Ray Tracing News

"Light Makes Right"

July 16, 2000

Volume 13, Number 1

Compiled by Eric Haines, 1050 Craft Road, Ithaca, NY 14850 [email protected] . Opinions expressed are mine, not Autodesk's.

All contents are copyright (c) 1999-2000, all rights reserved by the individual authors

Archive locations: text version at http://www.acm.org/tog/resources/RTNews/text/
HTML at http://www.acm.org/tog/resources/RTNews/html/

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



First things first: the SIGGRAPH Ray Tracing Roundtable SIG will be held at Thursday, 6-8 pm, at the Hilton Riverside, in the Oak Alley room. It's a short walk to the Papers/Panels reception from here.

The most significant news in ray tracing this year is the publication of Peter Shirley's book, Realistic Ray Tracing. See it at AK Peters' booth (#2621) at SIGGRAPH. Briefly paging through the book, it looks clear and precise, with many helpful illustrations and figures - more about it next issue. Really, though, you should just buy it; it brings together many disparate bits of information, especially on shading, sampling, and filtering. Pete says the book's subtitle could have been, "how Pete Shirley implements a ray tracer". He's too modest: the book looks like it gives a solid education in some of the foundations of the field, and all in 161 pages. At first I thought this a bit short, but I think that's just a sign of the effect of book selling psychology: some publishers like to sell trade paperbacks that are 1200+ pages, regardless of usefulness of the text, in hopes of looking authoritative. This book focuses on equations, which is a more dependable (and compressed) way to explain something than just pure text alone (I think of it like code vs. comments; code can be terse, but it is a more reliable source for what the program actually does than the comments about the code).

Other books I'm looking forward to: Dave Eberly's Game Engine Design, (Morgan-Kaufmann Publishers) which looks to have much useful information and code in it, see http://www.magic-software.com/3DGameEngineDesign.html (also, Dave's site http://www.magic-software.com/ is a national treasure). Mark DeLoura's Game Programming Gems (no relationship to the Graphics Gems series; from Charles River Media) collection sounds interesting, judging from the articles I've heard it will contain. Mathematics for Computer Graphics Applications by Michael Mortenson (Industrial Press) is another book which I want to look at. Another I'll definitely consider is David Rogers' An Introduction to NURBS, from Morgan-Kaufmann.

Another relatively new book worth getting is Apodaca and Gritz's Advanced RenderMan book. Even if you do not have any interest in RenderPerson itself, this book has much to recommend it, both from an algorithms standpoint and in learning about techniques used in motion pictures. I've also been enjoying Visual Intelligence: How We Create What We See by Donald Hoffman. It gives an overview of results in perceptual psychology for us ignoramuses, and does so without dumbing down the material. There are also copious references to the research the author draws his text from.

In case you missed it, the coolest graphics-related thing on the web this past half year is sodaconstructor: http://www.sodaplay.com/. The most insane graphics-related thing is Donkey Kong for the Kodak Digita digital camera: http://digita.mame.net/. The best user interface ever is at http://www.cs.unm.edu/~dlchao/flake/doom/. The Fantasy Graphics League results are at http://www.realtimerendering.com/fgl/. Hardest way to earn a million dollars: http://www.ams.org/claymath/. The most useful free web service I've found this year has been http://www.spyonit.com/; set up any URL you want for it to track, and the service will email you when the page has changed. It's great for seeing when a conference schedule gets posted, when new software has become available, when an annotated links page has been updated, etc. (Thanks to Craig Reynolds for this tip; he's an absolute links fanatic: http://www.red3d.com/cwr/links.html).

The other useful goodie has been GuruNet http://www.gurunet.com/, a free service that I use only occasionally but is wonderfully unobtrusive. Don't know a word, or exactly how to spell it, or just want to know about something? Hold down the Alt key and click on it and GuruNet pops up and tells you (after a few seconds of obligatory net delay), as well as giving thesaurus, encyclopedia, news, business summary, translations, and other relevant entries for the word. What's cool is that GuruNet cooperates with every program that has text that can be selected, not just web pages, so you just have to learn a single, trivial action and it simply works everywhere.

For the first time ever, I've noticeably changed a previous issue of the Ray Tracing News. I normally avoid doing this, disliking the "1984" concept of rewriting the past (definitely easy to do with web documents). This time it made sense to do so: Vlastimil Havran sent some updates to his octree summary article in RTNv12n2, and these have been folded in.

A puzzle invented by Paul Strauss: you're given a square sheet of paper, one side is blue and the other red. You can make folds in the paper, but the rule is that only red can touch red. Red touching blue or blue touching blue will destroy the entire universe. Folds are full 180 degree foldovers, so some piece of paper will always touch another part of the paper. So, what is the maximum number of folds you can make? There are no tricks here, it's just a straightforward puzzle. Hint: the answer is > 4 and < infinity.

Last issue's puzzle was: You have a cube and you select at random three (different) corners. What is the chance that the triangle formed by these corners is acute (all angle < 90 degrees)? is a right triangle (has one angle == 90 degrees)?

A quick answer is this (from Spike Hughes): there are 8*7*6 possible triangles, and divide by 3*2 since we do not care about the order of the vertices. This gives 56 unique triangles formed. Of these, 6 have right angles at a given corner: the 3 cube-edge/cube-edge pairs and the 3 edge/perpendicular-face-diagonal pairs. Each triangle can have at most one right angle, so with 6 right triangles at a given corner, then 6*8=48 triangles must be right triangles. So 48 of 56, i.e. 6 of 7 triangles formed are right triangles. This leaves a 1 in 7 chance that a random triangle is acute. No obtuse triangles are possible, interestingly enough. Other ways of thinking about this problem include proving that acute triangles must be formed entirely of cube face diagonals, and using dot products of 0 between edges to rule out right triangles.

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Ray Tracing Roundup

I've been compiling useful formulas and equations in a single document for people involved in global illumination research. It now has 34 pages and 86 items listed:


- Phil Dutre ([email protected])

[This thing's amazing, go grab it. -EAH]


3D Object Intersection page

A page giving references a wide variety of 3D object intersection references is now available:


Also at this site is my personal list of frequently used graphics related web sites:


It's geared towards real-time rendering, but has much that applies to graphics in general.


Optimizing Ray-Triangle Intersection

If you're interested in optimized ray-triangle intersection code and statistics, then take a look at:


I have optimized my and Trumbore's code from journal of graphics tools, and tried it on three different machines and with different percentages of triangle hits. The code is also available at the above mentioned site. Let me know if you have any suggestions for improvements.

-- Tomas Moller ([email protected])


Graphics Gems Repository update

The Gems repository has been expanded in two ways. New pages have been added:

    http://www.acm.org/tog/GraphicsGems/category.html - by category
    http://www.acm.org/tog/GraphicsGems/authors.html - by author
    http://www.acm.org/tog/GraphicsGems/gems.html - in book order

In addition, each of these pages has direct links to the relevant code for each gem.


Non-Photorealistic Rendering

Craig Reynolds maintains an excellent annotated links collection on the subject of NPR:



SIGGRAPH has the beginnings of a corrigenda (i.e. corrections) site for SIGGRAPH proceedings:


If you know of an error in a SIGGRAPH article, please let Stephen Spencer ([email protected]) know.

This year's SIGGRAPH papers are listed and online versions linked from:



A mailing list dedicated to realtime raytracing now exists:

You can join this community by going to:


Or you can join by sending email to:

     [email protected]

The archives are at:


This list has had some interesting discussions (see some later in this issue), though traffic has died down in recent months. Many of the subscribers are into the demo scene (see http://www.acm.org/tog/resources/RTNews/demos/overview.htm).


The Best Efficiency Scheme (BES) Project has its own page at:


The WWW page contains updated version of the project proposal, more than 200 possible scenes in VRML'97 format of different complexity, and technical report describing the first results of the project (testing on 30 SPD scenes X 12 acceleration data structures X 4 methods of shooting rays).

By the way, we are still looking for large scale scenes (over 500,000 primitives).

-- Vlastimil Havran ([email protected])


A software tool for interactive visualization of spatial data structures was designed to verify the correctness of implementations. Several static images from that visualization are available at:


-- Vlastimil Havran ([email protected])

[This is pretty interesting to look at, in fact. -EAH]


Art of Illusion

Art of Illusion is a free, open source 3D modelling and rendering studio. It is written entirely in Java, and should be usable on any Java Virtual Machine which is compatible with JDK 1.1 or later.

The current version is 0.4, released May 3, 2000. This is still a fairly early version, and some basic functionality is still missing. It is progressing quickly, however, and already has a number of advanced features including a powerful raytracer, a flexible and easy to use triangle mesh editor, and the ability to automatically generate smooth, organic shapes from triangle meshes.


-- Peter Eastman ([email protected])

[Peter also notes that this package has a triangulator in it: "The routine is Line.convertToTriangleMesh. I can't guarantee that it's the best algorithm, but it seems to work pretty well. Also, it will deal fine with concave polygons, but will sometime have trouble with self-intersecting ones."]


Real-Time Volumetric Rendering

We did some volumetric rendering with raytracing and then "mixed" that with a normal 3d scene (the scene was drawn using 3d acceleration).

Fair enough, it looks a bit blurry but it does give the impression of flying through 3d clouds.

Oh you can download the demo from http://www.incognita-hq.com if you're curious. it's called platipus (needs a "multimedia" machine with dx7, ram, the whole lot).

In the demo there's also my (now pretty old) rt engine, which is not too bad.

-- angel sastre ([email protected])

[Also, source code is available at the site.]


Year Three (1998/1999) results of the Internet Raytracing Competition are now available on CD ROM. The quality of some of the winning entries is impressive:



The global illumination bibliography now resides on:


as well as other related resources. There's also an image of the very first radiosity image ever computed (made with hand-cranked calculators). See http://www.helios32.com/resources.htm#History


Some interesting BIBTEX entries of lesser known papers can be found at:

    Ray tracing: http://www.cgg.cvut.cz/~havran/Bib/rtmame.bib
    Visibility: http://www.cgg.cvut.cz/~havran/Bib/vismame.bib
    Rendering: http://www.cgg.cvut.cz/~havran/Bib/renmame.bib

The lists are being regularly updated with new papers as they appear.

-- Vlastimil Havran ([email protected])


An extensive annotated collection of links to procedural texture synthesis is available at:



The PVMPOV patch gives POV-Ray the ability to distribute a rendering across multiple Unix platforms:


There is also a beta Win32 version at:



A nice summary of free L-system software out there is at:



A pleasant page covering free and shareware modeling software:



John Morris' lecture notes for his data structures class are nicely done, covering the basics of this subject. Worth a look at:



The mental ray specifications are interesting to read from a standpoint of looking at what features are part of a commercial ray tracing renderer:



In response to several requests, we have made available measured skin BRDF data from the 1999 EGWR paper,

"Image-Based BRDF Measurement Including Human Skin", by Steve Marschner, myself, Eric Lafortune, Ken Torrance, and Don Greenberg.

Data are in gzipped tar archives, each with a lot of binary data files containing raw BRDF samples in red, green, and blue channels. Data on the spectral response of the camera and the spectral balance of the source are included, and two of the three data sets include coefficients for rendering with the multi-lobe representation of Lafortune et al. (SIGGRAPH 97).

The URL is http://www.graphics.cornell.edu/online/measurements/

Have fun.

-- Stephen H. Westin ([email protected]ell.edu)


The Mops is a free open source modeler for Unix (Linux, IRIX) systems and (sort-of) Windows. Check it out at:



A great site for information on implicit surfaces is:


On a related note, Jules Bloomenthal's very useful implicit surface polygonizer article from Graphics Gems IV is now also available at his site:






is the Jungle project, run by Przemyslaw Prusinkiewicz and Brian Wyvill. It is focussed on modeling, animating and rendering blobbies. The software can be found at:


for SGI and Linux systems.


Robert Schneiders maintains an overwhelmingly large annotated links site on mesh generation:



The NURBS++ Package is source under GPL for manipulating and evaluating NURBS curves and surfaces. It includes an OpenGL-based editor, and can output POV-Ray, RIB, or VRML files. It's at:



A few useful resources are at:


A.T. Campbell, III's page. These include a small Java raytracer, a fairly complete BSP tree library, and other things.


The POV-Ray SuperPatch is an unofficial collection of extensions to POV-Ray:


Extensions include parametric surfaces, isosurfaces, splines, sphere sweeps, new pigment mappings, rational bezier surfaces, new mesh type, slope dependent textures, variable reflections, light groups, spherical/toroidal/ (etc)/ warps, and unicode text.


For a rundown of tools for POV-Ray, and other open source modeling and rendering software (as well as some nice images), see:



ParPov is a library for reading POV-Ray files. The first application of this library is Pov2Rib, a (surprise) POV to RenderMan RIB format converter. Open source at:


The quality of the conversion looks to be quite high, see:


for comparison images.


A basic particle system animator for POV-Ray (with source) is available at:



Hugo Elias' "The good-looking textured light-sourced bouncy fun smart and stretchy page" has some nicely done tutorials on Perlin noise, clouds, physically based animation, and many other tidbits:




Based on IBM's Visualization Data Explorer, this is an open source visualization package:



Graphics Course Notes

By Steve Seitz, Paul Heckbert, Andy Witkin, Joel Welling, the notes at:


have some interesting tidbits in them. For example, in the Ray Casting section there is a clear explanation of barycentric coordinates, and a cute geometric proof of the formula for the area of a triangle (I haven't seen it before).


The Graphics Muse columns in the Linux Gazette has all sorts of tidbits:


and see the bottom of the page for links to previous issues.


For some pleasing images of fractal terrain, see:



Information on IGES, STEP, and other CAD related formats is available at:


There are also links to other resources of interest to CAD programmers.

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BART: A Benchmark for Animated Ray Tracing, by Jonas Lext ([email protected]), Ulf Assarsson ([email protected]), and Tomas Moller ([email protected])

[The authors have come up with a useful new benchmark for measuring ray tracer performance. For static testing, these scenes address a problem the SPD/NFF benchmarks had: realistic environments. Little research has been done for ray tracing efficiency schemes including the fourth dimension, time. Given the growing interest in real-time ray tracing, I see this as an area ripe for some interesting work. For example, Reinhard, Smits, and Hansen just published a clever "modulo" grid scheme at the Eurographics Rendering Workshop this year; see the "Recent Papers" article at the end of this issue. - EAH]

Description of BART

Abstract: Due to the advent of ray tracing at interactive speeds and because there is an absence of a way to measure and compare performance and quality of ray traced scenes that are animated, we present an organized way to do this fairly and accurately in this proposal for BART: A Benchmark for Animated Ray Tracing. This is a suite of test scenes, placed in the public domain, designed to stress ray tracing algorithms, where both the camera and objects are animated parametrically. Equally important, BART is also a set of rules on how to measure performance of the rendering. We also propose how to measure and compare the error in the rendered images when some kind of approximation algorithm has been used.

Informal description: We were and still are interested in efficient algorithms for computer graphics. In this case we focused on ray tracing algorithms for animated scenes. The main reason for this is that real-time or interactive ray tracing is quite possible to do on multiprocessor computers. So, to be able to compare different acceleration schemes, we set out to compile a suite of test scenes for this. We currently have three scenes described below that we have put into the public domain. Each scene was designed to stress ray tracing algorithms in general. In order for the user of BART to make a fast implementation, we provide a simple C-parser for the file format (it takes about half a day to implement), and some animation code. The benchmark may also be used to: 1) compare algorithms using polygon rendering hardware (e.g., OpenGL), 2) compare global illumination and radiosity algorithms, 3) compare motion blur algorithms, but was not designed specifically for these areas of interest.

The BART benchmarks are at http://www.ce.chalmers.se/BART/.

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Global Illumination Test Scenes, by Brian Smits ([email protected]) and Henrik Wann Jensen ([email protected])

Henrik Wann Jensen and I have designed a set of scenes for testing energy transport. Our main goal was simplicity, both for the scenes, and for the type of transport tested by each scene. We wanted a set of very simple scenes that would exercise different aspects of rendering algorithms and make it fairly obvious when something wasn't working correctly. They range from the fairly simple (direct illumination without occluders) to the fairly challenging (reflection of a caustic from a secondary luminaire). We would appreciate any feedback you have.

Global Illumination Test Scenes

University of Utah Technical Report UUCS-00-013


The global illumination community has discussed having a database of scenes that could be used to compare and validate different global illumination algorithms. We present a set of test scenes for global illumination algorithms and propose that they be the beginning of such a database. These scenes are designed to be as simple as possible and still test a particular aspect of energy transport. The scenes are available on a web site in a variety of formats, along with images and pixel radiance data. We feel that the simplicity and availability of the models will make it easier for the community to use them, and that the field will benefit from a standard set of scenes. Additionally, we discuss classes of models with analytic solutions.

Tech Report:
Scene Repository:

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Realtime Techniques for Ray Tracing, by Ádám Nagy ([email protected]), Eric Haines, Russel Simmons ([email protected]), Paul Kahler ([email protected]), Factory ([email protected]), and Bruno Schödlbauer ([email protected])

[from the realtime raytracing email list, see the Ray Tracing Roundup in this issue. It starts on one topic, evolves to another, and ends with a web site. -EAH]

Ádám Nagy writes:

For intersecting decisions I always use 2 kind of 'Intersection' calculation:

- First is for rays with 1 fixed point. This is true for eye rays, and shadow rays also. This is much simpler for all primitives, because I precalculate coefficients which only depends on the fixed point of the ray, and the primitive.

- Second is the general intersection calculation, which I use only at reflections and refractions.

With a lot of light sources the number of shadow rays + number of eye rays can be so great that this kind of optimization is important.

I use basically sphere hierarchy, but for eye rays in addition I use another special optimization: I divide the screen into sectors (16*16) pixels: for all the sectors I precalculate its own sphere hierarchy which it intersects: I don't have to go through all the hierarchy for all the pixels.

I use adaptive techniques: I do full raytracing for some gridpoints on the screen, and for other pixels I do some heuristics (determined from the neighbor pixel infos) to save costly full hierarchy 'Intersect' calls. I am interested what kind of heuristics other guys are using.

My engine is a little bit slow because it is still in C++, but now I am in the middle of rewriting the important part to PIII Streaming SIMD based assembly code, I'm sure it will help a lot in terms of speed.

My basic idea regarding pIII Streaming SIMD is to maintain an 'Intersecting job queue', and I try to solve 4 intersect job in parallel fully using the PIII Streaming SIMD extensions. Does anyone have experience regarding to optimizing to PIII, or optimizing to Athlon?

Does anyone have experience in doing some part of ray tracing using integers instead of floating point?

Do you know better way than hierarchical spheres, and other kind of optimization techniques?


Ádám Nagy later adds:

I am trying to experiment with more complex heuristics, and I am interested in others' ideas about it. Here I show a very basic one, which is already implemented in my engine:

Let's do a classic raytracing for every fourth pixel (I put 1 where I do raytracing.)


Let's also store the trace information for those pixel. What does this mean? If for a pixel the eye ray intersected object A, and for the two lightsources the shadow rays intersected object B and object C, and the reflection ray intersected object D, we can store: A(BC)D. After it for the remaining pixels we can check its four neighbors, and if all the neighbors have the same stored structure, than we can simply assume that structure for that pixel without doing any intersecting calculation. In this case the only problem is that we sometimes miss some one-pixel objects, but these are not important (sometimes it is better not to show 1 pixel objects!)


Eric Haines replies:

That's an interesting approach, of knowing in advance what the ray tree is and shooting the rays only at those objects. Have you compared shooting the rays for the other 3 neighbors vs. just interpolating the already computed shade across the four corners? Interpolation would be bad if there are textures or a sharp specular highlight, I guess, but otherwise might be fine.


Ádám Nagy replies:

Actually, the best approach seems to make a compromise: I do not interpolate the pixel color directly, but I am not shooting the ray to the object: I am interpolating the normal vector (to enable shooting refraction rays...), the shade without texture, and the texture coordinates, and I multiply the texture color with interpolated shade: The textures look fine!


Russel Simmons responds:

You don't really need to interpolate the normal vector. I believe the 'standard' approach for grid-based raytracing (and the technique that I use in my engine) is just to treat rays that have been reflected no differently from other rays. In other words, your grid-interpolator routine knows nothing of rays being reflected rays or not; it merely interpolates shade, u, v, across the block. The higher-level tracing routines handle reflections. In addition to shade, u, v, I also store an id for each block. The id is essentially a surface id used to determine which texture to use, and also whether or not a block can be interpolated. If the 4 id's at the corners don't match, I don't interpolate the texture. Now, in the case of reflections, it would be possible to have 3 corners of a block hit object A, but the last corner hit object B, get reflected, finally hitting object A. Even though all 4 corners have the same surface id (of object A) we don't want to interpolate across (because there is an edge between object A and B in there somewhere). So to solve this problem you can flip some high bits in the id value if the surface is hit via a reflection - then the id's won't match. Also, if you want to not interpolate across shadow boundaries (you might want to subdivide) then a decent hack is to have a high bit in the id values represent in/out of shadow.

Getting back to normal vectors.. interpolating them like you say may have some advantages, but in my experience the method described above works and looks great.


Paul Kahler notes:

I find the following pattern works really well:


It's every 8th pixel, but my perception is that it's no worse than the 2x2 method you mentioned. That varies with texture, of course. At one point I did tests with a single object and found a disturbing amount of time being spent comparing colors of pixels and interpolating (I just tested color to see if more rays were needed, and interpolated just color) than tracing rays. I came up with a couple of great macros to compare and interpolate colors that cut the time way down - they're like MMX, but in C.


Factory responds to Paul's note:

I assume that interpolates along triangles, as opposed to being along parallelograms, i.e.


and not..


> I came up with a couple of great macros to compare and interpolate
> colors that cut the time way down - they're like MMX, but in C.

Hmm, looks like a nasty thing to optimize, I'd prolly try and make special case renderers for the four different rows that need to be rendered.

Another thing bad about that approach is that it prolly wouldn't lend itself to super sampling, but OTOH, I reckon it would look rather kewl if one was to do it with fairly large triangles.. (at least in a demo).


Paul replies:

No, actually if I follow your notation correctly it's:

343434343  Where the 2's are the average of 4 1's
132313231  The 3's are the average of a 2 and a 1
343434343  And the 4's I forgot how they work ;-)
231323132  probably just an average of 2 pixels (hmm which ones..)

Looking at this little diagram makes me think I'm not even averaging the best choices of pixels. OTOH I don't do weighted averages. Also, if the threshold for deciding to shoot more rays is low enough it doesn't mater how you interpolate. BTW, I'm doing this in windows with a default size of 512x384, so the pixels are small and the artifacts are not too bad. Shadows tend to look low-res because the outline of the shadow doesn't always trigger enough additional rays, but at higher speeds you still don't notice. The key is to always shoot rays along an objects edge. With solid colors, just about any interpolation will do for the interior.


Bruno Schödlbauer also responds to Paul's comments:

One thing you might try is not to interpolate, but instead to trace one frame every odd pixel, and next frame every even pixel. For every pixel not traced use the values of the previous frame. It probably looks strange with few frames per second, but with many frames one shouldn't notice it.


Paul ends on this note:

I just posted 3 small (72K) programs demonstrating my RTRT progress. Also available is an explanation of how I do my pixel interpolation - see the page describing DRE linked from the RTRT page (http://www.oakland.edu/~phkahler/rtrt/dre.html). All found at:


Choose the Realtime Raytracing link. BTW, these are Windoze programs and should run on 95,98, or NT unless there is some MFC dll version issue (I've seen some problems before).

You'll notice that my focus is not on getting the best possible performance on a particular scene, but on making a very high performance ray tracer that can be used for all sorts of things. Comments are welcome of course.

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Fastest Way to Generate Eye Rays in Ray Tracing, by Samuel Paik ([email protected])

[I'm sure I've seen this written up somewhere before, but can't seem to find it. This is the method I use, and I believe once Paul Heckbert also mentioned using something like it. I suspect it's been discovered by many others - time to have it written down somewhere. // Searching around, I finally found the reference, the poorly named (my fault) Ray Transformation, by Kendall Bennett, RTNv6n1. Still, it's worth repeating. -EAH]

Nicolas Delalondre wrote:
What is the fastest way to generate eye rays? (I don't want to apply a matrix to each vertex of the scene)

Fastest? There typically is no single fastest way to do anything in computing. Also, it seems highly unlikely that a single matrix-vector multiply is a major cost in your ray tracer.

However, try this:

Take the eye point and the projection plane and transform them into world space. Now simply march along the projection plane for each pixel, this gets you one point and the eye point is another for an eye ray. Viola! If your model & view transforms are nice (exercise to the student to determine what those conditions are...) then you can march along the projection plane for the cost of a single vector addition, and generating the eye ray costs another vector addition (if you need a normalized eye ray, that costs a little extra).

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On Transforming Planes, by Steve Hollasch ([email protected])

[From netnews. It's still a common error to transform normals by the rotation matrix, instead of its inverse transpose. You can often get away with this, see http://www.gignews.com/realtime020100.htm, and this topic was covered way back in RTNews1a. That said, I like Steve's little proof of how the matrix and its inverse cancel out. -EAH]

Here's my take on transforming planes that should make everything clear. If you represent points as column vectors (that is, translation components in the last column of a 4x4 matrix), then you transform a point by performing the matrix multiply M P = P', where M is the transformation matrix and P is the column-vector point. Given a plane Q, you'd transform by the same matrix by performing the matrix multiply Q M* = Q', where M* is the inverse of matrix M, and Q is a row vector [A B C D], where the plane is defined by Ax + By + Cz + D = 0.

Also note that Q M* = M*t Qt, where M*t is the transpose of M*, and Qt is the column vector form of Q. This is useful if your matrix multiply routine is geared to doing column-vector style multiplication.

If your system uses row-vectors to represent points, then just flip all of the matrix equations I've written above: P' = P M, Q' = M* Q = Qt M*t.

For a quick proof, consider that you want the plane equation Ax+By+Cz+D = 0 to hold true both before and after a transform. That is, for all points P that lie on the plane Q, then all of the transformed points P' should lie on the transformed plane Q':

    [1]    (Q') (P') = Q P    (it's an exercise to the reader to prove that
the matrix product of the row vector Q and the column vector P expresses the
plane equation Ax+By+Cz+D=0)

    [2]    (Q M*) (M P) = Q P    (Q' = transform Q, and P' = transform P)

    [3]    Q (M* M) P = Q P    (Associative law for matrix multiplication)

    [4]    Q P = Q P    (M* M = Identity)


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Intersecting a Ray and a Triangle with Plücker Coordinates, by Ray Jones ([email protected])

Testing for ray-triangle intersections with Plücker coordinates [1,2] has the benefit of being simple, reasonably efficient, and concise. Its main competitors are based on ray-plane intersection combined with barycentric tests. Various people have asserted to me that these methods are better because, as side effects of the intersection test, they give: a) the point of intersection b) the barycentric coordinates of the intersection both of which are often needed for further computations.

However, the Plücker intersection test can also be used to generate the barycentric coordinates. Paraphrasing from [1], given the Plücker coordinate R of a ray through points P0,P1 and the Plücker coordinates E0,E1,E2 for the edges of a triangle (A,B,C), the intersection test checks the signs of the permuted inner products of R and each of E0,E1,E2. If the signs agree, then the ray intersects the triangle.

The permuted inner product of the Plücker coordinates of two lines, P0->P1 and Q0->Q1, is related to the volume of the tetrahedron formed by (P0,P1,Q0,Q1). The volume of this tetrahedron (within a constant scale) is given by the determinant |P0 P1 Q0 Q1|:

               | P0.x P0.y P0.z 1 |
               | P1.x P1.y P1.z 1 |
               | Q0.x Q0.y Q0.z 1 |
               | Q1.x Q1.y Q1.z 1 |

The Plücker coordinates of the line P0->P1 are the determinants of the six 2x2 sub-matrices of:

               ( P0.x P0.y P0.z 1 )
               ( P1.x P1.y P1.z 1 )

The connection between the Plücker coordinates and the volume determinant is easy to see, and it can be shown that the permuted inner product is equivalent to |P0 P1 Q0 Q1| [3]. (I have assumed that the w coordinates for the points are 1, for reasons explained below.)

Getting back to the triangle intersection test, given the ray P0->P1, any
point P2 on the ray, and an edge Q0->Q1 of the triangle (A,B,C), it can also
be shown that the volume of the tetrahedron (P0,P1,Q0,Q1) is proportional
- The magnitude of (P0-P1)
- The dot product of (P0-P1) and the normal of (Q0,Q1,P2)
- The area of the triangle (Q0,Q1,P2)

The first factor above is the same for each edge. If we assume (without loss of generality) that P2 is in the plane of the triangle (A,B,C), then the second factor is also the same for each edge, since then the normal of (Q0,Q1,P2) is the same as that of (A,B,C).

Therefore, the volume of the three tetrahedra, and by extension the Plücker inner products of the edges and the ray, are proportional to the areas of the triangles (A,B,P2), (B,C,P2) and (C,A,P2). If P2 is in the plane of (A,B,C), then these are the subtriangles that define the barycentric coordinates of P2. Since the other two factors are the same for each edge, the Plücker intersection test gives (directly) the unnormalized (homogeneous) barycentric coordinates of the ray-plane intersection point.

There is one caveat: although the Plücker coordinates are homogeneous, for the equivalence to hold true, the Plücker coordinates of the edges of the triangle cannot be scaled independently.

(Another benefit to using the Plücker intersection test is explained by Amanatides and Choi[4], where tests are performed only once per edge and shared by adjacent triangles.)

[1] http://www.acm.org/tog/resources/RTNews/html/rtnv10n3.html#art11
[2] http://www.acm.org/tog/resources/RTNews/html/rtnv11n1.html#art3
[3] My thanks to Pat Hanrahan, who explained the equivalence of the tetrahedral volume and Plücker inner product as well as the connection to barycentric coordinates after I had found it by other (less clear) means. He knew of this connection well before I noticed it.
[4] http://www.cs.yorku.ca/~amana/research/mesh.pdf John Amanatides, Kia Choi "Ray Tracing Triangular Meshes", Proceedings of the Eighth Western Computer Graphics Symposium April 1997, pp 43-52.

(Two related articles are in RTNv15n1.)

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Volumetric Rendering, by Paul Kahler ([email protected])

[From the realtime raytracing list.]

I just thought I'd mention that I tried some very limited RT volume rendering. I used an oct-tree (however you spell that) and treated it as nested bounding *spheres*. A single sphere is circumscribed around the top level cube in the tree. Then 8 smaller spheres are circumscribed around the 8 subcubes. This pattern is repeated down to the smallest size sphere/cube to be used.

There really aren't any cubes or spheres defined. Only the orientation of the entire hierarchy. Each node in the tree has just 8 pointers to its children (NULL if an octant is empty). The bottom level objects have color information instead of pointers. You can use the recursive structure to do really fast intersection tests on the spheres. The ultimate may be to project the ray onto a plane perpendicular to the ray with the bounding sphere at the origin. The intersection test is then checking if the point is inside a circle, and subsequent tests of the children are just a matter of moving that point around the plane (via 8 vectors projected onto the plane).

I never got as far as the plane projection, but I did do a recursive intersection test and used one of 8 different orders to test the child octants depending on the ray direction (not optimal, but never used the worst-case order). I was able to trace a solid block of 64x64x64 balls at 1 fps or so. There was a LOT of aliasing going on because I was using the normal vector of each ball where the ray hit rather than some type of "surface normal" involving the overall surface geometry.

My conclusion was that each voxel should have a single normal vector associated with it (I've used this before in a non-RT voxel renderer http://www.oakland.edu/~phkahler) and with more optimization you could get good performance and large voxel counts. I've decided to put that aside until several gigabytes of ram are available and I have more time. RAM usage can be minimized if you can re-use even the smallest subtrees but a complex scene will still require a lot of storage and it will be static (those blades of voxel grass will not blow in the wind).

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Recent Papers, summarized by Eric Haines and Vlastimil Havran ([email protected])

This section is a collection of notices of some recent papers related to ray tracing and global illumination. Please feel free to send me other relevant abstracts to publish here.

Vlastimil Havran writes:

Several papers on ray tracing appeared at WSCG'2000(http://wscg.zcu.cz/wscg2000/wscg_2000_program.htm), SCCG2000(http://www.dcs.fmph.uniba.sk/~sccg/2000/program.htm), and EGWR 2000(http://www.fee.vutbr.cz/egrw2000/program.html), that could be of interest. I've selected only those closely related to ray tracing (many more of them are related to general rendering, particularly to global illumination):

Wilkie,A., Tobler,R.F., Purgathofer,W.: Raytracing of Dispersion Effects in Transparent Materials (Austria) http://wscg.zcu.cz/wscg2000/Papers_2000/W7.pdf.gz

Havran,V., Dachs,L., ra,J.: VIS-RT: A Visualization System for RT Spatial Data Structures (Czech Republic) http://wscg.zcu.cz/wscg2000/Papers_2000/X77.ps.gz

Revelles,J., Urena,C., Lastra,M.: An Efficient Parametric Algorithm for Octree Traversal (Spain) http://wscg.zcu.cz/wscg2000/Papers_2000/X31.pdf

Poitou,O., Bermes,S., Lecussan,B.: Laziness, a Way to Improve Distributed Computation of the Ray Tracing Algorithm http://wscg.zcu.cz/wscg2000/Papers_2000/T29.ps.gz

EGRW 2000:

Dynamic Acceleration Structures for Interactive Ray Tracing, Erik Reinhard, Brian Smits, Chuck Hansen

Direct Ray Tracing of Displacement Mapped Triangles, Brian Smits, Peter Shirley, Michael Stark

Ray Tracing Point Sampled Geometry, Gernot Schaufler, Henrik Jensen

Tapestry: A Dynamic Mesh-based Display Representation for Interactive Rendering, Maryann Simmons


What follows are abstracts which I (Eric) have collected from email, etc; by no means is it a comprehensive list.

Reinhard, E., Smits, B., and Hansen, C., "Dynamic Acceleration Structures for Interactive Ray Tracing," in Proceedings Eurographics Workshop on Rendering, (Brno, Chzech Republic), June 2000.

My own explanation: The idea of this paper is that as objects move outside the grid efficiency structure, they are (cheaply) reinserted into the grid itself, with a little resulting inefficiency. How is this possible? The grid is treated as a structure that repeats, filling space: now any object can be put into the grid, using the grid resolution as a modulus. In other words, something moving out through the left face of the grid "enters" the right face, and its larger location in the world is also noted. By doing so, the whole grid does not have to be recomputed each frame; rather, just the moving objects are deleted and reinserted. Only if the ray traveling through this grid matches the larger location of the object is the object tested for intersection. Paper available at http://www.cs.utah.edu/~reinhard/egwr/


"Interval methods for ray casting implicit surfaces with affine arithmetic," Affonso de Cusatis Junior, Luiz Henrique de Figueiredo, Marcelo Gattass, Proceedings of SIBGRAPI'99, p. 65-71. IEEE Computer Press.

Abstract: We study the performance of affine arithmetic as a replacement for interval arithmetic in interval methods for ray casting implicit surfaces. Affine arithmetic is a variant of interval arithmetic designed to handle the dependency problem, and which has improved several interval algorithms in computer graphics.

Full text, color images and other links available at http://www.tecgraf.puc-rio.br/~lhf/sib99/


"Hierarchical and Stochastic Algorithms for Radiosity", Philippe Bekaert, K.U. Leuven, PhD Dissertation, 1999.

The radiosity method is a physically based method to compute the illumination in a virtual environment with diffuse (matte) surfaces. It allows to generate very realistic images of such environments by computer, and it is suitable for quantitative predictions of the illumination.

In the radiosity method, a number of simplifying assumptions are made that can however lead to certain image artifacts. In this dissertation, the numerical error introduced by these assumptions is analyzed. The analysis allows to propose new algorithms in which this error, the discretization error, is efficiently controlled during the computations by means of hierarchical refinement.

The radiosity method also requires the solution of very large non-sparse systems of linear equations (about 100,000 equations is common). Moreover, the coefficients of these systems are non-trivial four-dimensional integrals. The main part of this dissertation is devoted to an in-depth study of how the Monte Carlo method can be applied in this context.

The Monte Carlo method is suitable for reliable computation of the coefficients of the systems of equations. It also leads to algorithms that do not require explicit computation and storage of these coefficients. A systematic overview of such algorithms is presented. Previously proposed algorithms of this type are compared and some new algorithms are developed. Next, the application of several variance-reduction techniques is described, and the use of low-discrepancy sampling in this context is discussed. Finally, new ways to incorporate higher-order radiosity approximations and hierarchical refinement are proposed.

The resulting Monte Carlo radiosity algorithms do not only appear to be more reliable, but also often lead more rapidly to usable images than their deterministic counterparts. They require significantly less computer storage, and they are more user friendly. It is expected that these algorithms will stimulate the use of the radiosity method in a wide spectrum of applications.

Overview and thesis at: http://www.cs.kuleuven.ac.be/cwis/research/graphics/CGRG.PUBLICATIONS/PHBPHD


"Incoming First-shot for Non-Diffuse Global Illumination," Szirmay-Kalos Laszlo, Mateu Sbert, Roel Martinez, Robert Tobler, Spring Conference of Computer Graphics, Budmerice, 2000


This paper presents a method that can replace the small and medium size lightsources by their effect in non-diffuse global illumination algorithms. Incoming first-shot is a generalization of a preprocessing technique called the first-shot that was developed for speeding up global diffuse radiosity algorithms. In order to reduce the prohibitive memory requirements of the original first-shot when it is applied to non-diffuse scenes in a direct manner, the proposed new method computes and stores only the incoming radiance generated by the lightsources and the reflected radiance is obtained from the incoming radiance on-the-fly taking into account the local BRDF and whether or not the actual viewing direction is in a highlight. Since the radiance function of the reflection is smoother and flatter than the original lightsource function, this replacement makes the integrand of the rendering equation have significantly smaller variation, which can speed up global illumination algorithms. The paper also discusses how the first- shot technique can be built into a stochastic iteration algorithm using ray- bundles, and provides run-time statistics.

Paper at: http://www.fsz.bme.hu/~szirmay/puba.html


"3D Visibility, analysis and applications", by Fredo Durand, PhD Dissertation, Universite Joseph Fourier, Grenoble

This thesis deals with 3D visibility, and also contains a huge survey of visibility techniques in different fields such as graphics, vision or robotics.

Abstract: Visibility problems are central to many computer graphics applications. The most common examples include hidden-part removal for view computation, shadow boundaries, mutual visibility of pairs of points, etc. In this document, we first present a theoretical study of 3D visibility properties in the space of light rays. We group rays that see the same object; this defines the 3D visibility complex. The boundaries of these groups of rays correspond to the visual events of the scene (limits of shadows, disappearance of an object when the viewpoint is moved, etc.). We simplify this structure into a graph in line-space which we call the visibility skeleton. Visual events are the arcs of this graph, and our construction algorithm avoids the intricate treatment of the corresponding 1D sets of lines. We simply compute the extremities (lines with 0 degrees of freedom) of these sets, and we topologically deduce the visual events using a catalogue of adjacencies. Our implementation shows that the skeleton is more general, more efficient and more robust than previous techniques. Applied to lighting simulation, the visibility skeleton permits more accurate and more rapid simulations. We have also developed an occlusion culling preprocess for the display of very complex scenes. We compute the set of potentially visible objects with respect to a volumetric region. In this context, our method is the first which handles the cumulative occlusion due to multiple blockers. Our occlusion tests are performed in planes using extended projections, which makes them simple, efficient and robust. In the second part of the document, we present a vast survey of work related to visibility in various domains.

Available at http://graphics.lcs.mit.edu/~fredo/THESE/

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Eric Haines / [email protected]