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Escher and Elliptic Curves
melquiades writes "Mathematician Hendrik Lenstra was struck by the blank spot in M. C. Escher's Print Gallery . Why is the spot blank there, he wondered, and what should go in it? Although Escher, who had only a high-school mathematics background, drew the picture by brilliant and methodical intuition, the mathematical machinery underlying the image turned out to be elliptic curves (which come up in factorization, cryptography, and the proof of Fermat's Last Theorem). Lenstra and his colleagues were able to generate several breathtaking possible completions for the missing space. Read the story at the ever-registration-required NYT."
It's supposed to make individuals think. Without the space it's just an optical illusion. Whats next, threories explaining Mona Lisa using computers? Morphing?
What?! They've already done that. Well, fuck it, I'll go back to coding...
Mirror picture here
I have trouble believing anyone will take tech people seriously these days without a degree, but I think it's great to see that there's still an opportunity for a true genius to break that belief.
I've nothing to say here...
Some theories are that he wanted people to look at it and wonder. Some say he just didn't quite know how to proceed. This guy seems to think he's done what Escher meant to do, but perhaps didn't quite have the mathematical understanding to complete. Escher was always known for not being very book-smart and sort of amazed at what mathemeticians found in his works. He knew he was making them with some structural intent, but never really knew the theories behind what made them seem to click.
I've nothing to say here...
This is a page of Escher images that are posted with permission of the copyright holder. It's one of the best collections on the web. http://www.cs.unc.edu/~davemc/Pic/Escher/
This is perhaps one of my favorite drawings by esher and has been so for many years. Oddly enough, when I first saw the picture I was sorely pissed off because the picture didn't seem complete. What the hell was in that spot? I wanted to know badly and I couldn't possibly like the drawing until I did.
It was only when I came back to the picture years later. I tried to figure out what I would put in the spot that I realized how excellent the drawing is. It is a stunning metamorphosis between images and I believe the spot only serves to compound that perfectly. If the spot was there you would spend more time staring at the spot them following the transforming images around the outside. The subtly of the picture would be lost on people who were fascinated by the damn spot in the middle (as it was with me).
I'm not denouncing their work. It is very impressive and interesting to read. However I have no intentions of ever hanging a print up without that damn spot. (insert appropriate Shakespeare joke here)
World of Escher
This brings up another debate which is more interesting than why did escher leave a hole in the picture. What constitutes genius or brilliance? Is the artist who draws instinctively a genuis? Or is the mathematician who applies complex theories to pictures and natural patterns a genuis? Are both the artist and scientist manifestations of two sides of a coin? Or are we just playing into stupid labels? In the end, does it really matter that escher left a hole in the picture, or that people wonder why the hole is there?
This is one of the few articles where the troll responses made more sense than the real ones.
1. It's art. Just enjoy it.
2. Not everything needs a higher meaning
My opinion is that it is the drain that the world is circling around, but that is just MY opinion.
You guys are missing the big point... Freedom of speech, this is hardly a troll post (like what we get from the CLIT)... This is a well thought out and good post, just because you don't agree with it doesn't make it flamebait. /. at one time was about unpopular opinions, now it's about Troll-Bashing... If you don't love X, and you think MS can make a product that runs your company's LAN then you don't have a say on slash. It's boring, it's old, and it's killing slashdot. If your goal is to prove to the world that linux is the operating system to use, and that open source software is the greatest thing since sliced bread then you are going to need an audience to preech too, but if you call them all fucktards then I don't know who is going to listen, and it will be nothing but a blow to the /. community.
Your mammas flamebait.
I have been watching the animation full screen on repeat for 15 minutes.
I now have a splitting headache and am dizzy and nauseous.
Elliptic Curves:
curves of the form y^2 = Ax^3 + Bx^2 + Cx + D
pick values for A B C and D, the locus in 2 space (the cartesian plane, or R2) is the type of curve Escher was using.
In analysis, which is where all of the headline making math using Elliptic Curves, A B C and D (as well as x and y) can be complex numbers.
At this point things get complicated. I'm not going to fill up 1000 words explaining Riemann surfaces, algebraic functions, etc.
There are a lot of good pages out there.
In Capitalist America, bank robs you!
I tried to follow the link, but it actually sent my browser to the page I visited before.
That's impossible. Wait.... if water can flow upwards..... damn Escher!
A little history on MC Escher here.
HTH HAND
I got a 1600 on the SATs.
I think Escher meant to leave that area blank since it seems like the rest of the drawing is being drawn towards it. It's the focal point of the picture since that's where the picture in the gallery actually connects to the gallery. If you look closely, you'll notice that the frame of the picture is on it's way to meet with the picture itself, which is infact the gallery (woah, I've fallen into the loop). I think this dot was left up to the imagination. There is no correct solution, but this method is a terrific idea.
- A real programmer uses $ cat > a.out
Escher psychic factorization.
"God fights on the side with the best artillery." - Napoleon, Marshal of France - speaking truth to power
Are you referring to the perfect reflections in the middle marble? Example Picture
Ant(Dude) @ Quality Foraged Links (AQFL.net) & The Ant Farm (antfarm.ma.cx / antfarm.home.dhs.org).
> No one reallly knows except Escher, who's
> unlikely this late in the game the disclose it
> (ie. he's passed on).
Hey, it never hurts to ask. I lit a candle. Now, everybody hold hands, close your eye's and concentrate.
Lenstra gave a talk on the subject at the HP Research Labs Colloquium last July:
m
http://www.hpl.hp.com/infotheory/lenstra071101.ht
Abstract:
Elliptic curves form one of the hottest topics in arithmetic algebraic geometry. Applications of elliptic curves range from a proof of Fermat's Last Theorem to the design of secure cryptosystems. In the lecture we present, as a novel application of elliptic curves, a mathematical analysis of Escher's lithograph `Print Gallery'.
When you have nothing left to burn you must set yourself on fire
Just got the time to save everything and mirror it here before the Slashdot effect doomed the whole thing...
having downloaded a few loops (they'll make great screensavers), i'm now torn between two equal desires: part of me wants to research the proof of fermat's last theoren, while another part of me wants to put on some hendrix, drop some acid and stare at an endless escher loop for a few days
i thought, therefore i was...
Seriously.
The point is, that you can perfectly see the sort of space that Escher draws, or that I dabble in, without too much mathematics.
I quite often see the curves that Escher drew in his pictures.
Also, one can even understand hyperbolic geometry without any great understanding of the mathematics. I have even made new discoveries out there.
The thing is, that the relations that describe these things can be found quite intuitively. In this light, one does not need a "formal education" to see them.
His circle-limits, for example, were gleaned from a drawing in H.S.M. Coxeters' book, of the symmetry group of a {6,4}. My understanding comes from a similar drawing of a {7,3}.
Also, there are some of Escher's drawings where he assembled ideas into distinctly non-mathematical drawings, such as his final lithograph, Snakes [which is a poincine projection, coupled with one that bends inwards as well].
The fact is, that Escher understood certian constructs of absolute geometry, and was also an artist. Having read a number of his notes, I can understand how he came to devise his drawings.
I can draw reasonably accurate projections in hyperbolic geometry even without any understanding of hyperbolic trig, etc...
OS/2 - because choice is a terrible thing to waste.
Perhaps. Some people's brains are better than other at extrapolating phenomenon after a cursory glance; mathematics simply attempts to formally describe the extrapolation, so people who are unable to extrapolate by themselves can do so by applying the formal principles. Maurits Cornellis Escher was amongst the former people, and university gratuates are amongst the latter.
The white space is there 'cause the server's slashdotted, Sir. Escher's painting should go in it.
Trollem mirabilem hanc subnotationis exigiutas non caperet
intentionally left blank.
Sorry, back to bed with me.
Nerd rage is the funniest rage.
...and I found this: /.ed. Just my $0.02.
"The secret of its making can be rendered somewhat less obscure by examining the grid-paper sketch the artist made in preparation for this lithograph. (picture here)Note how the scale of the grid grows continuously in a clockwise direction. And note especially what this trick entails: A hole in the middle. A mathematician would call this a singularity, a place where the fabric of the space no longer holds together. There is just no way to knit this bizarre space into a seamless whole, and Escher, rather than try to obscure it in some way, has put his trademark initials smack in the center of it."
The whole article can be found here. I didn't see the site, apparently
We're Doomed
Are both the artist and scientist manifestations of two sides of a coin?
Of the people I've known, a brilliant scientist and a brilliant artist are most frequently found in the same person. It really isn't two sides of something but two different words for the same thing.
It is unfortunate that our culture has separated art and science, because both are manifestations of knowledge, critical thinking, and ingenuity. For example, Ludwig van Beethoven and Sigmund Freud each had profound insight into human psychology, but they employed different vocabularies and reached different audiences.
Healthcare article at Kuro5hin
Add M.C. Escher to the list of people who got out of architecture and into something better, like Rick White of Pink Floyd.
The difference between a madman and a genuius is that we force the madman to live in our world while the genius forces us to live in his.
--Karl Evander Kaufeld
The Mongrel Dogs Who Teach
The Mongrel Dogs Who Teach
I don't think they've improved on Escher, any more than I think they've "ruined" him. They've just used his artwork as a springboard for their own. For a community that likes to rhapsodize about the value of the public domain and the intellectual commons, an awful lot of slashdotters seem to object to this.
The Mongrel Dogs Who Teach
1. Okay, haven't read the article yet because the site is effectively Slashdotted, but... During the Renaissance, I think it was (or maybe the 1700s), there was a genre of painting/drawing that involved using a highly reflective cylinder in the center of the work while creating it. The result was that the picture was distorted until you placed the cylinder in the center--then you could see in the cylinder itself the undistorted image. I've seen photos, but can't recall the name or the source. Anyhoo, that's what I thought of as soon as I saw this work. I'd be curious to know what shape cylinder you'd need, though, to make these images look normal!
c el laneous/Durer/Melancholia.html
6 /qid=1028039334/sr=1-1/ref=sr_1_1/103-5270790-4159 806)
2. My favorite work of art is Durer's (I don't know how to get an umlaut on that u) Melancholia I. It's got an angel of indiscriminate gender (when did we shift from thinking of angels as exclusively male to using the name "Angel" for women?), platonic solids, a magic square, a comet, a rainbow, drawing/navigational tools, and carpentry tools. But what is it about? I find it endlessly fascinating. Check it out at
http://www-groups.dcs.st-and.ac.uk/~history/Mis
3. Another example of mathematical interesting... um, -ness, in art is Celtic design. You can check out a discussion of it in "Turbulent Mirror" (http://www.amazon.com/exec/obidos/ASIN/006091696
Who moderates the meta-moderators?
Einstein had nominal education and apparently flunked math his first time through. Then he went and worked in a patent office... or so I've heard. I'm no expert.
::SNIP:: /-nE-"I/ ::SNIP::
M.C. Escher made money designing stamps and advertising pamphlets for a while. Seriously.
So maybe Genius is described as "intelligence" that is taught to one self. A kind of unexpected smarts that surpasses the expected level of smarts? But there are many other people who are geniuses that have had formal learning.
Maybe genius is defined as "intelligence at a level that is commonly accepted to be extraordinary". Lets see what the dictionary has for us:
Main Entry: genius
Pronunciation: 'jEn-y&s, 'jE-nE-&s
Function: noun
Inflected Form(s): plural geniuses or genii
Etymology: Latin, tutelary spirit, natural inclinations, from gignere to beget
Date: 1513
1 a plural genii : an attendant spirit of a person or place b plural usually genii : a person who influences another for good or bad
2 : a strong leaning or inclination : PENCHANT
3 a : a peculiar, distinctive, or identifying character or spirit b : the associations and traditions of a place c : a personification or embodiment especially of a quality or condition
4 plural usually genii : SPIRIT, JINNI
5 plural usually geniuses a : a single strongly marked capacity or aptitude b : extraordinary intellectual power especially as manifested in creative activity c : a person endowed with transcendent mental superiority; especially : a person with a very high intelligence quotient
42 - So long and thanks for all the fish.
Isn't the artist where art and reality meet? Maybe that's what Escher was getting at ... after all - it's not just a blank spot, he put his signature there. If so, then filling in the "spot" may actually change the point of the drawing.
It's quite possible a percentage of the population can intuitivly apply some mathamtical concepts without indepth analysis.
On the other hand, we try to beat that out of them as students in elementry school.
Looking at the print you are drawn to the spot, just as the person depicted on the right side of the work seems to be. What does he see? If you think about it you realize that he sees the same thing you do, the back of his head. He is observing the same work you are. By including the spot Escher makes you part of the picture. If the person on the right is "Observer #1" and the person he is looking at is "Observer #2" you are "Observer #0". If the spot were filled in it wouldn't have the same effect. Go to the site and spend some time looking at the original work and the filled in version. I find that the original give a different sense of wonder and point of view than the new one.
Lasers Controlled Games!
On page 717 in Godel, Escher, Bach, Hofstadter explains the "central blemish" as follows...
"Though the blemish seems like a defect, perhaps the defect lies in our expectations, for in fact Escher could not have completed that portion of the pircture without being inconsistent with the rules by which he was drawing the picture. The center of the whorl is -- and must be -- incomplete. Escher could have made it arbitrarily small, but he could not have gotten rid of it."
What Lenstra was able to do was to figure out the structure of the picture. From there, he was able to generate a suitable center so that none of the relationships between the four various pieces are disrupted.
This is the reason why this is some pretty neat work.
This is a painting by Escher, not some random pice of art. All of Eschers paintings are in some way mathematical and thought-inviting. They are supposed to be analysed this way, althought not to this degree of exactness. Read your GEB before you continue trolling!
--The knowledge that you are an idiot, is what distinguishes you from one.
What great timing! The Bridges conference was just held this weekend. It's all about work like this--stuff that bridges between math, art, music, and science. Neat stuff!
Here's a copy on Artchive. This site claims 25,000 hits a day, so they should be relatively Slashdot-Effect-resistent. Don't forget the tip jar!
Wow, I can't beleive that no slashdotter has mentioned Douglas Hofstadter's excellent book "Godel, Escher, Bach: An Eternal Golden Braid"
The book is over 20 years old and still a must-read for anyone who is interested in CS and/or art and/or music. It's fun, funny, and will make your brain hurt.
Grab it at your favorite book store|website.
Slashdot: come for the pedantry, stay for the condescension.
Is it possible that Escher's brain was wired in a particular way that allowed him to create works of art based on mathetic principles without knowing the underlying structure?
Yeah, same as everyone else's. Ever draw a circle? Know all the properties of a circle? Ever draw a cloud? Know all the properties of the fractal structure of clouds?
In the end, does it really matter that escher left a hole in the picture, or that people wonder why the hole is there?
In the end is the heat death of the universe. Sooner than that, in the end is our own death. In the end, none of it matters. But it's kinda cool no matter what.
In a flight to the Netherlands, Dr. Hendrik Lenstra, a mathematician, was leafing through an airline magazine when a picture of a lithograph by the Dutch artist M. C. Escher caught his eye. Titled "Print Gallery," it provides a glimpse through a row of arching windows into an art gallery, where a man is gazing at a picture on the wall. The picture depicts a row of Mediterranean-style buildings with turrets and balconies, fronting a quay on the island of Malta. As the viewer's eye follows the line of buildings to the right, it begins to bulge outward and twist downward, until it sweeps around to include the art gallery itself. In the center of the dizzying whorl of buildings, ships and sky, is a large, circular patch that Escher left blank. His signature is scrawled across it. As Dr. Lenstra studied the print he found his attention returning again and again to that central patch, puzzling over the reason Escher had not filled it in. "I wondered whether if you continue the lines inward, if there's a mathematical problem that cannot be solved," he said. "More generally, I also wondered what the structure is behind the picture: how would I, as a mathematician, make a picture like that?" Most people, having thought this far, might have turned the page, content to leave the puzzle unsolved. But to Dr. Lenstra, a professor at the University of California at Berkeley and the University of Leiden in the Netherlands, solving mathematical puzzles is as natural as breathing. He has been known, when walking to a friend's house, to factor the street address into prime numbers in order to better fix it in his mind. So Dr. Lenstra continued to mull over the mystery and, within a few days of his arrival, was able to answer the questions he had posed. Then, with students and colleagues in Leiden, he began a two-year side project, resulting in a precise mathematical version of the concept Escher seemed to be intuitively expressing in his picture. Maurits Cornelis Escher, who died in 1972, had only a high school education in mathematics and little interest in its formalities. Still, he was fascinated by visual mathematical concepts and often featured them in his art. One well-known print, for instance, shows a line of ants, crawling around a Moebius strip, a mathematical object with only one side. Another shows people marching around a circle of stairs that manage, through a trick of geometry, to always go up. The goal of his art, Escher once wrote in a letter, is not to create something beautiful, but to inspire wonder in his audience. Seeking insight into Escher's creative process, Dr. Lenstra turned to "The Magic Mirror of M. C. Escher," a book written (under the pen name of Bruno Ernst) by Hans de Rijk, a friend of Escher's, who visited the artist as he created "Print Gallery." Escher's goal, wrote Mr. de Rijk, was to create a cyclic bulge "having neither beginning nor end." To achieve this, Escher first created the desired distortion with a grid of crisscrossing lines, arranging them so that, moving clockwise around the center, they gradually spread farther apart. But the trick didn't quite work with straight lines, so he curved them. Then, starting with an undistorted rendition of the quayside scene, he used this curved grid to distort the scene one tiny square at a time. After examining the grid, Dr. Lenstra realized that carried to its logical extent, the process would have generated an image that continually repeats itself, a picture inside a picture and so on, like a set of nested Russian wooden dolls. Thus, the logical extension of the undistorted picture Escher started with would have shown a man in an art gallery looking at print on the wall of a quayside scene containing a smaller copy of the art gallery with the man looking at a print on the wall, and so on. The logical extension of "Print Gallery," too, would repeat itself, but in a more complicated way. As the viewer zooms in, the picture bulges outward and twists around onto itself before it repeats. Once Dr. Lenstra understood this basic structure, the task was clear: If he could find an exact mathematical formula for the repetitive pattern, he would have a recipe for making such a picture with the missing spot filled in. Measuring with a ruler and protractor, he was able to estimate the bulging and twisting. But to compute the distortion exactly, he resorted to elliptic curves, the hot topic of mathematical research that was behind the proof of Fermat's last theorem. Dr. Lenstra knew he could apply elliptic curve theory only after reading a crucial sentence in Mr. de Rijk's book. For esthetic reasons, Mr. de Rijk explains, Escher fashioned his grid in such a way that "the original small squares could better retain their square appearance." Otherwise, the distortion of the picture would become too extreme, smearing individual elements like windows and people to the point that they were no longer recognizable. "At first, I followed many false leads, but that sentence was the key," Dr. Lenstra said. "After I read that, I knew exactly what was happening." Escher was creating a distortion with a well-known mathematical property: if you look at small regions of the distorted picture, the angles between lines have been preserved. "Conformal maps," as such distortions are known, have been extensively studied by mathematicians. In practice, they are used in Mercator projection maps, which spread the rounded surface of the earth onto a piece of paper in such a way that although land masses are enlarged near the poles, compass directions are preserved. Conformal principles are also used to map the surface of the human brain with all the folds flattened out. Knowing that Escher's distortion followed this principle, Dr. Lenstra was able to use elliptic curves to convert his rough approximation of the distortion into an exact mathematical recipe. He then enlisted a Leiden colleague, Bart de Smit, to manage the project and several students to help him. First, the mathematicians had to unravel Escher's distortion to obtain the picture he started with. A student, Joost Batenburg, wrote a computer program that took Escher's picture and grid as input and reversed Escher's tedious procedure. Once the distortion was undone, the resulting picture was incomplete. Some of the blank patch in the center of "Print Gallery" translated into a blurred swath spiraling across the top of the picture. So, the researchers hired an artist to fill in the swath with buildings, pavement and water in the spirit of Escher. Starting with this completed picture, Dr. de Smit and Mr. Batenburg then used their computer program in a different way, to apply Dr. Lenstra's formula for generating the distortion. Finally, they achieved their goal: a completed, idealized version of Escher's "Print Gallery." In the center of the mathematician's version, the mysterious blank patch is filled with another, smaller copy of the distorted quayside scene, turned almost upside-down. Within that is a still smaller copy of the scene, and so on, with the remaining infinity of tiny copies disappearing into the center. Since Escher's distortion was not perfectly conformal, the mathematician's rendition differs slightly from his in other ways as well. Away from the center, for example, the lines of some of the buildings curve the opposite way. The researchers also used their program to create variations on Escher's idea: one in which the center bulges in the opposite direction, and even an animated version that corkscrews outward as the viewer seemingly falls into the center. After a recent talk Dr. Lenstra gave at Berkeley, the audience remained seated for several minutes, mesmerized by the spiraling scene. While Dr. Lenstra has solved the mystery of the blank patch and more, one question remains. Did Escher know what belonged in the center and choose not to represent it, or did he leave it blank because he didn't know what to put there? As a man of science, Dr. Lenstra said he found it impossible to put himself inside Escher's mind. "I find it most useful to identify Escher with nature," he said, "and myself with a physicist that tries to model nature." Mr. de Rijk, now in his 70's, said he believed Escher knew his picture could continue toward the center, but did not understand precisely what should go there. "He would be astonished to experience that his print was still much more interesting than was his intention," Mr. de Rijk said. He added that while he knew of another effort to fill in Escher's picture, it was not based on an understanding of the mathematics behind it. "He was always interested when somebody used his prints as a base for further study and applications," Mr. de Rijk said. "When they were too mathematical, he didn't understand them, but he was always proud when mathematicians did something with his work."
These are lithographs, stone etchings that were
used to print onto ink soluble media, right?
I hope the masters are safe somewhere, and that the technique for printing with them isn't a lost art already.
A gif of the image or an iron-on t-shirt simply doesn't have a certain quality that seeing an original print of this type of work does have.
I remember two life-changing moments in the development of my art appreciation:
1. Seeing a Van Gogh painting in a museuem gallery.
I had seen this picture many, many times, in books, on posters, etc. It is a very simple painting of sunflowers. But when I saw the real thing, it absolutely took my breath away. There is a third dimension to the paint that is utterly lost in a photograph of it, and there are qualities to the color of the original that would be impossible to describe, and a photograph only comes so close to capturing this, and only for one angle, one set of light conditions, etc.
2. Salvador Dali's Psychedelic Toreador.
I have had a print of this painting for years, and see something different in the detail every time I look at it. However, I never realized how very large Dali's original paintings are! There is extraordinary detail that might be well-reproduced on a laser print, but even if the details are there, something is lost.
I'm not an artist or an art student, so I surely lack the language to say what I'm trying to say here. I suppose it's that I would like to see this etching in person, and that I'm really interested in the process used to make this type of work. Oh, and that in 10,000 years, the alien archaeologists will find the stone originals intact.
-fb Everything not expressly forbidden is now mandatory.
The only reason the images on the site work is that they blow up the center so that you can see what's really going on. Without the zoom, you can't really make much sense of the center of the picture, anyway.
Richard Feynman is another fabulous example of this. Here's a link to Amazon for a book about him and his art. He became interested in drawing and got very good at it. If you can find this book, it's worth a look, if just for all the pictures and the entertaining introduction by Mr Feynman himself.
That's what you get for assuming. It's not a Droste effect picture at all.
It's the world projected flat as seen from the inside of a Klein bottle. The empty space in the middle is the mouth of the bottle.
That's also why the signature, litho number, and other information is in the center: it's the only unused portion of the surface, which encompasses the entirety of the world.
The clue is the embedding of the gallery in the outside world, but the outside world in the image in the gallery.
Escher is well known for projections of images onto/into objects (e.g. his self portrait on the reflective sphere held at arm's length).
It's amusing that some well-meaning person would come along and try to put a cap on Escher's Klein bottle.
-- Terry
Escher knew what he was doing -- and so did the people who did this research. I don't think they were trying to improve on the drawing; they just entered into the intellectually playful spirit of Escher's art, and started asking questions and making cool pictures.
As the NYT article points out, Escher himself was always thrilled when people found his artwork to be a springboard into new research and new ideas. He was always very derisive of highbrow artistic purism, and I imagine he would have been delighted with these new images.
The original print is great, and so is this research.
One does not need a formal education to read the papers on the subject, or to write to their authors. All the same, I can more or less make out the mathematics, and the pictures inspire me. Whether I could *write* maths like that is unlikely.
On the other hand, I am not restricted to their pictures. I can see other things in my mind, and my maths, although not rigourously proven, is as good for producing results as any other. It's just home-grown on a different soil, that's all.
Maths, unlike Linux or French, can be done without any contact with the products of the existing structure. You can not participate in Linux or French without access to an existing source of Linux or French (including source and read-only material).
If your whole experience is with Arabic and DOS, then you will not be able to invent French or Linux yourself. On the other hand, you may derive the whole body of mathematics without access to any mathematical work or mathematician. If you develop like this, and then come in contact with the established body, you can compare notes, and see if you are in front of them or behind them.
OS/2 - because choice is a terrible thing to waste.