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"Light Makes Right"
January 18, 1996
Volume 9, Number 1
Compiled by
All contents are copyright (c) 1995,1996, all rights reserved by the individual authors
Archive locations: anonymous FTP at
ftp://ftp.povray.org/pub/povray/Ray-Tracing-News/,
ftp://princeton.edu/pub/Graphics/RTNews/,
ftp://wuarchive.wustl.edu/graphics/graphics/ray/RTNews/,and others.
HTML at
http://www.povray.org/rtn/index.html, among others.
You may also want to check out the Ray Tracing News issue guide and the ray tracing FAQ.
I am glad to have an excuse to no longer track in the RT News all the various commercial packages, as that is a full time job and then some. Instead the RT News will focus more on research issues, on free or extremely cheap software, and on other resources both on-line and off. If you are interested in the commercial scene, check out the Tessellation Times (http://www.3dartist.com/tess/tessmain.htm), which is associated with _3D Artist_ magazine. It is an excellent bi-weekly source of information on commercial rendering trends, and amazingly enough is free.
jgt: I am one of around ten editors of the new "journal of graphics tools", from A.K. Peters (Alice Peters helped produce the first volume of _Graphics Gems_). Ronen Barzel (ronen@pixar.com) of Pixar is the Editor in Chief, and Andrew Glassner is the founding editor. Think "Graphics Gems" and "Numerical Recipes" and you have a sense of what sort of articles we are after: useful results, tutorials, guidelines for choosing algorithms, and so on. The journal is peer reviewed (academics take note), and now has a web site, courtesy of the ACM, at http://www.acm.org/jgt .
The only fun computer toy I have bought recently is the Snappy Video Snapshot (on the web at http://www.play.com/pages/snaphead.html). What a cute thing: for about $200 or less it is a phaser sized box with a 9 volt battery which plugs into your parallel port and hooks up to a video camera, VCR, or TV. Bring up the software and you can capture video shots on your computer at three different resolutions and up to 24 bits of color. It is fast to install and use (though catching exactly the right shot from a video can take a few tries). Also, it comes with Gryphon Morph, a nice image morphing package, and Fauve Matisse "lite", a reasonable natural paint program. It is pretty easy to take pictures of friends, pull them in, then morph one into the other. Why am I talking about it here? Because it is also a pretty good way to scan in materials and photos for use as textures. When you use a perfectly still video image you can up the scanning quality within Snappy's software and multiple frames are blended and filtered to give pretty sharp images. It beats a flatbed scanner for cost, speed (faster than a scanner, and no waiting to get film developed), and amount of desktop real estate it takes.
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The trivial answer is: 1/2, because I did not properly state that the 2D bounding box is axis aligned (sorry about that). So, what is the answer for axis aligned bounding boxes?
This problem turned out to be a lot trickier than I first thought. When I asked it, I figured out the answer for any given triangle where all three points of the triangle touch the bounding box. The gist is that you do not have to worry about the infinite plane for this case, as the ratio between the triangle and bounding box is all that matters. So, the bounding box can be thought of as size 1x1, and one corner of the triangle is always at one of the bounding box corners, and the other two points on the triangle can freely move along the two edges opposite this corner:
+-- this triangle point can be anywhere along this edge
V
1+---*-----+
| X |
| * <-- and this tri. point can be anywhere along this edge
| |
| |}Y
| |
*---------+
^ 1
+-- and this point must be at a corner
With the distances X and Y as shown, the area of the triangle is:
A = 1 - X/2 - Y/2 - (1-X)*(1-Y)/2 = (1-X*Y)/2
Doing a double integral of X and Y each over [0,1] gives the answer 3/8ths.
Unfortunately, it's a bit harder than that: what if just two of the triangle's points touch the bounding box (i.e. are at opposite corners of the bounding box) and the third point is somewhere inside the box? Call these "two point" triangles. This ratio is not so hard to compute, it turns out to be 1/6th (see solution). But, what is the ratio between these two point triangles and the previous, three points on the bounding box types? Personally, I struggled with this one and did not feel comfortable with my answer, as it did not closely agree with the empirical answer my C program found.
The person who won this contest is Arijan Siska, who came up with a good proof showing that the ratio between "two point" and "three point" types as 1:2. He made the same oversight that I did (forgetting the "two point" types), but when informed of this, quickly derived the rest and sent in a convincing answer, which follows. Jack Ritter (jritter@netcom.com) gets an honorable mention for getting the empirical answer first, 0.3055, by performing monte carlo testing. Joe Demers (joed@graphics.usc.edu) had a solution that was very close, the first to get 3/8th and 1/6th as the sub-answers, and almost but not quite coming up with a way to blend the two (like I said, nor did I).
No one took up the challenge of determining the average area of an arbitrary non-intersecting quadrilateral and its bounding box (nor do I envy anyone the task!).
The rest of the solution follows, in Arijan's words.
____
- [for two point triangles] you end up with a triangle with one side going from (0,0) to (1,1) and the third point being anywhere within the unit square.
- if the third point touches the unit square the triangle has already been processed by the case with all points on the b. box.
- area of this 2point on b. box triangle is:
+------------* (1,1) pt = 1/2 - (1-x)y/2 - x y - (1-y)x/2 | / | = (1-x-y)/2 | / / | | / / | average pt = 1/6 | / pt / | | / ---*------ |/ --- | | y *--------|---+ (0,0) | x
- now the portion of 2point and 3point on b. box triangles out of all triangles:
If we think of a triangle as a two point line with third point added to it, we can state the condition for triangle to touch b. box in only two points is to have the third point inserted between first two points in x and y dimension or inserted beyond first two points - see the picture:
A | | point x3 has to be in the regions A B C or D.
|x1 |
-----*----+---------
|\ B |
| \ |
| \ |
| C \|x2
-----+----*---------
| | D
| |
Let's take a one dimensional example:
1. We use finite region [0,1] in which we generate random points. This region can be later expanded to infinity without affecting the results (probabilities).
2. Generate 2 random points:
0 x1 x2 1
|----*======*---------|
probability of a third point x3 being placed between these two points is equal to the average distance between x1 and x2. That distance (= probability) is
Integrate[x-y,{x,0,1},{y,0,x}]*2 = 1/3 # this is Mathematica notation
3. In two dimensions the probability of the third point being in the regions
A B & C D A B C or D is (1/3)^2 + (1/3)^2 + (1/3)^2 = 1/3. This gives us ratio:
2 p. on b. box triangles : 3 p on b box tri. = 1/3 : 2/3
- total area can be stated as:
2/3 * 3/8 + 1/3 * 1/6 = 11/36 ~ 0.3055555555555555555
Hope this time I'm right!
I've done a random test myself, and the result for 33,619,968 triangles gives an average area ratio of 0.305586785790013. So my analytical solution should be a correct one.
P.S. Check out my homepage if you have some spare time: http://luz.fer.uni-lj.si/users/arijan-eng.html [there are lots of ray traced images, a ray tracer, and more. -EAH]
____
Arijan later noted:
My simulation is currently at 238,157,824 trigs with average ratio 0.305573593013203, but I doubt the precision due to random number generator properties and precision.
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[hint: actually, this question can be restated to get rid of the sphere, though this does not make it any easier to compute.]
I *think* this is pretty easy, but right now I have not sat down and figured it out myself.
The prize for the best correct answer: a copy of Syndesis' Avalon CD ROM (see RTNv8n1), kindly donated by John Foust of Syndesis, http://www.webmaster.com:80/syndesis/)
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________
FREE access to 'The official POV-Ray CDROM'
Well, I've finally done something I've been meaning to do for some time, but had little time in which to do it - and that is to HTML-ise the Official POV-Ray CDROM and place it onto the Web !
As of now, 'The Official POV-Ray CDROM' is available for free browsing and downloading at http://www.povray.org/pov-cdrom/. If you've been wondering what's really on the CD but never had a chance to find out, now's your chance!
[I've trimmed the announcement down to just this, but check it out - it's a fully HTML-ized version of the directory structure of the CD, with much material that cannot be found anywhere else on the Net. -EAH]
- Chris Cason (Chris.Cason@oaks.com.au)
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Font3D is a utility program for the generation of 3-dimensional text objects. Once created, these objects can be included in a scene description file for later modelling or rendering. Several different output formats are supported, including:
o Persistence of Vision 2.x
o Vivid 2.0
o AutoCad DXF
o RenderMan RIB
o RAW Triangles
It uses TrueType fonts for input. Find it at: ftp://ftp.povray.org/pub/povray/utilities/font3d11.zip
[This package includes C++ source code, which is cool. -EAH]
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PVMPOV allows distribution of POV-Ray 2.2 across a heterogeneous network of computers. While not ideal, and not even with all the features I'd like it to have, it does a pretty good job with long traces, and is relatively useful in any type of computing environment.
http://www-mddsp.enel.ucalgary.ca/People/adilger/povray/pvmpov.html
- Andreas Dilger (adilger@enel.ucalgary.ca)
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Nathan Loofbourrow has an interesting collection of links to papers available on the Web, including some on rendering and radiosity:
http://www.cis.ohio-state.edu/~loofbour/papers.html
- from Stephen Spencer (spencer@siggraph.org)
________
Here's the short address for my home page:
Have you seen "Deja News"? Go to my web page, and follow "misc links" at the top, then follow "Deja News" (near bottom of page) and you can search for:
haines
heckbert
(natas* | nasta*) & kinski
for example. It is basically a searchable index of recent USENET articles. Very cool for info junkies!
Also check out some of the other links on my misc page, like the computational geometry bibliography or the computational geometry software directory.
- Paul Heckbert (ph@hostess.graphics.cs.cmu.edu)
[Other searchers are also now adding netnews search capabilities. For example, try Alta Vista at http://www.altavista.digital.com, which searches the web and netnews. -EAH]
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An extensive survey of triangulation algorithms is given by:
Paul S. Heckbert and Michael Garland:
Fast Polygonal Approximation of Terrains and Height Fields.
Carnegie Mellon University tech report CMU-CS-95-181, August 1995.
http://www.cs.cmu.edu/afs/cs/user/garland/public/scape
- Yishay (timc@netvision.net.il)
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Cornell Sampler
A few of the great images from Cornell:
http://www.graphics.cornell.edu/sampler/
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I am updating my list of graphics pages at:
http://www.graphics.cornell.edu/~shirley/graphics.html
If you know of any I should add, please let me know.
Also, if you have online Monte Carlo code or papers, please let me know and I will add you to:
http://www.graphics.cornell.edu/~shirley/mc.html
- Pete Shirley (shirley@graphics.cornell.edu)
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The VRML Sourcebook rocks the house! It covers all of that evil nitty-gritty I purposely left out of my own book. But now I have a text I can recommend - in good conscience - to people who need something more technical.
- Mark Pesce (mpesce@netcom.com)
[For more info, check:
http://www.wiley.com/compbooks/k26.html
Every VRML programmer should own this one, period. -EAH]
________
Que Publishing, a division of Macmillan Computer Publishing, is proud to announce "Special Edition Using VRML," by Stephen Matsuba and Bernie Roehl. The book provides 800 pages of detailed information about VRML, including tutorials on creating VRML worlds, tips and tricks, optimization techniques, and useful insights into the design of the language. Also included is comprehensive coverage of the various tools that are available for creating VRML worlds, as well as the available browsers. Since the VRML specification is constantly evolving, several chapters are devoted to examining the future of the language.
For world-builders, over half a dozen worlds are detailed in the book, along with accompanying commentary by their creators. For programmers, there's a chapter on writing VRML translators and scene generators. Also included with the book is a CD-ROM with dozens of useful utilities, sample worlds, browsers, 3D Modelers, toolkits and other VRML resources.
Special Edition Using VRML should be available on store shelves by the middle of February.
- Benjamin Milstead
________
I wanted to advertise the URL of a page I am developing with surveys and links to research on parallel volume rendering:
http://www.cse.ucsc.edu/~craig/pdr.html
Please feel free to submit URL's and or comments. I'm also the maintainer of the FAQ for the mp-render community. This FAQ is reachable from the above site, or available by anonymous ftp: ftp://ftp.cse.ucsc.edu/pub/mp-render/mp-render.faq
- Craig Wittenbrink (craig@cse.ucsc.edu)
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There's a big RenderMan site out there, I think in California. Ah, here it is:
http://pete.cs.caltech.edu/RMR/rmTop.htmld/rmTop.html
- Stephen Spencer (spencer@emily.cgrg.ohio-state.edu)
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Quaternion Tutorial
Ken Offer (koffer@io.com) wrote: : Can anyone mention a good text for those wishing to comprehend the : intricate details of quaternions?
How intricate do you want to get? Sir Hamilton, who invented the beasts, wrote some books. I don't recommend them for the beginner and there is nothing about computer graphics in them. (That is only to be expected, since matrices were invented by Cayley after quaternions and computers only came in the next century.) You could try my brief tutorial at ftp://ftp.cis.upenn.edu/pub/graphics/shoemake/. If you can find a copy of the Siggraph 85 Proceedings (tough, I know), my paper there may help. There are also a number of articles in the various Graphics Gems books.
Quaternions are simple to program and use, but deep comprehension still beckons me 10 years after my first paper on them.
- Ken Shoemake (shoemake@bambi.cis.upenn.edu)
________
Now available (for free) at http://www.ledalite.com:
* 200+ photometric data files for fluorescent light fixtures * Documentation and ANSI C source code for an IESNA LM-63 photometric data file parser (public domain) * Demonstration radiosity renderer for MS-Windows 3.1 and Windows '95 * Radiosity bibliography * Color quantization bibliography * ... and more
- Ian Ashdown (iashdown@ledalite.com)
________
Per Christensen, Eric Stollnitz, David Salesin and I have been doing some research on extending clustering to glossy global illumination. Last month we submitted a paper "Clustering for Glossy Global Illumination" summarizing our research to ACM TOG. The paper is available online via anonymous ftp from ftp://espresso.cs.washington.edu/pub/danix/clustering .
An earlier version of this paper was submitted to Siggraph 95 (and was rejected). Fortunately, the comments of the anonymous reviewers challenged us to substantially improve both the paper and the implementation.
The highlights of the results section are: a comparison with RADIANCE on a glossy "sphereflake" model and nice global illumination solutions for an architectural environment with ~8000 initial surfaces, most of which are glossy.
If you happen to read the paper and have any comments, please e-mail them to me.
- Dani Lischinski (danix@cs.washington.edu)
________
BMRT Update
[BMRT is a RenderMan compliant ray tracer/radiosity renderer. See RTNv8n1.]
BMRT is still going strong. In fact, I just updated the FTP site (ftp://ftp.gwu.edu/pub/graphics/BMRT) with a new revision that has some enhancements and bug fixes. I've had a WWW site for a while, too: http://www.seas.gwu.edu/student/gritz/bmrt.html. One new feature is that I use the Mesa library (freeware OpenGL clone), so all the non-SGI ports now support the polygon previewer and other features that were previously only enabled on my SGI version.
The software is quite widely used, including by a lot of well known studios (who mostly use PRMan, but occasionally use BMRT for particular effects that are easier with ray tracing). It was used recently for some effects on the show "Star Trek: Voyager", and also was used to render the new video logo for the Utah Jazz (NBA team), which apparently is shown whenever their games are broadcast.
- Larry Gritz (lg@pixar.com)
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Rayshade on Linux:
Direct URL http://www.carsinfo.com/~eric/rayshade-linux.tar.gz Indirect http://www.carsinfo.com/~eric/ < Has link to it
- Eric Bardes (eric@carsinfo.com)
____
Netshade
I'm a little uncertain about releasing this to the world at this point, but there have been many recent questions about it (and about Rayshade in general) so I thought I should just do it.
This was a project I started some time ago, when I had a different job and free time. Now I have little free time, so I cannot easily continue serious work on it. So, here it is. Perhaps the net can make it cleaner and I can be a patch-applying droid. ;)
If any changes are made to the source, PLEASE mail them to me as well.
Netshade 0.0.3 (really beta :) can be found on:
ftp://packrat.flame.org/pub/netshade/
Comments, suggestions, and flames to me. I'll apply the usual mail filter and ignore the latter. :)
- Michael Graff (explorer@iastate.edu)
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For a free blobby modeler, with source code, visit:
http://beebalm.cit.cornell.edu/Info/People/pbenton
Alex Benton's homepage. Source is available (soon in a zip file), and it's pretty fun to use: you move the spheres around and watch the implicit surface form on the fly (turn on Blobby Surface under View). It is more of an educational demo right now, but since the code is available it could be hacked to export to DXF, etc.
- Eric Haines
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Free Models
Well, my web page has moved one more time to:
http://www.loop.com/~hhc
Hopely, connection will be much better.
In case you have never been to my web page, the following is a summary of what it has.
1. Free 3D Models in .3ds format
* Victorinox Swiss Army Knife
* Deoxyriboneucleic acid
2. Free Star Wars related Models in .3ds format
* 16 Models of Rebel Alliance Weapons
* 16 Models of Empire Weapons
* 7 Other Models
3. Thumbnails of my Architectural Works
4. thumbnails of my Industrial and Miscellaneous Models
5. Gallery of raytraced images of my models and animation
Do enjoy my free models. All models on my site are created by me. [A nice little site, the knife is great. -EAH]
- Harry H. Chang (hhc@loop.com)
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Crisis in Perspective, Inc. announced JOEY 4.0, an integrated programmers tool kit for Windows 95 and NT, that promises to enable developers to create 3D applications without any prior 3D graphics knowledge or experience. It sells for $250; it's available for evaluation at:
http://www.teleport.com/~cip
[If you use Visual C++ and MFC, you should give JOEY a look; I saw it at SIGGRAPH and it looked pretty cool, a nice way to get some 3D controls in place quickly. There is a book from Springer Verlag (http://www.springer-ny.com or http://www.springer.de/) by Roy Hall and Danielle Forsyth about JOEY, with some of the system's code and development tools included on a floppy. "Interactive 3-D Graphics in Windows," Approx. 390 pp. 3 1/2" disk, Softcover ISBN 0-387-94573-3. I plan to have a short review of this book and software here soon. -EAH]
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For a tutorial on constraint based modeling, see:
http://www.cs.purdue.edu/homes/cmh/electrobook/intro.html
It is from Purdue, and my modeling friends tell me it is quite good.
- Eric Haines
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The Proceedings of the Rendering Workshops 5 and 6 are available now!
5th Rendering Workshop 1994:
Photorealistic Rendering Techniques (ed. Sakas, Shirley, Mueller)
ISBN 3-540-58475-7
6th Rendering Workshop 1995:
Rendering Techniques'95 (ed. Hanrahan, Purgathofer)
ISBN 3-211-82733-1
order from: Springer-Verlag, http://www.springer.de/ or http://www.springer-ny.com
- Werner Purgathofer (wp@stellaris.cg.tuwien.ac.at)
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Radiosity system
I'd like to announce the initial release of the radiosity code developed in our research group. Our group is at:
http://www.cs.kuleuven.ac.be/cwis/research/graphics/graphics-E.shtml
Features: ---------
- It does both shooting (progressive refinement radiosity) and gathering. - Hierarchical refinement if you like (for both shooting or gathering) - Importance-driven if you like (for both shooting or gathering) - Formfactor computation by numerical integration. The ray-casting for visibility determination is accelerated by using hierarchical bounding boxes, shadow caching and shaftculling. - handles both triangles and (convex) quadrilaterals. - T-vertex elimination and many other rendering options. - reads Greg Ward's MGF scene description files (see http://radsite.lbl.gov/mgf/HOME.html) - a more or less nice user interface and interactive view manipulation. - ...
It requires - Motif, - Silicon Graphics' Iris GL (or a clone) or PEX - an ANSI-C compiler (gcc 2.7.0 was used to develop it) and GNU make (or another make which offers the .include feature, unless you change the Makefiles a bit) to compile it.
You can obtain this code for free on URL:
http://www.cs.kuleuven.ac.be/~philippe/rad.html
- Philippe Bekaert (Philippe.Bekaert@cs.kuleuven.ac.be)
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Form Factor of a Cow
[This is a radiosity joke, "what's the form factor of a cow?", and the fact that someone actually computed it once made it truly strange. What adds to it is to find out that the simplification for a cow is, well, read on. -EAH]
I did look up the cow paper once, and it is for real. The motivation was designing heating systems for the care of livestock.
Here is a summary:
"Uses mechanical integrator to measure factors from various wall elements to a cow, and presents some results for size of equivalent sphere that gives same factor as cow. It is found that the sphere origin should be place at one-fourth of the withers-to-pin-bone length back of the withers, at a height above the floor of two-thirds the height of the withers, and the equivalent sphere radius should be 1.8, 2.08 or 1.78 times the heart girth for exchange with the floor and ceiling, sidewalls, or front and back walls respectively. Also discusses exchange between cows and entire bounding walls, floor and ceiling and between parallel cows."
I don't think I kept a copy of the paper, so please don't ask me for it. As I recall, the authors were from California, and the paper was initially read at an agricultural meeting held at Cornell.
Pastorally, - Holly E Rushmeier (holly@cam.nist.gov)
____
Ian Ashdown notes:
%A R. L. Perry %A E. P. Speck %T Geometric Factors for Thermal Radiation Exchange between Cows and their Surroundings %R Technical Report 59-323 %I Journal of the American Society of Agricultural Engineering %D 1959 %K form factors %Z a seminal paper in computing cow form factors
________
My favorite image manipulation program on Unix is still ImageMagick, at:
http://www.wizards.dupont.com/cristy/ImageMagick.html
It's no PhotoShop, but it *is* free and with source, and you can run batch jobs. It and the old but good Utah Raster Toolkit are two of the many reasons our systems administrator can take my Unix workstation away only by prying it from my cold, dead fingers.
On PC's the shareware Paint Shop Pro and LView Pro are both reasonable image manipulation programs, though both have their gaps. I prefer PSP, but LView Pro allows you to make GIFs transparent (good for HTML hacking). Search for the latest versions of these (and just about any other shareware and freeware for PC's) at the Virtual Software Library:
http://vsl.cnet.com/
________
The coolest graphics java applet for me is Paul Haeberli's impressionist paint program. See:
http://reality.sgi.com/grafica/
It is based on Paul's "paint by numbers" work presented at SIGGRAPH '90. His 3D interactive java applet here is also impressively fast.
- Eric Haines
________
Other sites of interest:
http://www.essi.fr/~diard/photon4d.html - Photon 4D, a Unix based ray
tracer, z-buffer renderer, and Motif interface. The author is
looking for beta testers at this point.
http://www.cyberstation.fr/~dolivier/povlab.html - POVLAB modeler page,
a modeler for POV-Ray.
http://www.lightside.com/3dsite - 3D Site's new address, a great
clearinghouse for all things computer graphical
http://graphics.rent.com/ - A graphics user related site, lots of
pointers and graphics package mailing list addresses and
material.
http://www.PrimeNet.com:80/~grieggs/cg_faq.html - computer graphics FAQ
http://www.research.microsoft.com/research/graphics/glassner -
Andrew Glassner's home page, interesting graphics research
http://www.sdsc.edu/SDSC/Partners/vrml/software/textures.html -
sources of free textures on the Web.
http://www.exit109.com/~jimh/ - a nice little gallery; the glass
sphere experiments are different and interesting.
http://www2.cinenet.net/GWEB - professional computer animators' site
http://intranet.on.ca/~sshah/booklist.html#Graphics - list of graphics
books and ISBNs, pointers to some publishers now on line.
http://www.dcs.ed.ac.uk/home/mxr/objects.html - another free 3D model
site.
http://rushmore.JPL.NASA.GOV/~ndr/tiff - Unofficial TIFF homepage
- Eric Haines
________
If you are interested in having your computer look at you, check out:
http://www.cs.cmu.edu/~cil/vision.html
a home page pointing at many computer vision related resources.
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For those who've never heard of Sced (probably most of you): - Runs on almost any UNIX platform with X11R5 or better. - Constraint based modelling interface. - Support for CSG objects. - Exports to POVray, rayshade, radiance, renderman and now VRML.
There's lot's to say but I won't say it. Find out more at:
http://http.cs.berkeley.edu/~schenney/sced/sced.html
[There is also a patched version of Sced called Sceda, at:
http://www.cyberus.ca/~denism/sceda/sceda.html
which adds keyframing abilities. - EAH]
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I've been working on fast but thorough hashing.
OK, the major problem with FP numbers is that are 17.000000000 and 17.000000001 the same number? If no (in other words, ANY difference makes them distinct) you run into problems if any operations have been applied to them. Copying usually does not corrupt them.
A strategy for FP I expect would deal with the 4 (or 8) bytes that make up the value - converting to some integer/exponent or whatever is horribly slow.
So basically you have a 3 sets of 4 bytes, and you want to mash all twelve bytes into a unique hash. Lots of ways to do this, some fast, some very good.
Probably the fastest version would be something like:
float position[3]; unsigned char *byte;byte=(unsigned char *)position; /* lets us address the floats as 12 bytes */
unsigned long hash; hash = 3*byte[0] + 5*byte[1] + 7*byte[2] + 11*byte[3] + .... (prime)*byte[n]
This is very fast. However it can suffer from aliasing, especially when the input float values have zero bytes in their binary representation.
The high quality way is to use an "S-box" to mash the bits around thoroughly. [This is THE theme of cryptography]. An S-box is just a way of taking one byte and outputting another, with a (random, usually) truth table. In practice an 8x8 S-box (8 bits in, 8 bits out) can be made really easily by the pseudocode:
for i=0 to 255
sbox[i]=i;
next i
for i=0 to 255
j=random(0,255)
swap sbox[i] with sbox[j]
next i
Then the (very high quality) hash of a byte x is sbox[x]. All an S-box is is bit-swizzling. It is mapping sets of bits to new sets of bits. However it does it completely nonlinearly, which is what you want.
To combine this with many bytes, you can do a lot of things. For your purposes, it's probably enough to do...
hash = 3*sbox[byte[0]] + 5*sbox[byte[1]^123] + 7*sbox[byte[2]^47]...
+ (prime)*sbox[byte xor some random (fixed) number];
The mults probably are not even horribly necessary.
Actually in both cases you may want to use larger primes, like 101, 247, etc, not 3, 5, 7.
Ken Perlin uses the algorithm:
hash = (sbox[(X+sbox[(Y+sbox[Z%256])%256])%256];
This is a compromise, since it has excellent behavior locally (as the LSB changes) but it causes repeated areas, i.e. hash(x,y,z)=hash(x+i*256,y+j*256,z+k*256). It is more efficient than a longer hash since it needs only three sbox lookups. This hash is good for generating noise samples on an integer lattice but is terrible for finding matching vertices (e.g. 1.0, 1.5, and 1.99 as inputs all give the same hash number out).
There are lots and lots and lots of variations. But the sbox method is extremely good as a basic tool to use. If you are looking for cryptographically secure hashing, you can chain rules, like hashing each byte into a new byte with a new sbox, then shifting the full 12 bytes right 4 bits and hashing again. In crypto, they often do 10 "rounds" of this kind of iteration.
I should mention that my main hashing has been for stuff like fractal noise values at gridpoints.
It is surprisingly easy to get bad aliasing on a uniform grid!! Aliasing you can actually SEE as repeated patches. This is why I was researching all this s-box stuff. Luckily s-boxes only take 256 bytes of ram and a little precompute.
Trick #3: make a 512 byte array of your sbox appended to itself, so sbox[256]=sbox[0], sbox[257]=sbox[1]...
Then you can do:
hash= sbox[byte[0]+sbox[byte[1]+sbox[byte[2]+... ]]]]
[the 512 byte array is just so that you do not overflow when adding. -EAH]
This is a REALLY good hash, but only gives you 0-255 out.
One other note, you can move to a 16x16 S-box, same idea. Your memory usage jumps from 256 (or 512) bytes to 120 or 240K. But then you are chunking two bytes at a time. It's a better hash, and nearly twice as fast (after the sbox is built).
There are even more details than this: stuff about collisions from "pipelines" when things are just one bit different that for the best hashing you want to avoid.
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http://www.inf.tu-dresden.de/~cn1/gx.html
I find this work to be interesting as it explores two relatively new and unexplored ray tracer efficiency schemes. Because the connection to JuHu's site (at least from here) is tenuous and slow, I reprint the results of his efficiency scheme exploration here. -EAH]
Surprisingly, BSP isn't always faster than ABVH, especially when over-partitioning occurs. From my experiments, the max. number of candidates in a node can be adjusted to any given number, but the max. level of subdivision should be between 8-10.
The results from my BSP- and 5D-Implementation show that 5D is faster than BSP only if the scene contains many low-reflective/refractive polygons with small changes in curvature (teapot.nff, for example). For other .NFFs of SPD, such as balls.nff, jacks.nff etc., 5D is much slower than BSP, and the required memory for 5D is enormous compared to BSP. Another problem is that the main advantage of 5D over BSP is not having to compute the voxel-path, but if the BSP-tree is balanced, it is possible to find the closest intersection very soon after few voxel-tests. And, the beam/box overlap-test for the reclassification of candidates lists undermines the efficiency of 5D, too. I think 5D is only attractive for eye-rays and shadow-rays, because with these coherency is high enough.
Hardware : Iris Indigo w/ 33 MHZ IP12 Processor, Main memory size: 48 Mbytes
- Object-CH : shadow cache hits by last object - Voxel-CH : shadow cache hits by last voxel - BSP-Tree is generated w/ X/Y (means the max. depth is X and the max. number of objects in a node is Y) - Preparation and tracing times are given in seconds.
For these tests, GX uses BSP as a first method and ABVH as the second, which generates an ABVH for each BSP-node. Pure BSP or pure AVBH can be applied by setting the max. number of candidates in a BSP-Node to 1 or to a large number.
max. subdivision depth =
ceil(log(number of primitives / max. num of prim. in a node)/log(2)) + 1 )
gears4.nff =============================== Eye rays : 40000 Reflection rays : 13335 Refraction rays : 16749 Shadow rays : 238681
---------+------+-------+-------+------+------+-------+-------+------- BSP-Tree | 15/1 | 11/10 | 10/20 | 9/50 | 8/90 | 7/200 | 6/400 |5/1000 ---------+------+-------+-------+------+------+-------+-------+------- Preparing| 74 | 52 | 60 | 65 | 56 | 55 | 76 | 86 ---------+------+-------+-------+------+------+-------+-------+------- Tracing | 651 | 308 | 298 | 293 | 259 | 340 | 418 | 597 ---------+------+-------+-------+------+------+-------+-------+------- Object-CH| 40376| 41524 | 41572 |42145 |42454 | 41866 | 4183 | 40253 ---------+------+-------+-------+------+------+-------+-------+------- Voxel-CH | 2578| 5889 | 7077 |10568 |12296 | 16492 | 20374 | 24056 ---------+------+-------+-------+------+------+-------+-------+-------
balls4.nff ====================== Eye | 40000 Reflection | 30889 Refraction | 0 Shadow | 150967
---------+-------+-------+-------+------+------+-------+-------+------- BSP-Tree | 14/1 | 11/10 | 10/20 | 9/50 | 8/90 | 7/200 | 6/400 | 4/1000 ---------+-------+-------+-------+------+------+-------+-------+------- Preparing| 37 | 45 | 40 | 65 | 44 | 43 | 47 | 65 ---------+-------+-------+-------+------+------+-------+-------+------- Tracing | 634 | 352 | 261 | 293 | 228 | 264 | 348 | 891 ---------+-------+-------+-------+------+------+-------+-------+------- Object-CH| 23270 | 23660 | 23718 |42145 |23971 | 2382 | 23897 | 23429 ---------+-------+-------+-------+------+------+-------+-------+------- Voxel-CH | 594 | 1470 | 2162 |10568 | 3942 | 4985 | 5973 | 8617 ---------+-------+-------+-------+------+------+-------+-------+-------
[As it turns out, Matt Quail just tipped me off that George Simiakis,
who also has explored 5D ray classification
(RTNv6n1,
RTNv7n5), has just put
his thesis on the net (it's 4 megs, he should be compressing it soon):
ftp://ftp.sys.uea.ac.uk/pub/ah/G.Simiakakis_PhD.ps
gzipped (much smaller! 0.7 megs) at:
http://hardy.ocs.mq.edu.au/~mquail/G.Simiakakis_PhD.ps.gz
Paging through it, this thesis looks like a good overview of existing RT efficiency research, and Matt said the research looks well done. The thesis examines pure 5D and hybrids using this scheme, along with an in-depth study of making such algorithms parallelizable.
George Simiakis writes: "If you ask me what I really think about 5D, I believe it is good for low complexity scenes with lots of ray coherence, but I prefer 3D methods (or the 2D grid, see my thesis) as an all-around method. In other words I prefer scalability, general robustness and predictability of performance. Ray tracing may not be solved 100% but new improvements are bound to be insignificant compared to improvements in the past. I hope to work on parallel radiosity now. There is still some efficiency work to be done there."
-EAH]
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Yep, I have written a little IBM Demo called 'The Attic', maybe you'll find it on some demo collection cd as ATTICFIX.ZIP, which does just that: an image of a sphere with some cylinders (difficult to scan convert) was raytraced, including some 'cool' procedural texturing (not possible in realtime) and with this stored, fixed zbuffer, a realtime rotating cube was mixed. Looks good.
[This is actually a fun trick, and can even be useful. I once wrote a demo for an HP graphics box where they had a weird mode where you could compare with the z-buffer but not write to it, and could render into an overlay plane. The effect: you could have a fully ray traced camshaft or whatever and move a semi-transparent (screen-door) square around through this scene and the object - cool effect, you could get a good sense of the contours of the object, while having a nice rendering on display. -EAH]
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This dissertation concerns efficient computation of realistic images. To compute realistic synthetic images, the effect of global illumination is essential. Ray tracing algorithms solve the global illumination problem for specular interreflections, and radiosity algorithms solve it for diffuse interreflections. But computing a solution is more complicated when the surfaces have glossy reflection.
This dissertation describes hierarchical techniques for efficient solution of the glossy global illumination problem. Two types of hierarchy are utilized: wavelets to accurately represent radiance distributions on surface patches, and clusters to approximately represent radiant intensity from groups of surface patches. Without hierarchical techniques, the solution time would be quadratic in the number of patches and O(b^{1.5}) in the number of basis functions b. The hierarchical techniques make solution time linear in both the number of patches and in the number of basis functions. This reduction is significant since the numbers of patches and basis functions are large for accurate solutions in realistic environments. Furthermore, directional importance is used to focus refinement of the solution on parts that contribute significantly to a particular view of the scene.
Our method is the first finite-element method capable of handling complex glossy scenes.
____
If you would like to read more than the abstract, go to
http://www.cs.washington.edu/homes/per/publication.html
and click on "dissertation" or "smaller version". The smaller version is much faster to transfer, but has no color images.
[A paper on this topic by Per Christensen, Eric Stollnitz, David Salesin and Dani Lischinski is available online via anonymous ftp from ftp://espresso.cs.washington.edu/pub/danix/clustering . - EAH]
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There were about 20 of us attending, and I won't embarrass myself by misspelling (or forgetting) everyone's name, but instead I'll just mention some of the topics we covered and what was said about them.
1. Sharing models and data
To address need for shared models that are usable and useful for global illumination, a new format has been developed called MGF (for Materials and Geometry Format) and models and libraries are now available from:
http://radsite.lbl.gov/mgf/HOME.html
Or, if you only have ftp access, try:
ftp://hobbes.lbl.gov/www/mgf/
This format supports a reasonable reflectance model, CIE XYZ and spectral colors, and the IES luminaire data format for light sources. A number of models from Radiance and other sources have been collected, and we encourage people with big hearts to add their own models to our collection. Write to me if you have any questions or want to get on the mailing list for how to improve this format to meet our long-term needs.
2. Setting up light sources
Dan Wexler [see his article] pointed out that one of the biggest time sinks in animation and rendering for film is the setting up of light sources, and that methods for making this more interactive were critical to this application. Specifically, he felt that a "deep raster" approach (similar to that developed by Sequin et al and presented at Siggraph 4 years or so ago) was a good way to go. In general, he felt there needed to be more of an emphasis on animation and interaction than there was in our current focus on static environments and generating one really nice image.
3. Image-based rendering
Pete Shirley addressed a general question to the group about the relative importance and promise of image-based rendering (ala Quicktime VR) versus polygon-based methods. The overall response seemed to indicate that indeed image interpolation was a significant direction to head in the future, though polygon rendering will continue to be important when object manipulating is required and for nearby objects. It seemed that at least five people there were actively pursuing such techniques. (Check out this year's Siggraph proceedings on Plenoptic Modeling and Quicktime VR, also Nimeroff et al in Eurographics Rendering Workshop '95.)
4. Visibility processing
Several people talked about the difficulty of geometric simplification and other statistical or computational geometry methods for detecting visibility changes or approximating visibility. Christine Piatko of NIST reminded folks that tracking visibility is an O(N^6) problem and not something you want to do explicitly, and there was an interesting paper by Francois Sillion and George Drettakis (similar in some respects to the one Francois presented at EGRW this June) on approximating shadows. Jeff Nimeroff of UPenn also mentioned the work he had done with Julie Dorsey and Holly Rushmeier and presented at EGRW on statistical measures of object visibility for looking at this problem. It was agreed by most that statistical measures or simplification were necessary in large environments, though there is still much work to be done in this area.
5. Calibrated input and output
Holly Rushmeier (who by the way is next year's Siggraph papers chair) talked a little about the difficulty of obtaining accurate physical data from real-world scenes, and a number of people lamented about the undetermined nature of image-to-output mappings for computer monitors, printers and film recorders. It was suggested that these devices be measured, which is certainly possible, and that this data be shared with the larger community. (Hear, hear, let's all do it, etc.) Holly and I also reminded people to look at the conference room image measurements and model we have made available on the web site:
http://radsite.lbl.gov/mgf/compare.html
From there, you can also pick up an online copy of our EGRW '95 paper on image metrics if you haven't purchased the Springer-Verlag proceedings yet.
6. Getting papers into Siggraph
Henrik Wann Jensen of Denmark asked about the lack of papers covering global illumination this year at Siggraph, and if this was a trend or what? Holly answered that there's a bias against incremental work in any field, and that Siggraph tends to accept papers that are new directions rather than the logical next step in a technique previously published. Ian Ashdown of Ledalite said that he had submitted something that reviewers liked except that it didn't have any pretty pictures and was therefore rejected. (No one had anything to say to console him on this.) I made the unsubstantiated claim that it can help to provide evidence of a real-life application along with an incremental improvement in one or more existing algorithms, and Holly agreed that there must be a balance between application and innovation.
There is a lot of useful information by the way on next year's Siggraph in New Orleans. Look for the "new" icons on the web site:
http://www.s95.siggraph.org/conferences/siggraph96/siggraph96.html
In particular, you should be aware of the new electronic abstract requirement for technical papers.
7. Oracle-assisted rendering
Another trend that people discussed (me for one) was the use of oracles to decide when and where to apply which algorithms (a.k.a. "hacks") to improve the speed and quality of renderings. Some information may be derived from the scene itself, but other assistance will always require the user to help the program to guide the calculation. This is not as bad as it sounds, since users are after all necessary to rendering, so they may as well guide it. I directed people to my EGRW '95 paper on the subject, which I can make available online if people are interested. Although I don't plan to pursue this too much myself, I think it continues to be an interesting and promising direction for investigation.
8. Rendering code available
Philipp Slusallek of Erlangen Univ. mentioned to the group that they have a renderer that extends Renderman input to include global illumination calculations (physically-based) and that code is available from him. His address is (slusallek@informatik.uni-erlangen.de) if you're interested. (He's going to hate me when he gets back.)
Remember also about the Blue Moon Ray Tracer, written by Larry Gritz (lg@pixar.com) and available at:
http://www.seas.gwu.edu/student/gritz/bmrt.html
It also does some global illumination calculations, though I'm not in a position to compare the two.
As long as I'm plugging other people's code, must mention my own for those of you who have been asleep or locked up for some time:
http://radsite.lbl.gov/radiance/HOME.html
Well, that's about it I guess. Anyone else who was there and remembers stuff I forgot or points I left out, please pipe in. I tend to do a really bad job at these sorts of summaries, and I neglected to take notes during the meeting, so I'm relying here on my less-than-adequate memory.
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I've run into an interesting implementation problem (again) which I've found annoying. Imagine you've got something with a big index of refraction, like a diamond, and you shoot a ray at it. A refraction ray is spawned and passes through the diamond. Due to total internal reflection (TIR) (i.e. the ray's past the critical angle) no refraction ray exits when this refracted ray (the one passing through the diamond) now intersects the surface, only a reflection ray is generated which bounces back through the diamond. The normal correction for TIR is to simply boost the reflection ray's contribution if the refraction ray is not generated.
However, what happens if this reflection ray again hits, TIR occurs, and again only a reflection ray is spawned? Say this happens again and again until the maximum level of computation is reached and we call it quits. What happens at this point? That is, how do you shade this bottommost ray? Very little color has been picked up along the way, as it is expected that the reflection ray eventually hits something mostly diffuse and takes up that color.
Really, this is just a special case of the general problem of what should happen when the maximum ray level is hit. Currently my kludgy fix is to say when the maximum level is hit we simply don't shoot the reflection ray but return the background color times the reflectivity and call it a day. I've also tried just taking the bottommost ray's surface and boost the diffuse component, but this is scary. I am interested to hear how other ray tracer coders have handled this problem. (Perhaps I can glue up the answers into an article for the Ray Tracing News).
____
Greg Ward (greg@hobbes.lbl.gov) replied:
I don't think you will see black faces due to TIR because the light in a mutifaceted gem eventually finds a surface at a passable angle. If the light started from inside a pane of glass or something, it might not ever get out, it's true, but that's the way it is. The light that can't get out also couldn't get in, i.e. any incident angle up to 90 degrees refracts to less than the TIR angle.
[Greg focusses on the gem case in particular, but I am interested in some general solution for the situation. Thinking about it later, I realized that in some cases there just is no good answer: TIR is how fiber optics work, and rays down a fiber optic cable yields a huge ray depth. - EAH]
____
Chris Larson (larson@kingston.cs.ucla.edu) replied:
Two comments. First of all, I don't hard code any "maximum level" for rays; rather, I maintain a fraction for each ray, indicating its contribution to the intensity (color, whatever) assigned to the root ray. Recursion stops when this fraction falls below one bit of intensity in the final color. I suppose this is still a hard limit in one sense, but it allows as many reflections/refractions as necessary to obtain this approximation of the root ray's color. It also has the nice property that reflected/refracted rays which don't contribute much can be solved without recursing to a hard maximum level. Although this could just as easily be done with the maximum level approach, my method provides this without any additional complexity.
However, this method can still lead to infinite total internal reflection. To combat that problem, rays are attenuated based upon the distance they travel through any transparent (or translucent) media. This attenuation is based upon the physical characteristics of the media in question (exponential decay for non-conductors, different for conductors because the index of refraction is complex for an electrically conductive medium; I think there is another thread around here discussing this very point). Such attenuation is in addition to the typical division of energy performed between reflected and refracted rays.
Nothing about either of these approaches is new and different; it was simply my chosen solution to the same problem.
[For more on adaptive depth, see IEEE CG&A, Roy Hall, A Testbed for Realistic Image Synthesis, v.3, n.8, Nov. 1983. This method bears repeating here as it is a good technique to save time; I use it too. But without a maximum depth you can potentially bounce a ray forever, as with the fiber optic case. After getting people's insights, my conclusion for TIR for now is what Hall concluded, that a maximum depth is called for just to avoid a near-infinite compute time for each TIR ray. However, this leads back to my original question: how to shade once I reach this maximum level? If you have written a ray tracer, please let me know how you treat this case. -EAH]
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"Anomalous dispersion", the peculiar asymptotic curves for refractive index vs. frequency which I mentioned before, can yield a refractive index of less than 1.0, but apparently only occurs at frequencies higher than visible light (e.g., UV and X rays). Or so say Born & Wolf in "Principles of Optics", 1964 edition, pp. 93. Another reference I read contradicts this, saying that it happens in dyes -- which are necessarily colored, due to the presence of an absorption band in the visible frequencies.
We suspect that the 6000-8000 Angstrom area of the spectrum holds a roseate coloured light which we can utilize in making spectacles. By having an index of refraction which is less than 1.0, the light itself then must be moving faster than the speed of light in vacuum. As you no doubt have surmised, such spectacles would have the property of being able to look forward in time, though admittedly only a small amount. However, we believe with an elaborate application of smoky dispersive materials and mirrors we can make a marketable product.
Ken suggests the names:
"Pretemporal imaging systems"
or
"Supersimultaneous transmission optics"
for the industrial version, but what we need is a good consumer brand name, and that's where you can come in (my suggestions of Turbo-X-Lenses and Tachyon-Lenses are lame in the extreme). Just send us your suggestions along with all your loose change and convertible bonds and we'll let you in on the ground floor (or even the basement) of our enterprise.
I think we could have an initial public offering at SIGGRAPH at a nickel a share (for all those nickels which Ken Perlin should have made for developing turbulence functions). I say we put all the initial money into the exploration of amber colored zymurgical beverages and their optical properties when placed in prism sided glass mugs.
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To find out who is leading the way with PNG, check out Greg Roelf's page at:
http://quest.jpl.nasa.gov/PNG/
Greg is one of the PNG spec developers (I'm a minor one also) and is one of the zip/unzip developers. The page cited above will lead you to lists of software that already supports PNG (there's a lot) as well as portable source code libraries if you want to implement PNG in your own application. These are the Libpng and Zlib libraries, which are written in C and have been tested on many platforms.
CompuServe has endorsed PNG, and Spyglass Mosaic supports inline PNG now. Any browser can support PNG via helper applications. Under UNIX, there are patches available for adding PNG support to Mosaic and Netscape.
PNG incorporates all the features that make GIF popular, and provides a badly-needed update including support for true color, gamma, and alpha channels. PNG uses the same deflate/inflate algorithm as zip/unzip/gzip that is as patent safe as is possible these days. PNG and JPEG are complementary; the main difference being the use of lossless vs lossy compression. Unlike GIF, both support true color (PNG also supports palette based color.)
____
Tom Lane (organizer of the Independent JPEG Group) notes:
PNG is always going to be much faster at decompression than compression, same as everything else that's based on the "zip" compression method. This is the right tradeoff for most applications: you make an image file once and then look at it many times.
PNG is strictly lossless. On photo-like images it will never be able to beat JPEG for file size; that's not the point of PNG. The other side of the coin is that JPEGed images can turn to mush after a few editing cycles due to lossiness. PNG files won't.
PNG is smaller than other lossless image formats. For instance, it can store a palette-color image in less space than GIF. But lossy compression methods will always beat lossless methods on file size; that's the only reason people put up with the obvious drawbacks of a lossy method.
____
And this just in from Tim Wegner:
The W3C is now releasing the PNG Specification, Version 0.92, as a W3C Working Draft. You can see the draft through the link at the W3C Technical Report page: http://www.w3.org/pub/WWW/TR/
[This is important for PNG acceptance, as it is now aimed to become a Web standard. -EAH]
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Here are a couple of my favorite references:
American Cinematographer Manual
Published by the American Society of Cinematographers (ASC), this is the
Foley, vanDam (F&H) of all cameramen and lighting experts. Complete
descriptions of all types of cameras, lights, lenses, filters and supports
are given along with comparison tables. Techniques for special visual
effects (background plates, compositing, miniatures, travelling mattes,
optical printers), and special techniques (arial, underwater, arctic,
tropical, day for night, stereo 3D, sound, television) are briefly
discusses. There's even a section on Computer Graphics. This is a small
book that can be found in many good book stores and sells for about $70.
ISBN 0-935578-11-0
[Sing-Choong Foo (sfoo@graphics.cornell.edu) notes that "the price is only $49.95. You can order it from ASC (American Society of Cinematographers) (213)969-4333.]
Millerson, Gerald; "Lighting for Television and Film"; 3rd Ed.; Focal Press;
This is a very good introduction on how to light for film and video.
Excellent discussions of the aesthetic qualities of light and perception.
The basic principals of lighting (key, fill, shadows, balance, direction,
continuity) are summarized. Detailed chapters on lighting people, the
production process, atmospheric lighting, light sources and equipment,
color temperature, picture control, scenery, and visual effects. A great
book to learn how to light a scene to achieve a specific mood. First
published in '72, third edition revised in '91. ISBN 0-240-51299-5
Malkiewicz, Kris; "Cinematography"; Simon & Schuster; '73 & '89
A shorter, but thorough treatment of the basic techniques. Chapters
include Cameras, Films and Sensitometry, Filters and Light, Picture
Quality Control, Sound Recording, Cutting and Lab Work, The Basics of
Optical Printing, Vehicle and Underwater Cinematography, Production, and
Video Transfer. ISBN 0-671-76220-6
Other excellent texts include:
Lowell, Ross; "Matters of Light & Depth"; Broad Street Books; '92 ISBN 1-879174-03-0
Brown, Braun & Grondin; "Lighting Secrets for the Professional Photographer"; Writers Digest Books; '90; ISBN 0-89879-412-9
Hart, John; "Lighting for Action"; AMPHOTO; '92; ISBN 0-8174-4225-1
These contain other references, and I will happily send you more if you send me mail. I think I might like the third one because the author makes special mention of Haskell Wexler, who is, of course, not related to me in any way I know of. I am indebted to Jean Cunningham, one of our local lighting experts for most of these references.
I cannot emphasize enough how much there is to learn from this body of knowledge. Good lighting with a bad renderer always looks better than bad lighting with a good renderer, IMHO.
Daniel Wexler
R&D Staff, Pacific Data Images
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Three possible ways of getting rid of the divide are:
1 : lines of constant z (that's q to you, for some reason)
First, find the 2-d, screenspace, line along which the polygon has
constant z (or q). There exists such a line for any polygon in any
orientation. Then scan convert your polygon along this line. As z
is constant along this line, linear interpolation of the texture
coordinates does give you the more or less correct result (as
long as you do the divide along the poly's edges to get the correct
values to interpolate between).
pro's : - potentially very fast.
- if all your poly's are walls/floors (ala doom/marathon),
this gets a lot easier because then the lines of constant
z are vertical/horizontal lines across the screen.
- very easy to do light diminishing with distance (or fog).
con's : - nasty implementation. You have to be *very* careful how
you scan convert along these lines in order to avoid holes
and/or multiple writes to the same pixel. This is especially
true for the edges.
- as the on-screen line along which you are scan converting
is (necessarily) an approximation of the real line of
constant z, the texture coordinates will not be absolutely
correct.
2 : (scanline) subdivision.
This is what you mention above: the divide and conquer approach.
You linearly interpolate the texture coordinates, while making sure
the distance across which you interpolate doesn't get too big. You
can do this by cutting up big polygons into lots of smaller ones, or
(the more popular approach) by sampling the texture coordinates at
a couple of points for each scanline (by doing the divide) and
interpolating between them.
pro's : - relatively easy implementation.
- when correctly implemented, easy to trade a bit of speed
for accuracy and vice versa. That is, you can simply
decide which combination of speed and accuracy looks best
after you've implemented it.
con's : - there are usually still artifacts from the linear
interpolation visible. These are in part caused by:
- hard to find a good rule for the subdivision process (how
to cut up the polygons/scanlines). This must be continuous
from frame to frame as sudden jumps in the places where
you cut/sample will produce very strange looking results.
(I think this is where SlipStream 5000 (a new 3D racing
game for the pc, there's a demo somewhere) breaks down.
Just fly close to a wall and move around a bit, it makes
me want to cry :^).
3 : n-th order approximation.
Here you do not linearly interpolate the texture coordinates (a
first order approximation), instead you use a higher order of
approximation. Usually, this means a quadratic approximation, where
you pick a third point somewhere between the endpoints and "draw" a
2nd order polynomial (a parabola) through these points. The
mathematics of this are not completely straightforward, but they
aren't exactly rocket-science either. If you want me to explain this
in more detail, mail me, or, better yet, post here so others can read
it as well.
pro's : - very nice, clean implementation. Your inner loop can look
as simple as :
for (x=0; x < dx; x++){
*screenptr++ = texture[u][v];
// well, ok, you'll have to round
u += du; // u and v, but that's no real
v += dv; // problem with fixed point math.
du += ddu;
dv += ddv;
}
- (almost) as fast as approach 1).
con's : - we're approximating a hyperbola by a parabola, so the
results will not be exact. (but pretty close, if you're
careful)
- not so easy to pick a midpoint. It's not hard to formulate
in mathematical terms where the ideal midpoint is, but
calculating it in real time is very expensive, You can,
however, get quite good results with some simple rules.
These are not the only ways of doing it, but I haven't heard of another method that will ge reasonable results at a reasonable frame rate. All of these approaches are approximating, i.e. will not look as good as the real thing, but you can get pretty close. At the moment, there is no realtime free direction texturemapper for the pc, ppc, or any other "low-end" machine that does do the divide, and some of them (descent, magic carpet...) do look quite good.
My personal favourite is approach nr. 3 (possibly combined with 2), although this is probably because I like a clean, elegant, solution more than anything else. I've heard somewhere that Descent uses this approach, but I'm not sure about this.
And while we're on the subject, these are just my ideas/opinions on the subject, and while I don't think they are wildly of the mark, I don't think they are 100% correct either. If anyone sees things differently, just say so.
____
Bob Pendleton (bobp@bga.com) comments:
Linear interpolation with runs of 16 pixels seems to be the best trade off between speed and image quality for perspective texture mapping using current PCs.
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