Archive for the ‘Computing’ Category.

Second Life

I visited Svend today at my old dormitory, [Skejbygård][]. This was to retrieve a table plate I had stored there while I was in Switzerland. I was glad to see that it was still in good shape.

Second Life logo But that was not the most interesting thing I saw there — Svend showed me a video about Second Life which looked really facinating. I have never seen something like this! Second Life is a vast online world where anybody can join and live, well…, a second virtual life.

To make this interesting, everybody can develop their own stuff. What is different from other such places is that you own your creations, and so you can start selling them to others. You can build 3D objects and script their behaviour, so people have created anything from moon rockets (that launch up into the air with flames and all…) to guns to artificially intelligent clown fish (think Nemo…).

There are now more than 200,000 residents in Second Life, and they are trading those virtual items like mad — a certain anshe Chung currently holds virtual land for some 250,000 US dollars! They explain in the movie how a software developer in some poor country makes half of his income in Second Life…

This whole idea is crazy and fantastic at the same time! :-) It will be very interesting to see where this leads…

Multi-Touch interfaces

Interactive lava lamp! I just saw a post on the Enlightenment mailinglist with a link to this page. There you will find a movie showcasing something they call multi-touch interfaces: touch screens where you can move multiple mouse cursors simultanously.

That gives you the ability to have an interface very much like what we saw in Minority Report where you can drag, scale, and rotate your photos with the touch of your fingers. I think it looks incredibly cool!

Mencoding movies

A beautiful MPlayer logo I found... Yesterday I made a small five-minute movie with my camera to show Stéphanie around in our new appartment (which she hasn’t seen since she’s still in Switzerland). I thought the movie was small, but what is small in terms to minutes and seconds quickly becomes very large in terms to bits and bytes.

And as we all know: size does matter :-) (I’ve always wanted to include that in one of my posts…)

Seriously, size does matter, especially if you plan on sending the movie to someone. The five minutes of 640×480 MPEG movie weighted in with a whopping 114 MiB. With an upstream bandwidth of only 256 Kb/s (and here is really is Kb/s and not Kib/s since network equiptment don’t follow the binary units) it would take at least an hour to transfer the movie.

So what do you do? You bring our your mencoder and resize it. That was surpricingly easy to do:

mencoder a.mpg -ovc lavc -lavcopts vcodec=mpeg4 -vop scale=320:240 -oac copy -o b.mpg

That did the trick for me, converting a.mpg into b.mpg which was only 23 MiB. The new movie has about a quarter of the pixels in the old one, and the size reduction fits nicely with that. I guess I could have squizzed it some more by compressing it harder, but I have not investigated how to do that.

Initial C# impressions

(This was originally meant as a comment for Janus’s post about C#, but it turned out to be longer than that…)

Mikkel and I have to learn C# for understanding the code from SCET, a project running before our project. Anyway, we’ve been looking at a tutorial, the one over at (just a random one).

And so far my impression of C# is that it is like C++ with garbage collection (so it’s a bit like Java) and with new keywords such as ref, out, etc for parameter passing. The ref is nice and clearer than pointers or C++ references so that’s good.

But the out keyword? I think it’s ugly. First it’s not necessary, just pass a ref parameter and have the method update it. A better solution would be to let methods return multiple values in a — ohh — tuple! That cannot be so hard, especially because it could be implemented as syntactic sugar for the out parameters. Several languages (Python, ML and Haskell comes to mind) have such a construct and it is very useful.

Also, who uses goto these days? I think that is a bit too low-level. But maybe it reflects that C# is mostly C++ with a standard garbage-collector?

The choice of having methods being non-virtual as default is also unfortunate. For you simply cannot predict if a given method will need to be specialized in the future. It hurts extendability if you have to go change the base class when you want to specialize a function in an derived class — the source for the base class might not even be available in which case you are stuck. The efficiency concern of virtual methods is not valid, IMHO. First, the compiler should be able to look at the class tree and note that method foo() isn’t redefined and thus make code that jumps to foo() just as efficiently as if it had been declared non-virtual. Second, the extra pointer indirection incurred for virtual functions shouldn’t be a problem with our modern fast CPUs and with normal programs that spend 99.9% of their time waiting for user input anyway.

In the book Object-Oriented Software Construction it is described how it is always right to call the redefined method B::foo() and often wrong to call A::foo() if class B extends A and if you want to take advantage of polymorphism. The problem is with invariants: since foo() is redefined in B it presumeably adds some functionality to A::foo(), it might update some internal state in the object to maintain a class invariant in B. Class A does not have the same class invariant as B and so calling A::foo() destoys it whereas calling B::foo() would have been the right thing to do. The method B::foo() is always safe to call since it must fulfill both the invariants of class A but also of B. Even though class invariants is something you mostly see in Eiffel code, the principle applies to all languages.

I also don’t get why delegates was invented — aren’t we just talking about a function pointer? It seems that I can declare a delegator for comparing numbers like this

public delegate int Comparer(object obj1, object obj2);

after which I can say

Comparer cmp = new Comparer(CompareFirstNames);

when the CompareFirstNames() method has a signature matching with the delegate just declared. One can then of course pass cmp to a sort() method as usual. But why could I not just pass CompareFirstNames to sort() directly? Why do I need to make a new delegate type (if it is a type, I’m not really sure…)? The notion of agents in Eiffel seem much more elegant and powerful. I would be happily surpriced if someone tells me that you can pass a semi-instantiated function as a delegator in C#. Agents in Eiffel work much like lambda functions in functional languages, giving you the possibility to easily adapt functions for use as callbacks. (Agents are described and compared with .NET delegates here, see also the OOSC course homepage.)

That was a lot of initial comments about what I’ve seen from C#. Don’t take this as meaning that it’s all bad — the idea of properties is a good one, for example. A garbage collector is also something which I’ve come to expect from a modern language, so that too is a welcome change from C/C++.

Shaking up the Planet

Still spinning... Thomas has been bugging me for not having as a link to our Planet. Well, bug me no more for now it is! :-) The old location also still works, of course, since we all know that cool URIs don’t change.