Archive for June, 2011

better congestion control in linux

Note to self:

modprobe tcp_westwood
sysctl net.ipv4.tcp_available_congestion_control=westwood

plenoptic cameras and the meaning of photography

Raytrix introduced the R11 Lightfield Camera not too long ago. It is still low-res and expensive, but improved versions of these should eventually catch on — they make too much sense not to.

The idea of plenoptic cameras has been thrown around for quite a while. Instead of a conventional camera with a single lens focusing a single object plane onto the in-camera image plane (i.e. the sensor), a plenoptic camera attempts to capture enough additional information so as to be able to reconstruct “all possible” images that can be obtained from the light entering the same aperture. The most talked-about application is subsequent refocusing; if it were just this, then multi-capture with mechanical focal-length sweeps using a conventional camera would suffice. Another is stereograms, but again, two spaced shots would suffice for that. A plenoptic camera does more in one shot and makes these merely special post-processing cases. The simplest conception of a plenoptic camera is essentially an array of mini-cameras (microlens + micropixel array for each logical pixel) that separately captures light from all directions at each point in the image. In between conventional cameras and plenoptic cameras are perhaps smarter, sparser non-regular arrays like these coded aperture systems that hark back to old radio-astronomy literature. These have good signal processing properties from a deconvolution perspective, but the full-array plenoptic camera like the R11 seems fully general, and with some future industrial scaling, the saved expenses of a compromise may be inconsequential.

Fine, so a plenoptic camera may make clever use of its given aperture size, but do we really get something for nothing? To answer that, first a digression.
(Read the article)

risk matching in gambling

An argument for playing a game such as poker with “real” money is that it forces people to play with true risk-reward calculations. While this is certainly better than playing without risk, there exists the question of how to match risk profiles among players. With enough players (large liquid market), they can self-sort by stake size, and this seems fair. With only few people though, the situation is turned around, where a stake size has to be agreed upon at some clearing size (so that enough people agree to play the game) rather than chosen individually, and that same amount of money may be considered as very different values by different people. A pauper and a millionaire do not see $100 as the same value, and will adjust their utilities accordingly, and this will materially affect wagering. Since risk is measured in utility units, it is desirable to match utilities rather than dollar amounts. But there isn’t an agreed-upon utility currency. Or is there?
(Read the article)

pessimism, or risk aversion?

I found the title of this article, “Honeybees might have emotions,” hard to believe. But then what is “emotion”?

Bateson and Wright tested their bees with a type of experiment designed to show whether animals are, like humans, capable of experiencing cognitive states in which ambiguous information is interpreted in negative fashion.

and from the referenced paper itself:

We show for the first time that agitated bees are more likely to classify ambiguous stimuli as predicting punishment. Shaken bees also have lower levels of hemolymph dopamine, octopamine, and serotonin.

If an “ambiguous” stimulus is one that is unassociated with anything but is interpreted as a mixture of two known stimuli (or if it is explicitly some mixture-state stimulus… is it in this case?), then isn’t all this simply risk-aversion? If so, then basically bees have concave utility like probably all evolved organisms naturally should — to stay alive. Indeed, the fact that the actual chemical signaling mechanism for risk-aversion is associated with “pessimism” or “depression” in higher organisms is the more intriguing (but on second thought, obvious) result, suggesting that emotions are just internal (psychological) reward adjustments that drive external (behavioral) risk adjustments. Emotions bend the utility “function” into shape by applying compensatory rewards to the actual external rewards: a risk and reward add-on circuit.

on abstraction

I’ve come across the word “abstraction” in exactly three contexts. The first is in computer science, where “abstraction” is the hiding away of details within a black box whose interfaces are well defined. The second is in mathematics, where “abstraction” is the induction from special cases. The third is in art, where “abstraction” seems to be independence from concrete meaning. Abstract is “drag away” in Latin, so it is likely to be defined by its opposition — that which it is “dragged away” from. No wonder it has so many different meanings. Interpreted thusly, the first context for “abstraction” may better be termed “opacity” in opposition to “transparency”; the second, “genericity” in opposition to “specificity”; and the third, “notionality” in opposition to “concreteness.”

What is the point of abstraction? Is there something terrible about transparency, specificity, or concreteness; are they not qualities that we praise, for the clarity that they provide?
(Read the article)

on yak dung electricity generation So I was looking into how many yak dung pies are required to charge an iPad once, but I couldn’t find how much a dried yak dung pie weighs.

I did get some information like, an efficient yak dung stove can produce heat at a rate of 17198 kJ/kg (of dried yak dung). And that it burns at 400 degrees Celcius, which works out to a Carnot efficiency of about 60% presuming the outside environment is the Qinghai-Tibet Pleateau at 0 degrees Celcius. Okay, a little optimistic, I grant. An iPad battery is rated at 24.8 W-hr, which is like 89.28 kJ of energy, and that means it just takes 9 grams of dried yak dung to do a full charge. Incredible, at first glance.

Then I realized that an iPad doesn’t take much power to run at all. Even more of a killjoy to my nascent yak dung entrepreneurial instincts is HP Labs, which already designed a megawatt datacenter fueled by dung. They say that a single dairy cow produces 125W in recoverable electricity in dung alone (yes, I did the math). That’s not even counting making cows run circles to generate more electricity from kinetic motion. Alas.