Re: [R] Worm distribution :-)
Alberto Monteiro wrote: I don't know if anyone has heard this tale, but it runs more or like this way: A biologist was studing a (semi-spherical) cave where bats lives. He fell asleep in the cave, and he woke up in the middle of the night. Half-dreaming, he thought that he was outside, because glow-worms were living in the walls, and they looked like stars. However, he noticed that, unlike a real sky, these stars had no _pattern_: there were no recognized images like The Cross, a Scorpion, The Hunter, etc. When he woke up, he conjectured that the reason we _can_ see patterns in the real sky is that the stars are randomly distributed, while the glow-worms tried to keep a distance to each other. My question: what is the best way to generate a glow-worm-like distribution? I imagine that using a Latin Hypercube would leave too many holes in the (x,y) plane. Hi Alberto, I once had to do this to generate random dot stereograms for perception experiments. One easy way is to build from one corner or edge (almost all these are rectangular, you can just clip out the figure you want later). The user defines a hit area for each successive point that contains the minimum and maximum allowable distances from the two nearest points (typically a small square). Points can be generated with two uniform random numbers having a mean of the distance to the center of the hit area and a range spanning the hit square. It was easy to churn out constrained random dot patterns on the fly with this method. I no longer have the FORTRAN code that performed this, but I could probably knock it up in C or R if you're stuck for a method. Jim __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Re: [R] Worm distribution :-)
Jim Lemon wrote (in response to a question from Alberto Monteiro): I once had to do this to generate random dot stereograms for perception experiments. One easy way is to build from one corner or edge (almost all these are rectangular, you can just clip out the figure you want later). The user defines a hit area for each successive point that contains the minimum and maximum allowable distances from the two nearest points (typically a small square). Points can be generated with two uniform random numbers having a mean of the distance to the center of the hit area and a range spanning the hit square. It was easy to churn out constrained random dot patterns on the fly with this method. This appears to be: (a) closely related to the ideas of ``partially ordered Markov models'' developed by Noel Cressie et al (see, e.g. Cressie, N., Zhu, J., Baddeley, A. J., and Nair, M. G. Directed Markov point processes as limits of partially ordered Markov models. Methodology and Computing in Applied Probability, 2, 5-21). (b) re-invention of the wheel. As I pointed out in a previous posting to the list there are a brazillion ways of generating point patterns with interpoint inhibition, readily available in R. cheers, Rolf Turner [EMAIL PROTECTED] __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
[R] Worm distribution :-)
I don't know if anyone has heard this tale, but it runs more or like this way: A biologist was studing a (semi-spherical) cave where bats lives. He fell asleep in the cave, and he woke up in the middle of the night. Half-dreaming, he thought that he was outside, because glow-worms were living in the walls, and they looked like stars. However, he noticed that, unlike a real sky, these stars had no _pattern_: there were no recognized images like The Cross, a Scorpion, The Hunter, etc. When he woke up, he conjectured that the reason we _can_ see patterns in the real sky is that the stars are randomly distributed, while the glow-worms tried to keep a distance to each other. My question: what is the best way to generate a glow-worm-like distribution? I imagine that using a Latin Hypercube would leave too many holes in the (x,y) plane. Alberto Monteiro __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Re: [R] Worm distribution :-)
You are talking about random point patterns, since the glow-worms appear as ``stars'' (= points). See the package ``spatial'' (which comes with R) and try simulating a pattern using Strauss(). Or install the package ``spatstat'' from CRAN --- in this package there is a variety of ways to simulate ``regular'' random point patterns --- rMaternI, rMaternII, rSSI, and rmh (which simulates several point pattern models depending on the specified Papangelou conditional intensity function). cheers, Rolf Turner [EMAIL PROTECTED] __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Re: [R] Worm distribution :-)
Have you heard of Boids? (google it if not) Depending upon what you want this for a simulation approach might be appropriate. Regards, Ana Nelson On 23 Oct 2006, at 18:18, Alberto Monteiro wrote: I don't know if anyone has heard this tale, but it runs more or like this way: A biologist was studing a (semi-spherical) cave where bats lives. He fell asleep in the cave, and he woke up in the middle of the night. Half-dreaming, he thought that he was outside, because glow-worms were living in the walls, and they looked like stars. However, he noticed that, unlike a real sky, these stars had no _pattern_: there were no recognized images like The Cross, a Scorpion, The Hunter, etc. When he woke up, he conjectured that the reason we _can_ see patterns in the real sky is that the stars are randomly distributed, while the glow-worms tried to keep a distance to each other. My question: what is the best way to generate a glow-worm-like distribution? I imagine that using a Latin Hypercube would leave too many holes in the (x,y) plane. Alberto Monteiro __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting- guide.html and provide commented, minimal, self-contained, reproducible code. __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
