.ca)

A few months ago I read some articles on superquadrics here on the net. It interested me and I decided to try to implement a modeler based on superquadric shapes. Since the inside-outside functions are so easy to use in determining intersections and so forth I thought it would be somewhat easy to do. But I stumbled into a small problem.

Parametrically, a superellipsoid is defined by

	x = c(u,e1) * c(v,e2)
	y = c(u,e1) * s(v,e2)
	z = s(u,e1)

where -pi <= u <= pi, -2pi <= v <= 2pi, and c(u,e1) = cos(u)^e1, s(u,e1) = sin(u)^e1. O.K., this is the easy part. By varying u and v through the ranges we generate a bunch of points on the surface of the ellipsoid. But, the only place that the functions are defined for real numbers is in the positive octant because once the sin or cos function becomes negative and e1 and/or e2 are not integers, the function moves out into the complex plane.

Then I tried to calculate everything in the complex plane. There are two problems here. 1) speed, 2) how do you map back to image space?

Then in Franklin and Barr's paper on "Faster calculation of superquadric shapes", they say that using an explicit equation and reflecting 47 times is much faster. Sure I can see that, but the patch that is generated by the explicit equation is small and odd shaped, and what 47 directions does one have to reflect it?


From Robert Skinner (robert@texas.esd.sgi.com)
Organization: Silicon Graphics Inc., Entry Systems Division

I'm going to suggest this without poring over the references, so I'll apologize ahead of time:

Try using the same identities for c(u,e) and s(u,e) as for sin() and cos():

	c(-u,e) == c(u,e)
	s(-u,e) == -s(u,e)
	s(pi/2 - u, e) = c(u,e)
	c(pi/2 - u, e) = s(u,e)

you can make this restriction 0 <= u,v <= pi/2 and solve only the easy cases. This also means that you only have to compute 1/4th of u's range, and 1/8 of v's range, a reduction of 32. Define your basic patch over the range above, then define what the other ranges would be in terms of that:

	0 <= v <= pi/2
	-pi/2 <= u' <= 0		(i.e. u' = -u)

 	x' = c(u',e1) * c(v,e2) = c(u,e1) * c(v,e2) = x
 	y' = c(u',e1) * s(v,e2) = c(u,e1) * s(v,e2) = y
 	z' = s(u',e1) = -s(u,e1) = -z

so just reflect your basic patch by -1 in the Z to draw this one.

Repeat for all other sections of the total range. This should work, but it looks like you only get 32 sections, not 48.

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Ray Tracing Penumbral Shadows, Prem Subrahmanyam (prem@geomag.gly.fsu.edu)

Organization: Florida State University Computing Center

I would like to hear how people who have done the above have succeeded at this. Presently, I am working with DBW_Render which uses the following basic algorithm.

Find the direction to the light source and determine distance to this source (for inverse square shading). Now, create a random unit vector and scale this into the direction to light vector using the radius of the light source as the scaling factor. Test this new vector for shadows, etc.

This generates very poor shadows except when the anti-aliasing is turned way up (6 or greater rays per pixel) since we are either in shadow or not (no in-betweens). Does anyone else have any usable suggestions as to how this can be done where we vary the amount of light depending on how much in shadow it is (short of firing multiple rays at the light source--pretty much the same as anti-aliasing).

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Ray with Bicubic Patch Intersection Problem, Wayne Knapp (wayneck@tekig5.PEN.TEK.COM)

Organization: Tektronix Inc., Beaverton, Or.

Time for a hard problem. Anyone have a great idea on how to compute the a ray intersection with a general bicubic patch? The methods I've found in papers so far tend to be very slow. Seems like most papers take one of two approaches:

    1. Sub-divide the patch in many small polygons and ray-trace that.  Works
       but when you have thousands of patches you can end up with millions of

    2. An Iterative numerical approach, chosing a x,y,z point on the ray and
       checking to see if it intersects the patch by using the x,y,z values
       in the system of equations given by the four cubic equations forming
       the patch.  This of coarse normally requires many trys.

Does anyone have any better ideas?


John Peterson (jp@Apple.COM)
Organization: Apple Computer Inc., Cupertino, CA

I did my MS thesis on comparing techniques #1 and #2. #1 was definitely the winner, both in terms of speed and robustness. #2 requires root finding, which can have convergence problems (not finding a root, finding the wrong root, etc). Also, it performs the surface evaluation (which is expensive) in the very inner loop of the ray tracing process where it is executed literally billions of times for a complex image.

Reducing to polygons first allows the ray tracer to deal strictly with simple, fast and stable linear math. It also does the surface evaluation (to generate the polygons) only once as a pre-process. Once the polygons are generated, there are several very effective schemes for reducing the ray-surface search problem for large quantities of small, simple primitives (e.g., octrees, bounding volume trees, 5D space, etc).

For the gory details, see "PRT - A System for High Quality Image Synthesis of B-Spline Surfaces", MS Thesis, University of Utah, 1988.


Lawrence Kesteloot, (lkestel@gmuvax2.UUCP)
Organization: George Mason Univ. Fairfax, Va.

Check out the book "An Introduction to Splines for use in Computer Graphics & Geometric Modeling", by Richard H. Bartels, John C. Beatty, and Brian A. Barsky. (Morgan Kaufmann Publishers, 1987). It has a section entitled Ray-Tracing B-Spline Surfaces (p. 414). It goes into several steps to speed up the intersection:

(I have not read this yet. I'm summarizing as I read.)

   1.  Refinement Preprocessing - This breaks the image down into many smaller
	 splines.  Each spline covers several 100 pixels.

   2.  Tree Construction - Break the new (smaller) spline into a bunch of
	 boxes, starting with one box for the whole spline, then break that
	 down (put all this into a tree).  Intersection with boxes is easy.
	 You can find out which of these boxes (check only the leaves of the
	 tree) intersects the ray.  This will give you the starting point for
	 Newton's iterations.

   3.  Do newton's iteration to find the exact distance.

I'm sorry if I've made any errors in the above description. You're going to have to get the book, of course, to implement it. I'm going to implement it in my own ray-tracing program in the next few weeks, so I'll post the source if anyone is interested. It seems like a complicated algorithm, but it may speed things up quite a bit. [I never did see the source posted - EAH]


Mark VandeWettering (markv@gauss.Princeton.EDU) writes:

Well, there is another solution to this problem which people haven't fleshed out a great deal: generate triangles and raytrace those.

I hear groans from the audience, but let me put forth the following reasons:

1. ray/bicubic patch intersection IS floating point intensive. The best figures I have seen quote around 3K floating point operations per ray/patch intersection. I have a hard time believing you couldn't do better with a good hierarchy scheme + good triangle code.

2. Even if you can convince me that your bicubic patch intersector worked faster than my combination, dicing into triangles is very simple and easy to implement. The same code could also be used to generate views of nearly any parametric surface with minimal modification.

There are (of course) some difficulties. You would like to subdivide appropriately, which means being careful about silhouette edges and shadow edges. Barr and Von Herzen had an excellent paper in the 1989 siggraph to illustrate how to implement subdivision. You might want to check it out.

(Of course, all this is moot, cuz I never HAVE managed to implement real ray/patch intersection code)


Thomas Williams ({ucbvax|sun}!pixar!thaw) replies:
Organization: Pixar -- Marin County, California

Another problem which I haven't seen solved is subdivision for reflections or transmissions which magnify the surface intersection of this secondary ray. For example, what is a suitable subdivision algorithm for surfaces seen through a magnifying glass? Adaptive techniques that use gradient differentials can generate gillions of unneeded polygons. Also the continuity you lose by approximating surfaces with triangles for curved objects with more than one transmitting surface (like a bottle, or thick glass) can cause some pretty horrible artifacts. If it is important to avoid these problems the only way I know that you can do it it with ray-surface intersection.


Mark VandeWettering (markv@gauss.Princeton.EDU) then replies:

The problems you list are legitimate. However, I would counter with the following arguments:

1. How often do scenes which have magnifications through reflection or refraction REALLY occur. The answer to this question for me was: never. Much more difficult is to solve problems with shadow edges, which can project edges which are irritatingly linear. Two things will help soften/alleviate problems which shadow edges:

	a.	using distributed raytracing to implement penumbras.
		fuzzy boundaries can be more piecewise without causing
		noticeable effects.
	b.	We can help eliminate artifacts by treating light sources
		specially, and subdividing on silhouettes with respect
		to the light source as well as the eye.

2. Remember, your screen has on the order of 1M pixels. Each patch will probably cover only a vanishingly small fraction of these pixels. If a patch covers 100 pixels, any subdivision beyond 10x10 is probably overkill. Use expected screensize as a heuristic to guide subdivision.


Thomas Williams ({ucbvax|sun}!pixar!thaw) then replies:

Don't forget problems with areas of high curvature. Especially animated sequences where specular highlights "dance" on sharply curved edges. A hybrid approach might work well but, you had better have a _lot_ of memory for all the polygons you generate. Thrashing brings the fastest machines to their knees. So, I still think there is a place for ray-surface intersections.

Of course, I guess which approach you take depends on the audience your playing to.

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Rendering Intersecting Glass Spheres, John Cristy (cristy@eplrx7.uucp)

Organization: DuPont Engineering Physics Lab

I am (still) looking for a renderer (raytracer, radiosity, scanline, etc.) that can render two intersecting semi-transparent glass spheres and realistically show the area of intra-penetration. I have been looking for a couple months now and have not found a renderer that does this well (or at all). Suggestions of public domain or commercial renderers that solve this problem was welcome. Please send Email to cristy@dupont.com. Thanks in advance.


Craig Kolb (craig@weedeater.uucp) replies:
Organization: Yale University Department of Mathematics

>To accurately model nested objects (for example, if your two
>spheres had different indices of refraction), you also need to
>maintain a stack of refraction indices, since you can't assume that
>when you exit an object, you exit into `air'.

Rayshade does exactly this. But there are still a couple of problems with rendering intersecting transparent objects. First, the renderer needs to keep track of solid body color in order to achieve the proper "filtering" effect of, say, white light passing through green glass and then blue glass.

The second and more fundamental problem is how to treat the volume corresponding to the intersection of the two solids. Given that solids A and B each have a set of properties (solid body color, index of refraction, etc.), what properties should be used in rendering the volume (A ^ B)?

Doing The Right Thing means resorting to CSG techniques so that one can specify the properties of (A ^ B) as well as those of A and B.

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DKBPC Raytracer, Tom Friedel (tpf@jdyx.UUCP)

Organization: JDyx Enterprises (Atlanta GA)

The guys on Compuserve seem to have endorsed a (new) ray tracer called DKBPC, which is available as source. It appears to support CSG and Textures from what little I've seen. Has anyone evaluated it (i.e., compared it to rayshade, vort, mtv, etc.) Is it archived anywhere?

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New release of IRIT solid modeller, Gershon Elber (gershon@cs.utah.edu)

Organization: University of Utah CS Dept

IRIT is a polygonal C.S.G. based solid modeller originally developed on and for the IBM PC family of computers. Version 1.x has been released about a year ago for the IBM PC only. Since then, it has been ported to unix environment (SGI Irix 3.2 and BSD4.3) using X11, and all known bugs has been fixed.

This is release 2.x of the solid modeller and its accompanying utilities which include a data viewing program (poly3d), hidden line removal program (poly3d-h) and simple renderer (poly3d-r). Thanks to Keith Petersen, all the sources (Ansi C) for these programs (and executables for the IBM PC) are available on simtel20.arpa, directory PD1:MSDOS.IRIT :

IRIT.ZIP        Full CSG solid modeller, arbitrary orientation
IRITS.ZIP       Turbo C ver 2.0 sources for IRIT
IRITLIBS.ZIP    Libraries for IRIT sources
POLY3D.ZIP      Display 3D polygonal objects, part of IRIT
POLY3DS.ZIP     Turbo C ver 2.0 sources for POLY3D
POLY3D-H.ZIP    Create hidden line removed pict., part of IRIT
POLY3DHS.ZIP    Turbo C ver 2.0 sources for POLY3D-H
POLY3D-R.ZIP    Render poly data into GIF images, part of IRIT
POLY3DRS.ZIP    Turbo C ver 2.0 sources for POLY3D-R
DRAWFN3D.ZIP    Display 3D parametric surfaces, part of IRIT
DRAWFN3S.ZIP    Turbo C ver 2.0 sources for DRAWFN3D

All above sources are for the unix system as well, but DRAW*.ZIP which has not been ported (MSDOS only). In order to unpack ZIP archives in unix environment you will need to ftp from directory PD3:MISC.UNIX the file UNZIP30.TAR-Z.

[list of changes deleted - EAH]

Elber Gershon gershon@cs.utah.edu 918 University village Salt Lake City 84108-1180 Tel: (801) 582-1807 (Home) Utah Tel: (801) 581-7888 (Work)

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Easier Use of Ray Tracers, Philip Colmer, Mark VandeWettering, Jack Ritter

Philip Colmer (pcolmer@acorn.co.uk) writes:

As someone who has used QRT and MTV, and is about to try RayShade, could I make a couple of suggestions to the authors of these packages, and any other budding ray tracer programmers.


Mark VandeWettering (markv@gauss.Princeton.EDU) answers Philip's points:

In article <...> pcolmer@acorn.co.uk (Philip Colmer) writes:

>1. Please try and provide an option to produce a quick and dirty outline
> image. This would not do any reflections, shadows or any of the other
> time consuming elements of ray tracing. Instead, it would just show
> where the objects are. This would allow the basic picture to be checked
> for accuracy. Not everyone can cope with visualizing a 3D world!

	Yeah, this should be configurable from within the data file,
	or via command line options.  Things like raydepth and stuff are
	not run-time configurable on MTV.

>2. MTV has a very nice colour database (pinched from X11). How about a
> similar database for materials, ie just what ARE the parameters that
> should be given for metal, glass and so on?

	Well, colors are a little easier than things like metals.  We should
	actually shift from an RGB representation of color to a more
	realistic wavelength model, and then convert.  Somewhere I have a list
	compiled by Andrew Glassner of reflection curves for a number of
	materials.   Perhaps these will work their way into Son of MTV.

>3. How about a fixed point integer system? This would make ray tracers go
> one hell of a lot faster, but I'm not sure if this is a viable option.

	Guess what folks, this probably won't help.  Mainly because modern
	machines are spending alot more time to do fp multiplies than integer
	multiplies.  Note that on a machine like the i860, a double precision
	multiply can be done every two cycles.  A 32 bit integer multiply
	takes between four and eleven.  Net result: You lose.
	Similar things happen with the MIPS R3000.

	Another big lose for most machines, using single precision fp in
	C.  Doesn't help one iota in speed for every machine I tested, and
	hurt the accuracy.


Jack Ritter (ritter@versatc.versatec.COM)
Organization: Versatec, Santa Clara, Ca. 95051 Summary: speed up tricks for approx ray tracer

In article (2413@acorn.co.uk) pcolmer@acorn.co.uk (Philip Colmer) writes:

>1. Please try and provide an option to produce a quick and dirty outline
> image.

I have an option to scale the scene into an arbitrary NXM pixel area. For initial renderings, where I just want to the overall effect, & make sure objects don't penetrate each other, I have found that even a 30X30 pixel rendering is revealing. 30X30 sure beats the hell out of 512X512, or whatever these darn kids are using these days.

I also bound objects in screen space, which makes things very fast when you're not doing reflections & refractions. Some fairly complex scenes have taken under a minute with all these trick in use. Screen space bounding is described in the upcoming book "Graphics Gems", which will no doubt also contain many other speed-up tricks that I will wish I had thought of.

>3. How about a fixed point integer system? This would make ray tracers go
> one hell of a lot faster, but I'm not sure if this is a viable option.
> Guess what folks, this probably won't help. Mainly because modern
> machines are spending alot more time to do fp multiplies than integer

Yes, the latest processors have on-chip floating point, and are fast. However, on the processors I have used: Motorola 68000, 68010, 68020, I have found that well thought-out fixed point code always beats the floating point coprocessor, algorithm for algorithm.

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Raytracer Glass, F. Ken Musgrave (musgrave-forest@CS.YALE.EDU)

Organization: Yale University Computer Science Dept, New Haven CT 06520-2158

In article <...> pwh@bradley.UUCP writes:
>What are the spectral properties of glass
>that I could use in a raytracing program?
>I've a friend who's been working on the problem
>for a while now, and it's given some interesting results,
>but nothing that actually looks like glass....

  Glass is not so easy to do - I got a Master's degree for doing it!

Three things are necessary: (1) The proper index of refraction (1.5-1.9). (2) The proper reflection function - Fresnel's Law. (3) Dispersion. Also, you should propagate rays spawned by total internal reflection - many ray tracers quash such rays outright; this can lead to ugly artifacts in (glass) objects with planar surfaces.

The first two things can be standard features in a ray tracer, the third is uncommon. There are two published solutions (that I know of):

	Thomas, S. W., "Dispersive Refraction in Ray Tracing", Visual
	Computer, vol. 2, no. 1, pp 3-8, Springer Int'l, Jan. '86

	Musgrave, F. K., "Prisms and Rainbows: a Dispersion Model for
	Computer Graphics", Proceedings of the Graphics Interface '89,
	London, Canada, June '89

Neither of these references is easy to get. Perhaps UC Santa Cruz would provide a copy of my thesis:

	Musgrave, F. K., "A Realistic Model of Refraction for Computer
	Graphics", Master's Thesis, UC Santa Cruz, Santa Cruz CA, Sept. '87

As an alternative, I will put the troff sources for my GI paper where you can get them via anonymous ftp on weedeater.math.yale.edu - but you won't get any of the nice illustrations.

At any rate to get dispersion into a ray tracer requires some hacking, and will in general slow down the rendering a *lot*. Thomas & I used quite different approaches; his would probably be faster for scenes without much dispersion, and vice-versa.

A future version of Craig Kolb's RayShade may feature dispersion... (I'm not at liberty to distribute my ray tracer with dispersion.)


Michael A. Kelly (mkelly@comix.cs.uoregon.edu) replies: [Organization: Department of Computer Science, University of Oregon]

In article (8600001@bradley) pwh@bradley.UUCP writes:
>What are the spectral properties of glass
>that I could use in a raytracing program?

Try "Color Science" by Wyszecki & Stiles (1982). I don't have the book with me but I'm pretty sure it has the information you need.

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Ray Intersection with Grid, Rick Speer (speer@boulder.Colorado.EDU)

Organization: University of Colorado, Boulder

In article 12598 of comp.graphics, aiadrmi@castle.ed.ac.uk (Alasdair Donald Robert McIntyre) writes:

> I am trying to raytrace rippling water and need to solve the following
> problem:
> Given a surface defined by heights on an square grid, find the
> closest intersection of a ray with the surface thus defined.
> I wonder if anyone knows of an efficient method to do this?
> Replies by mail, or to the net.
> Thanks in advance

You might check the following-

1. "Shaded Display of Digital Maps", by S. Coquillart and M. Gangnet in IEEE Computer Graphics and Applications V. 4 No. 7 (July 1984), p. 35-42.

2. "Vectorized Procedural Models for Natural Terrain: Waves and Islands in the Sunset", by N. Max in Computer Graphics V. 15 No. 3 (Proceedings of SIGGRAPH '81), p. 317-324.

These should give you some good ideas.

[My own two cents: Also look at "The Synthesis and Rendering of Eroded Fractal Terrains" by F. Kenton Musgrave, Craig E. Kolb, and Robert S. Mace, SIGGRAPH 89. Towards the end they describe their method to ray trace height fields. - EAH]

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Database for DBW-Render, by Prem Subrahmanyam (prem@geomag.fsu.edu)

Organization: Florida State University Computing Center

Ok, here is a description file for a trio of balloons over reflective water with fractal mountains in the background. It should be pretty interesting.

& 0 400
R 24.0
a .5
b .8 .4 .4
e 0 10 100  0 -5 -200  0 1 0
w  0 0 -200  7 .1 1 0.00
w 0 0 0  5 .2 1 0.00
w 0 0 200  20 .4 1 0.50
w 200 0 0  2  .1  1  1.00
w -200 0 0  10 .15  1 1.00
w 50 0 0  6 .2 1 0.00
w 30 4 60  15 .3 1 .75
l  1 1 1  2 10 5
g .5 0 .8  15 15
f 4  0.1 0.5 0.7  3
f 4  .7 .6 .6  3
f 4  .5 .5 .7  3
{s 50 .2 0 1  0 0 0  .3 .3 .3  .6 .8 .2  0 29 0  10
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 t 50 .2 0 1  0 0 0  .1 .1 .1  .6 .87 .2  3.7 14 1.2 -1.4 0 2  3.6 10.2
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{q 3 .5 0 1  0 0 0  .1 .1 .1  .3 .4 .5  -3.7 6 1.2  3.7 0 2.7  0 -5 0
 q 3 .5 0 1  0 0 0  .1 .1 .1  .3 .4 .5  0 6 3.9  3.7 0 -2.7  0 -5 0
 q 3 .5 0 1  0 0 0  .1 .1 .1  .3 .4 .5  -3.7 6 1.2  2.9 0 -2.7  0 -5 0
 q 3 .5 0 1  0 0 0  .1 .1 .1  .3 .4 .5  -.8 6 -3.9  4.5 0 2.7  0 -5 0}}

 r 4 0 .55 1  0 0 0  0 0 0  .1 0 0   0 1 0   1 0 0  0 0 1  0  200

{s 70 .2 0 1  0 0 0  .3 .3 .3  .5 0 .2  20 30 -10  10
{t 70 .2 0 1  0 0 0  .1 .1 .1  .5 0 .2  11 25.2 -10  1.7 0 5.3  5.3 -10.2 1.2
 t 70 .2 0 1  0 0 0  .1 .1 .1  .5 0 .2  12.7 25.2 -4.7  4.5 0 3.3  5 -10.2
 t 70 .2 0 1  0 0 0  .1 .1 .1  .5 0 .2  17.2 25.2 -1.4  5.6 0 0  2.8 -10.2
 t 70 .2 0 1  0 0 0  .1 .1 .1  .5 0 .2  22.8 25.2 -1.4  4.5 0 -3.3  -.5 -10.2
 t 70 .2 0 1  0 0 0  .1 .1 .1  .5 0 .2  27.3 25.2 -4.7  1.7 0 -5.3  -3.6
-10.2 -4.1
 t 70 .2 0 1  0 0 0  .1 .1 .1  .5 0 .2  17.7 15 -6.8 -1.4 0 -2
-5 10.2 2
 t 70 .2 0 1  0 0 0  .1 .1 .1  .5 0 .2  20 15 -6.1 -2.3 0 -.7  -2.8 10.2 4.7
 t 70 .2 0 1  0 0 0  .1 .1 .1  .5 0 .2  22.3 15 -6.8  -2.3 0 .7  .5 10.2 5.4
 t 70 .2 0 1  0 0 0  .1 .1 .1  .5 0 .2  23.7 15 -8.8 -1.4 0 2  3.6 10.2
{q 0 1 0 1  0 0 0  .1 .1 .1   .1 .1 .1  20 15 -6.1  .4 0 0  0 -8 0
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 q 0 1 0 1  0 0 0  .1 .1 .1   .1 .1 .1  19.2 15 -13.9  .4 0 0  0 -8 0}
{q 3 .5 0 1  0 0 0  .1 .1 .1  .3 .4 .5  16.3 7 -11.2  3.7 0 2.7  0 -5 0
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 q 3 .5 0 1  0 0 0  .1 .1 .1  .3 .4 .5  19.2 7 -13.9  4.5 0 2.7  0 -5 0}}

{s 5 .2 0 1  0 0 0  .3 .3 .3   0 .5 .8  -30 40 -20  10
{t 5 .2 0 1  0 0 0  .1 .1 .1   0 .5 .8  -39 35.2 -20  1.7 0 5.3  5.3 -10.2 1.2
 t 5 .2 0 1  0 0 0  .1 .1 .1   0 .5 .8  -37.3 35.2 -14.7  4.5 0 3.3  5 -10.2
 t 5 .2 0 1  0 0 0  .1 .1 .1   0 .5 .8  -32.8 35.2 -11.4  5.6 0 0  2.8 -10.2
 t 5 .2 0 1  0 0 0  .1 .1 .1   0 .5 .8  -27.2 35.2 -11.4  4.5 0 -3.3  -.5 -10.2
 t 5 .2 0 1  0 0 0  .1 .1 .1   0 .5 .8  -22.7 35.2 -14.7  1.7 0 -5.3  -3.6
-10.2 -4.1
 t 5 .2 0 1  0 0 0  .1 .1 .1   0 .5 .8  -32.2 25 -16.2 -1.4 0 -2
-5 10.2 2
 t 5 .2 0 1  0 0 0  .1 .1 .1   0 .5 .8  -30 25 -16.1 -2.3 0 -.7  -2.8 10.2 4.7

 t 5 .2 0 1  0 0 0  .1 .1 .1   0 .5 .8  -27.7 25 -16.8  -2.3 0 .7  .5 10.2 5.4
 t 5 .2 0 1  0 0 0  .1 .1 .1   0 .5 .8  -26.3 25 -18.8 -1.4 0 2  3.6 10.2
{q 0 1 0 1  0 0 0  .1 .1 .1   .1 .1 .1  -30 25 -16.1  .4 0 0  0 -8 0
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 q 0 1 0 1  0 0 0  .1 .1 .1   .1 .1 .1  -33.7 25 -18.8  .4 0 0  0 -8 0
 q 0 1 0 1  0 0 0  .1 .1 .1   .1 .1 .1  -30.8 25 -23.9  .4 0 0  0 -8 0}
{q 3 .5 0 1  0 0 0  .1 .1 .1  .3 .4 .5  -33.7 17 -18.8  3.7 0 2.7  0 -5 0
 q 3 .5 0 1  0 0 0  .1 .1 .1  .3 .4 .5  -30 17 -16.1  3.7 0 -2.7  0 -5 0
 q 3 .5 0 1  0 0 0  .1 .1 .1  .3 .4 .5  -33.7 17 -18.8  2.9 0 -2.7  0 -5 0
 q 3 .5 0 1  0 0 0  .1 .1 .1  .3 .4 .5  -30.8 17 -23.9  4.5 0 2.7  0 -5 0}}
 x 60 0 0 1  0 0 0  .1 .1 .1  .4 .4 .4  -100 0 -170  0 30 -200  100 0 -170
 x 61 0 0 1  0 0 0  .1 .1 .1  .4 .4 .4  -50 0 -170  -150 50 -132  -180 0 -85
 x 62 0 0 1  0 0 0  .1 .1 .1  .4 .4 .4  50 0 -170  160 30 -132  180 0 -85
 k .8 0 .9  5 5 5  0 0 0

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Eric Haines / erich@acm.org