Tuesday, 29 September 2009

Bash hates enthusiasm

I often feel good when I get to "svn commit" something. I want to type something like:

svn commit -m "Example ready to go!"

But this enthusiasm is not welcomed by the bash shell, which says:

bash: !": event not found

So I have to type:

svn commit -m "Example ready to go\!"

and reaching for the '\' key just doesn't seem spontaneous. Especially on a Mac, where it's in the wrong bloody place!

So I end up being firm, and typing the ploddingly unenthusiastic command:

svn commit -m "Example ready to go."

Monday, 14 September 2009

A parallel algorithm which has been intriguing me

When I was in 3rd year at the University of Glasgow, I learned about the Floyd-Steinberg error diffusion algorithm, a method for performing digital half-toning on images.  This is an extremely simple algorithm which produces a wonderful effect. It converts a greyscale image, with many shades of grey:

into a black and white image which essentially looks the same; in particular which looks the same when viewed from a distance:

More generally the technique can be applied to a colour image using a large palette of colours, producing a similar image in a reduced palette.

The technique is pretty well-known, but I'll quickly summarise how it works for greyscale images: pixels are visited left-right, top-bottom.  A pixel with greyscale value p is replaced with 0 if p <= 128 (assuming 0 is black, 255 is white and 128 is mid-grey).  This has the effect of finding the “best match” for p in the reduced palette.  But if p goes from say 100 to 0 this has darkened the pixel significantly, and if p goes from 130 to 255 this has lightened the pixel significantly.  So, to compensate for this, the error associated with the palette decision for the pixel (old value minus new value) is propagated to surrounding pixels in the following way:

In the following diagrams, pixel 22 is set to 0 since it is less than 128. Then 22x7/16=10 is added to the pixel immediately to the right, 22x1/16=1 to the pixel to the bottom right, 22x5/16=7 to the pixel below, and 22x3/16=4 to the pixel to the bottom left.

(The Wikipedia entry for Floyd-Steinberg dithering gives some pseudo-code showing how the algorithm can be implemented.)

Parallelizing error diffusion

I'm no expert on this technique, and I don't know where the authors got these coefficients from, or why they work so well, but when I started working at Codeplay Software Ltd. in 2007 I was looking for interesting algorithms to try parallelising using their Sieve technology. I remembered this algorithm as being particularly cool, and wondered if it was parallelizable. I started from scratchy, trying to work out by myself if it could be parallelized, but I got stuck, so I did a bit of searching and came across this paper, which presents a parallel version of the method, by Panagiotis T. Metaxas.

The paper makes the following observation: the top-left pixel in the image must be processed first, and the pixel to its right second. But once these two pixels have been processed, either the third pixel in the top row could be processed, or the first pixel in the second row. By continuing to reason in this way, we can number each pixel with an integer representing the time step at which it could be processed in a fully parallel machine:

Observe that there is a skewed diagonal of pixels which can be processed in parallel, and the length of this diagonal increases, reaching its peak as it crosses the middle of the image, then decreases. For a large image there would clearly be a fair bit of opportunity for parallelization.

When I read this paper I didn't feel too stupid for having got stuck with this problem: his algorithm is pretty clever, and apparently even Donald Knuth believed the Floyd-Steinberg algorithm to be inherently serial - see a quote in the paper. (It occurs to me that perhaps Knuth has been somewhat mis-quoted - Knuth states that the algorithm is inherently serial because the pixel in the bottom right hand corner depends on processing of all other pixels. While the point about dependences is true, Metaxas's approach shows that there is opportunity for parallelism. But maybe Knuth simply meant that there is no way to process every pixel in parallel.)

To be continued.

Thursday, 3 September 2009

Trouble getting Mac Book Pro to connect to wireless networks

My mate Colin Riddell from Codeplay just helped me fix a problem with my beautiful new Mac Book, which I thought I'd write down the solution to before I forget, and in case other people have the same problem (looking online, a lot of people seem to have trouble getting their Macs to connect to wireless networks).

Before that, I'll tell you something that really pisses me off - the whole "A is better than B" issue with computers and operating systems.  The reason I got a Mac is that I have never used one before (well, actually my secondary school was kitted out with Mac Classics, but I haven't used one within living memory) and people often big them up.  When I was recording an album with my band Latonic, the studio we worked in used a Mac and swore by it (even though it crashed all the time, but in my recording experience, music software seems to push any machine to the limit).  So I've got a Mac Book Pro, and it is in many ways really lovely.  But it does crash.  People often say things like "My Mac has never crashed", or even "Macs don't crash".  But in my experience the former is not true, and the latter obviously cannot be true - of course one can write a program for the Mac which crashes!

Anyway, so far I really love the Mac, apart from its AirPort program, which is used to manage wireless networks, and which in my opinion is really crap.  In contrast I really like the Windows wireless network manager.

The problem I had over the last couple of days is that, having returned from holiday, I could not connect to my BT HomeHub wireless network using my Mac.  My PC laptop connects fine, and nothing has changed with the network.  I have MacOS 10.5.7.

My problem was that AirPort would say:
Status:  On
AirPort does not have an IP address and cannot connect to the Internet
Network name: BTHomeHub2-XXXX

At the top, Location is set to Automatic.

No matter what I tried, I could not get the computer to connect to the network (even though this had been working fine before I went on holiday).

After a lot of messing around, this is the solution my mate Colin came up with:

  • Under the "Location" drop down (in Network preferences) click "Edit Locations"
  • Click "+" to add a new location
  • Give this a name (I used NewLocation) and click "Done"
  • Click on "AirPort" on the left hand side, and make sure that your new location is selected under "Location"
  • You should see the same error message as usual about no IP address.
  • Click "Advanced", which will open a new dialogue box, and select the TCP/IP tab.
  • On the right, click "Renew DHCP Lease"
  • Click "OK"
You should now get connected fine to the wireless network.

The weird thing is that you might expect to be able to solve this problem by selecting the "Automatic" location, then clicking "Advanced->TCP/IP->Renew DHCP Lease", but the weird thing is that the "Renew..." option isn't there for this location.  You can make it appear by clicking "Using DHCP" again, but that didn't fix the problem for me.

I'm relieved this works, but I don't think it is a great fix to the problem - I still don't know what has gone wrong with the "Automatic" location, so I'd be interested to hear from any Mac experts who know about this.

How to plug in a monitor

I have frustrating problems with computers all the time, and am totally reliant on internet sources to fix my problems.  So I thought I'd start writing down some of the problems I've had and how I fixed them, in case this can be useful to others.

I'll start things on a high by telling you about how I couldn't work out how to plug in a monitor.

I just had a refreshing 2 week holiday away from Oxford, then a week of conference/summer school work, and got back to the Oxford Computing Laboratory to find that my office had moved.  This was good news - my office mates and I are now in a larger, nicer office in a more sociable part of our building.  The minor downside was that I had to set up my computer (which was ready set up when I arrived at Oxford).

For someone with a degree and PhD in Computing Science, plus 2.5 years of experience with a very practical software company, this shouldn't be a problem, but being a bit of a dufus when it comes to real computers I couldn't get the monitor to show anything.

To cut a long story short, what I found out was that my machine has multiple DVI sockets on the back - there's a white DVI socket which looks like the regular one you should plug the monitor into, and that's what I was trying.  But then there are also 2 red DVI sockets (which look a bit different, they looks like they take more pins) at the bottom of the machine's case, on the back, which are apparently due to it having an external graphics card.

So, if you are like me and have this silly problem then look out for these mysterious additional DVI sockets!!  Of course you'll need a monitor to be reading this....