Ray Tracing News

"Light Makes Right"

August 29, 1989

Volume 2, Number 5

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

All contents are copyright (c) 1989, 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 Mother of all Ray Tracing Pages.


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


Now that the "RT News" is posted on comp.graphics, I'd like to make a few changes. First of all, you really don't need to get on the subscription list of the RT News if you're an avid reader of comp.graphics. However, I do want to maintain an emailing list of people interested in ray tracing.

So, I'll be keeping a single address list of interested people (which I will call the "contact list"), and only some of the people on this list need to be sent copies of the RT News. Furthermore, it would be nice if various schools, etc, would make a single email address the place where they will receive the RT News. For example, Duke has "raycasting@duke.cs.duke.edu" as the address to which I should send the RT News. They also have individuals on the address list, but individual copies are not sent to them. Instead, the "raycasting" account they have remails the RT News to all people that are interested at Duke. In this way I can cut down on having issues sent out unnecessarily, of worrying about bounced mail, of maintaining a long mailing list (currently about 100 people), etc etc.

	To summarize, there are then a few different ways you can be listed.

	1) Individual subscriber - You don't read comp.graphics and want to
	   get the RT News.  Your name is put on the subscriber list and the
	   contact list.  To subscribe, simply send me your name, email and
	   snail mail addresses, and a one line summary (that I'll put next to
	   your name) describing any special areas you're interested in (e.g.
	   "parallelism, radiosity, filtering" might be one).  You might also
	   send me a few paragraphs about what you're up to nowadays, and I'll
	   include this in the News.

	2) Group subscriber - Like Duke.  This way you have full control over
	   who will automatically get an issue, and I won't have to change my
	   subscriber list each time someone else wants to get the RT News.

	   Some people have already switched over.  I just received this:

	   We went ahead and implemented the local alias.  It's "raytrace",
	   which you can reach as raytrace@cpsc.ucalgary.ca or
	   calgary!raytrace.  At the moment Dave Jevans and I (Bill Jones) are
	   the only subscribers.

	3) Contact list only - You read comp.graphics and so do not need to
	   subscribe.  However, you want to be on the list of people interested
	   in ray tracing.  Another advantage of the contact list is that
	   people may send you mail - for instance, I wrote all the people on
	   the list and invited them to come to a get-together for ray tracers
	   at SIGGRAPH (more on this later).  Everyone who is a subscriber is
	   also automatically on the contact list, but not vice versa.  You
	   can also be listed as an individual on the contact list if you're a
	   group subscriber.

	4) The silent majority - You don't need to be on any list, and are
	   happy just reading comp.graphics (or have hit the "n" key by now).

Currently almost everyone on the contact list is also a subscriber. So, if you read comp.graphics fairly constantly and are on the subscriber list, please tell me to put you on only the contact list. Clear as mud? Good. I'll probably send the above to all people asking for subscriptions just to make sure they know what's what, so don't be offended if you get one.

Whew! Well, with that done, I should mention that anyone can ask for a copy of the contact list. If you want to subscribe to the RT News, hardcopy edition, that's another coastline entirely - contact Andrew Glassner at glassner.pa@xerox.com for details, as he's the editor of that one. The email and hardcopy journals are mostly non-overlapping, so it's worth your while to get both if you're seriously interested in the subject.

I can't afford to send out the back issues of the email RT News, but they are available via anonymous FTP from:

	cs.uoregon.edu - in /pub, also has lots of other ray tracing stuff, etc
	freedom.graphics.cornell.edu - in /pub, also has Xcu menu creator

One other resource: I've been updating Paul Heckbert's ray tracing bibliography while he's been busy at grad school. If you would like the latest copy of this list, or have anything to add or change in it, just write me. I also will be posting it to the two ftp sites realsoonnow.

back to contents

A SIGGRAPH Report, by Eric Haines

Another SIGGRAPH has come and gone, and I had a good time. My only frustration was meeting many researchers for a minute and not getting to talk with them. I noticed that although the ray tracing session had but three papers, there were about nine others throughout the proceedings that used ray tracing techniques in various ways. Ray tracing (and ray casting) as a graphics tool has come into its own.

A few more ray tracers are out on the market, such as "Sculpt 3D" by Byte by Byte for the Macintosh and Amiga and "LazerRays" by Lazerus. Hewlett Packard is now shipping radiosity and ray tracing software bundled in with every high-end graphics workstation. Intergraph has been getting a fair bit of airplay out of their new ray tracing package, though I haven't gotten details yet. Alias should be offering a ray tracer sometime soon.

Ray tracing researchers got together informally for a "ray tracing roundtable" for an hour and some. Like last year, it was a large gathering, with around 50 people attending. We went around the group, with each person giving a brief introduction. I took an informal poll on a few questions, Andrew noted that the ray tracing book was out, and asked for contributions to "Graphics Gems" (more later), then we broke up and talked about this and that.

Given the size of the gathering, it was a bit frustrating: there are all these people that I've wanted to meet and talk with in the same room, and after half an hour they're all gone and I've spoke with only a few. Basically, the roundtable meeting is too big for my tastes. One possibility that you all might consider is to invite people in your own special interest to a lunch or dinner. For example, I went to a pleasant "global illuminators" lunch this SIGGRAPH, where there were about twelve people - a good size.

Oh, some nice books came out this SIGGRAPH. I'll plug the ray tracing book later. Others worth note (i.e. I bought them) are _Mathematical Elements for Computer Graphics, Second Edition_ by Rogers and Adams, which has been expanded to more than twice its original length, and _The RenderMan Companion_ by Steve Upstill, which looks like a nice book no matter what your feelings on RenderMan itself. I'm looking forward to the much expanded second edition of Foley & Van Dam's _Fundamentals of Interactive Computer Graphics_, which should be out early next year.

back to contents

Ray Tracing Poll, from the roundtable discussion

At the roundtable people were polled on a few questions.

1. What efficiency schemes have you used?

	Grids			   - 20
	Octree/BSP		   - 26 (4 were specifically BSP)
	Bounding Volume Hierarchy  - 22
	Hybrid			   - 17
	Greater than 3D (4D or 5D) - 13
	Other			   -  5 (what were these, anyway?)

2. How many processors have you used?

	One processor		   - 21.1
	Multiprocessor		   - 29.1
	More than 10 processors	   - 21
	More than 100 processors   -  4

	The most processors used was 1024 (2 people).

3. How long do you usually wait for a picture?

	Less than a minute	  -  1
	Less than ten minutes	  -  6
	Less than an hour	  - 13
	Less than ten hours	  - 25

4. What is your favorite background color?

	UNC "Carolina Blue"	  - 12
	Black			  - 15

back to contents

_An Introduction to Ray Tracing_, Announcement and Errata

Well, the book is finally out. It is edited by Andrew Glassner, and sells for $49.95 from Academic Press. It includes sections by Andrew, Pat Hanrahan, Rob Cook, Paul Heckbert, Jim Arvo, Dave Kirk, and myself. The book is essentially the same as the 1988 SIGGRAPH Course Notes, with the addition of some figures and images and some minor corrections. Incidentally, if you have the book and find any glaring errors, please notify Andrew Glassner; enough copies sold at SIGGRAPH that another edition will be printed soon. Considering that the authors are scattered around the country, the book flows fairly well and covers most areas of ray tracing theory and practice. Also, there's finally a text to suggest when someone wants to know whether a point is inside a polygon (Sedgewick's _Algorithms_ didn't solve it efficiently). If you don't have a copy, buy a few and make us all fabulously wealthy beyond our wildest dreams (i.e. then I could afford the Sears' hibachi instead of the K-Mart version).

Incidentally, the 1989 "Intro to RT Course Notes" included the book and some additional notes. The notes included reprints of classic articles, the code for the SPD package (God forbid that anyone have to type it in, though), and an article I whipped off called "Tracing Tricks", which I'll post sometime soon, maybe during a slow month.

Some bugs have already been reported to me about my section, so I'll pass them on:

From Tim O'Connor:

If you'll flip to page 66 you'll note a little algorithm for ray/box testing. About halfway through you say:

	If T1 > Tnear, set T1 = Tnear
	If T2 > Tfar, set T2 = Tfar

Then you never use T1, or T2 again in that loop. The example bears out my suspicion that one should actually "set Tnear = T1" and "set Tfar = T2".

From Ehood Baratz <{world}!hpbbn!hputlaa!ehood>:

In chapter 2 page 52 on formula (C9) it is written:

		If Pn * Rd  < 0
		(in other words, if Vd > 0)
		   then ....

I think it should be "if Vd < 0" because Pn*Rd is Vd (look at page 51 after (C4)).

back to contents

_Graphics Gems_ Call for Contributions, by Andrew Glassner


Contributions are solicited for a new book, tentatively titled GRAPHICS GEMS. This book will be a collection of short notes by and for computer graphics programmers and researchers. The basic idea is to create a book similar to the CRC Mathematics Handbook, only tailored to the subject of computer graphics.

The motivation for Graphics Gems comes from a desire to document and share the many techniques that are a necessary part of every graphics programmer's toolbox, yet don't appear in the standard literature. Each Gem represents a solution to a problem: not necessarily a research result, nor even a deep observation, but simply a good, practical technique for dealing with a typical computer graphics programming problem. A typical Gem may be a code fragment, a short analysis of an interesting problem, a bit of mathematics, a data structure, or a geometric relationship.

Here are some appropriate topics for Gems - this list contains only a few suggestions for topics that might be covered by interesting Gems, and is far from complete:

Two Dimensions: Fill, smooth, blur, dither, 2d plots, line drawing, curve drawing, bounding boxes, overlapping boxes, efficient bitblit (example: automatic selection of tick marks on a plot).

Three Dimensions: Scan conversion, highlight detection, shading, isosurfaces, ray intersection, form factor calculation, visibility, texturing, transformations, deformations, smoothing, 3d plotting, parameterizations, surface subdivision, texturing functions, bounding boxes (example: fast shading formulae).

Graphics: Colormap hacking, object manipulations, sampling, filtering, optics, interaction techniques, modelling primitives, efficient rendering, edge detection (example: reconstruction from stochastic sampling).

General Math: Algebra, calculus, geometry (e.g. why normals don't move under the same transformations as surfaces).

Programming: Numerical integration, root finding, root polishing, data structures (objects), data structures (programs), inner loops, interactive debugging, graphical debugging, color map hacking, over- and under-flow detection and correction, unusual functions (e.g. polynomial root-finding).

Most Gems will be about 1 or 2 final printed pages (4 or 5 pages of typewritten, double-spaced manuscript), though if you choose to include source code the listings may run longer. Rough figures and equations will be professionally redrawn by the publisher. Each contributor will have a chance to review the final copy for his or her Gems before publication. Each Gem will be clearly identified with the name and affiliation of its contributor(s).

If you have developed a nice solution to a problem that others might encounter, be it a data structure, an inner loop, or even an algebraic simplification that makes your programs shorter and more robust, then it would probably make a splendid Graphics Gem. Write it up and send it to the editor at the address below, either in hardcopy or electronic mail. Acceptable formats are plain text, nroff, TeX, MacWrite, and Microsoft Word (Macintosh). I would like to receive a rough draft of all Gems by November 1989.

Contribute and share your favorite tricks and techniques with the rest of the community! Send your Graphics Gems to:

Andrew Glassner
Editor, Graphics Gems
Xerox PARC
3333 Coyote Hill Road
Palo Alto, CA  94304  USA
email: glassner.pa@xerox.com
phone: (415) 494 - 4467

back to contents

New People and Address Changes

There were many new people at the roundtable at SIGGRAPH. In the interest of brevity, only new people who've sent an intro are listed. If any of you (or anyone else out there) would like to send in a paragraph or two of introduction, it will be printed here. You can always request the latest contact list from me.


We just received the 7 editions of "The Ray Tracing News" you posted recently at USENET. This is exactly the kind of information we need in our research project an rendering software. Not just the latest research news, but also discussion on the results of them.

So PLEASE put us on your mailing list!

Just let me describe our past and future work to justify the costs for you. We are a working group at the Institute for Interactive Graphic Systems at the University of Tuebingen (West Germany) and are part of the faculty of physics. The main future research aspect will go in direction of combining raytracing and radiosity. We come from the background of geometric modelling and graphics hardware (here Phong shading in realtime). I will start working on this project 4Q89 as my PhD.

If you can give any additional information that might be interesting to us (including other research going on in this area) please let us know.

Surface mail: Wilhelm-Schickard-Institut fuer Informatik
	      Graphisch Interaktive Systeme
	      z.Hd. Philipp Slusallek
	      Auf der Morgenstelle 10
	      D-7400 Tuebingen
Email       : philipp@infotue.uucp
Tel         : x49 7071 296356


(internet)  zmel02@trc.amoco.com
(usenet)    uunet!apctrc!mlee
(snail mail)    Mark Lee
		Amoco Production Company
		Tulsa Research Center
		PO Box 3385
		Tulsa, OK  74102
	(phone) (918)-660-3556  or (918)-660-3000 for operator

A short introduction. My interests lie in the areas of illumination models, faster ray tracing, photorealism, numerical and statistical methods. My real work is more in the area of rendering algorithms and scientific visualization. The areas that I enjoy working in, I pursue whenever I can squeeze it in.

Incidentally, did you find a copy of the article by Levner, Tassinari, and Marini, "A Simple Method for Ray Tracing Bicubic Surfaces"? [no] Is this in a textbook or such? How could I find of copy of this paper? [Can anyone help? Has anyone seen it, and can at least summarize?]

Some questions to post to the mailing list...

Did the paper "The Ray Tracing Kernel" by Jim Arvo, Dave Kirk and Olin Lathrop ever get published?

Does anyone have a copy of Blinn's notes from the Siggraph '84 tutorial notes on "The Mathematics of Computer Graphics" that they could send to me? [hey, I'd like one, too: I was a student volunteer at this course, and they ran out of course notes and I never got a copy.]


I am now doing a dissertation on realistic rendering for complex scenes, with extensions for nonisotropically scattering gasses. I am also part of a project that uses raytracing for visualization of volumes of scalar data.

Peter Shirley
University of Illinois at Urbana-Champaign

	816 E. Oakland, #206
	Urbana, IL  61801
	(217) 328-6494


Mike Sweeney - rendering, splines, object-oriented languages Softimage Inc. 3510 Blvd St. Laurent, Suite 214 Montreal, Canada H2X 2V2 (514) 845-1636 [write to Kaveh Kardan at larry.mcrcim.mcgill.edu!vedge!kaveh to reach Mike]

Hello net-land, it's been a while. Eric says he wants an introduction so here goes....

I've been writing renderers for the last 6 years. The Waterloo CGL Raytracing Package was your basic naive ray tracer. It did contain an iterative solution to ray/bspline intersection, but I no longer believe in this approach - it's much faster to break the spline into triangles. My second attempt, the Alias renderer, war a raycaster. It was slow period.

Then came Abel, where I started playing with octrees. The code was about twice as fast as any implementation of Kay/Kajiya slabs I could come up with, and at least ten times as fast as my implementation of Fujimoto's algorithm. After Abel folded, I wound up at Softimage. I added a modified Watkin's front end, and rewrote the tracer to make the maximum use of empty space.

I've not tried to implement the Kirk/Arvo algorithm yet, but have an instinctive distrust of anything that mixes preprocessing with the rendering (the cost will go up with the sampling rate). What is the group's experience with this method?


# Jerry Quinn - PhD research in raytracing at Dartmouth
# Dartmouth College
# Department of Math & Comp Sci
# Hanover, NH 03755
# (603) 646-2565

I am a PhD student in computer science at Dartmouth and am in my second year. I'm currently interested in increasing efficiency in raytracing. I'm also looking at parallelism in RT, radiosity, and the combination of both.


I have taken a job at Princeton, and thought I'd send you my new address for the purposes of the RT News.

Mark VandeWettering (markv@acm.princeton.edu)
c/o Program in Applied and Computational Math
Fine Hall
Princeton University, Princeton NJ, 08544

Another question you might know off the top of your head: Alvy Ray Smith has written a tech memo on Volumetric Rendering at Pixar. Do you have his e-mail address, or otherwise know how I might request Pixar Tech Memos? [does anyone else know? - EAH]

The ray tracing archive on skinner.cs.uoregon.edu still will remain there. I have permission from the higher-ups at the U of O to keep it there. If I lose that permissions at some future date, it will probably move to Princeton somewhere....


Pat Hanrahan has also moved to Princeton, and is now at:



And one more for Princeton:

alias marshall_levine mplevine@phoenix.princeton.edu

[I asked about the ray tracing demo at SGI:]

The ray-tracing demo that you saw on an IRIS at SIGGraph is called Flyray. It was written by Benjamin Garlick in June-August 1988. The underlying voxel ray-tracer was written by Paul Haeberli in July 1983. The demo is standard around here; it should be on every demo machine. I would guess that it would be in the /usr/src/cmd/demo directory on most demo machines, but it depends on that particular machine's configuration. It is easily accessible through Buttonfly, the SGI menu program. Buttonfly displays menus of demos on 3D buttons that twist, flip, and fly towards the screen when selected (with text on them!). You will probably find Flyray on Buttonfly under the SGI/CPU-Intensive button (it will be listed as Ray-Tracer or Flyray). If you have any questions or want a description of the demo, just let me know and I'll send you any information that I can dig up. While you're at SIGGraph, you should take a look at the newest version of Flight (By Rob Mace, one of the guys in my group) on the IRIS 4D computers. Take a look at the F-14. I'll let it be a surprise, and trust me, you'll be very surprised!


Please change my email address to palmer@ncsc.org (was palmer@ncifcrf.gov).

Thomas C. Palmer		North Carolina Supercomputing Center
Cray Research, Inc.		Phone: (919) 248-1117
PO Box 12732			Arpanet: palmer@ncsc.org
3021 Cornwallis Road
Research Triangle Park, NC


Another address change:

# Mark Reichert
# Program of Computer Graphics
# 120 Rand Hall
# Cornell University
# Ithaca, NY 14853
alias mark_reichert mcr@venus.graphics.cornell.edu

back to contents

Bugs in MTV's Ray Tracer, by Eric Haines

Craig Kolb mentioned that he was having problems with Mark VandeWettering's ray tracer. Since I've been touting it all this time (but having run it only once), I felt obligated to find the bugs. There were two: one was that the up vector was sometimes not perpendicular to the view vector, which results in the "balls" image having a "comin' at ya" kind of distortion (kind of interesting, but incorrect). The other was that the frustum width was affected by the distance to the hither, which it should not be. One result is that the "mountain" scene would get zoomed in on something fierce. Finally, I changed the statistics output a bit to give information that I like (e.g. the number of reflected and refracted rays actually shot are counted separately). Anyway, at least apply the fixes to main.c and the first part of screen.c.

diff old/main.c new/main.c
< 	printf("number of rays cast:	   %-6d\n", nRays);
> 	printf("number of eye rays:	   %-6d\n", nRays);
diff old/screen.c new/screen.c
< 	VecNormalize(upvec) ;
> 	 * Make sure the up vector is perpendicular to the view vector
> 	 */
> 	VecCross(viewvec, leftvec, upvec);
> 	VecNormalize(upvec);
> 	/*
< 	frustrumwidth = (view -> view_dist) * ((Flt) tan(view -> view_angle)) ;
> 	frustrumwidth = ((Flt) tan(view -> view_angle)) ;
< 			Trace(0, 1.0, &ray, color);
> 			Trace(0, 1.0, &ray, color, &nRays);
< 			Trace(0, 1.0, &ray, color);
> 			Trace(0, 1.0, &ray, color, &nRays);
< 				Trace(0, 1.0, &ray, color);
> 				Trace(0, 1.0, &ray, color, &nRays);
diff old/shade.c new/shade.c
< 		nReflected ++ ;
< 		Trace(level + 1, surf -> surf_ks * weight, &tray, tcol);
> 		Trace(level + 1, surf -> surf_ks * weight, &tray, tcol,
> 			&nReflected);
< 		nRefracted ++ ;
< 		Trace(level + 1, surf -> surf_kt * weight, &tray, tcol) ;
> 		Trace(level + 1, surf -> surf_kt * weight, &tray, tcol,
> 			&nRefracted) ;
diff old/trace.c new/trace.c
< Trace(level, weight, ray, color)
> Trace(level, weight, ray, color, nr)
>  int *nr ;
< 	nRays ++ ;
> 	(*nr) ++ ;

back to contents

Bug in SPD, from Pete Segal (pls@pixels.att.com)

Pete Segal reported a particularly subtle bug in my Standard Procedural Databases package. Turns out a "w" component needs to be initialized. If you have a machine that initializes everything to 0, you'd never notice it. The patches to the README and lib.c files are below. I should be putting the latest and greatest version on cs.uoregon.edu soon.

diff old/README new/README
< Version 2.5, as of 10/19/88
<     address: 3D/Eye, Inc., 410 East Upland Road, Ithaca, NY 14850
> Version 2.6, as of 8/28/89
>     address: 3D/Eye, Inc., 2359 N. Triphammer Rd, Ithaca, NY 14850
> Version 2.6 released August, 1989 - lib_output_cylcone fix (start_norm.w was
>     not initialized).
>     The SPD package is also available via anonymous FTP from cs.uoregon.edu.
diff old/lib.c new/lib.c
<  * Version:  2.2 (11/17/87)
>  * Version:  2.6 (8/28/89)
> 	start_norm.w = 0.0 ;

back to contents

Solid Textures Tidbit, by Roman Kuchkuda

One piece of research that you might mention in RTN:

I had used procedural wood textures for a while and wondered whether you could use "real" wood textures. Through some connections at the UNC hospital I had a block of pine CT scanned.

Sure enough, the grain showed up very well. The resulting pictures were more "interesting" than procedural texture. The structure is more complex than the procedural model.

There were a few problems though:

1) There is a lot of data in the CT scans and memory paging slows down the
   ray tracing like crazy.

2) Reality doesn't look nearly as "real" as a neat clean procedural model of it does.

The results were so mixed that I never tried to get this published anywhere.

back to contents

Sundry Comments, by Jeff Goldsmith (jeff@Iago.Caltech.Edu)

About the ray-tracing-as-a-sleazy-sales-gimmick idea:

The whole point about my suggestion to include a ray tracer as part of a CG system is that it is, in fact, mostly useless, and most buyers don't know that. Thus, it works as a great gimmick, which is exactly "promising someone something that they think they want, but don't really." No one will use it after finding out how long it takes, so it doesn't have to have as many features as the rest of the system. Yes, this is a very cynical view, but sales is a non-technical problem, and (at least around here) expensive systems are usually purchased by people who know nothing about them, so it ought to be an effective technique with not a whole lot of resource expenditure. Besides, most CG programmers enjoy hacking ray tracers so you boost morale at the company at the same time. ...by the way, this is not entirely a joke, but...

Euclidean distance calculation: Iterative approaches work great on this problem. A short summary (and code for one) is in Tom Duff's SIGGRAPH '84 Tutorial on Numerical Analysis (a terrific article, by the way, as is his spline piece in the same place) most of which he got from Moler and Morrison's "Replacing Square Roots by Pythagorean Sums" IBM Journal of Research and Development, 27/6, Nov. 1983. The method he describes has cubic convergence, so it works well to unroll the loop and do a set number of iterations. 4 iterations (2/, 4*, 2+) yield 62 digits of precision.

back to contents

Texture Mapping Question, by Susan Spach

What are good techniques for implementing texture mapping within raytracing? Is point sampling with a good sampling strategy most commonly used? Has mipmapping been extended to secondary rays? [I'm interested, too]

back to contents

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

Ray Traced Image Files, Prem Subramanyan

Reply-To: prem@geomag.UUCP (Prem Subramanyan)
Organization: Florida State University Computing Center

We have quite a collection of rasterfiles of all sizes here at geomag. You can use the fbm package by Michael Mauldin to convert from Sun rasterfiles to GIF files. The main reason why I have chosen to keep them as rasterfiles, rather than convert them to GIF is that on the Sun the rasterfile viewers are neater. In any case. anonymous ftp to geomag.gly.fsu.edu ( cd to pub/pics and download any pics you want. In the future, once I get the latest fbm package, I will post it there as well. We have a good collection of files ray-traced on the eta-10g with QRT by Steve Koren. The longest time taken (for blue.rst.Z) was 1 1/2 hrs. The small ones (640x400) went in usually under 15 minutes. In any case, they are quite interesting.

back to contents

Image Collection, by Paul Raveling

From: raveling@venera.isi.edu (Paul Raveling)
Organization: USC-Information Sciences Institute

With gobs of satisfaction, I'd like to announce the addition of some pictures of my own to the "Img" collection available on venera.isi.edu. Along with this go sincere thanks to Nic Lyons and HPLabs for the opportunity to digitize these photos.

venera's pub directory now contains 2 compressed files to facilitate retrieval of images in this collection and of the imglib code. These are:

	pub/img_ls-RAlF.Z       Directory listing of everything
				in the [~ftp/] images hierarchy

	pub/img.tar.Z           Code for imglib and the various
				simple programs using it.

The "root" directory, [~ftp/]images, and each of the four subdirectories containing images has its own README file with additional info.

Credits for 2 images that weren't from my own pictures are:

	Window Rock:    My wife spotted and captured the natural
		spiral pattern at Window Rock, Arizona.

	Solings:        This is from the 1982 International Soling
		Association calendar.  I used to own and race one
		of these, but wouldn't risk taking a camera aboard.

Here's a subject summary of the new images in images/color_mapped:

aspens          Aspens in San Juan Mts, between Ouray & Silverton
blue_tigers     Blue Angels, in F11F Tigers
ds_train        Durango & Silverton narrow-gauge train
elk             An elk in Banff National Park
ghost_house     House in ghost town, somewhere between Ouray and Silverton
graycard        Kodak gray card, grayscale, and color patches
halfdome        Halfdome in polarized infrared light (Yosemite Nat'l Park)
harvey          Harvey, an African lion who lived at the L.A. Zoo
high_line       Durango & Silverton narrow gauge train on the high line
model           Model at one of Frank's photo day shows, probably in 1978
model_fullscale Model at one of Frank's photo day shows, probably in 1978
old497          Old 497:  One of Durango & Silverton's narrow gauge steamers
porcupine       Porcupine @ ranger cabin, Mosquito Flats, Banff
puff1 - puff2   Puff (the cat, not the dragon)
san_juans       San Juan Mountains, between Ouray and Silverton
smurf1 - smurf4 Whitesmith, aka "The Smurf"  [another cat]
snake           Non-digital snake (not an adder)
solings         Solings, from 1982 International Soling Association calendar
stream          Stream in San Juan Mountains, between Ouray and Silverton
tbirds          Thunderbirds
window_rock     Window Rock
x-1e            X-1E at NASA Ames Dryden Flight Research Center, Edwards AFB

back to contents

MTV-Raytracer on ATARI ST - precision error report, by Dan Riley

From: riley@batcomputer.tn.cornell.edu (Daniel S. Riley) Organization: Cornell Theory Center, Cornell University, Ithaca NY

In article (1171@laura.UUCP) wagener@unidocv.UUCP (Roland Wagener) writes:
>I have ported the MTV-Raytracer on the ATARI ST using the Turbo-C-
>Compiler. The Program works fine and it uses about 3 hours CPU-Time
>to create the Balls-Picture in 320x200 Resolution.

>But there is a bug somewhere in the program. There are white spots in
>all reflecting surfaces. This bug does not appear on a IBM-PC programmed
>with Zortech-C++. But the PC needs 5 hours for a 200x200-Picture ...

This sounds like a floating point precision problem. I've been playing with a number of ray tracers, including MTV, on my Amiga. I've seen white spots and other sorts of splotches if I use the Motorola ffp format floating point routines, which are single precision (32 bit) only. They go away if I use IEEE math libraries (all calculations done with 64 bit doubles). 3 hours cpu for a 320x200 picture on an 8 MHz 68000 sounds like single precision to me, but I don't know the ST or Turbo-C that well.

I suppose there must be papers on controlling round-off errors in ray tracing algorithms, but none of the ray-tracers I've seen make any special efforts in that regard. Of course, all the ones I have source code to are meant to be clean and simple, not fast and convoluted...:-)

-Dan Riley (riley@tcgould.tn.cornell.edu, cornell!batcomputer!riley) -Wilson Lab, Cornell U.

back to contents

Question on Ray Tracing Bicubic Parametric Patches, by Robert Minsk

From: ccoprrm@pyr.gatech.edu.UUCP (Robert E. Minsk)
Organization: Georgia Institute of Technology

Does anyone have a routine of know of any pointers to articles to find a intersection between a ray and a bicubic parametric patch besides a recursive subdivision algorithm? I am trying to speed things up a bit in my ray tracer.

back to contents

Eric Haines / erich@acm.org