from the April 2011 issue of Lightspeed

Definitely one of the more memorable stories from this year’s Lightspeed. I’m also very happy to see fellow Eastern Europeans gain more exposure

At first I thought this would be a story set inside a computer running Linux: “The gnomes live in the cellar. They’re short and green and wear big fluffy hats with their names on them, like GUI 1, GUI 2, GUI 3″. Which isn’t really a novel idea, save for maybe the Linux part. But the story is about something completely different and ends up being much more ingenuous than just another virtual-reality tale, so keep on reading! Without giving away much, neuroplasticity is a keyword.

Be sure to read the author interview as well once you’re finished. All magazines should have similar extra features!

To be honest I disagree with him (spoiler cut):

“What if you took a very young human brain and placed it in a complex simulated quantum-like environment? Might it learn to adapt to this environment and predict its behavior in real time as accurately as we predict the behavior of the real world (for a quantum-mechanical, probabilistic meaning of “predict”)? If it did, that would be a very strong indication that the human brain does indeed rely on quantum phenomena to make sense of the world”

I think the human mind can model environments whose behavior has no real-world analog; there are some really far-out ideas in experimental gameplay etc. that IMO seem to show this… but I don’t know if this issue has ever been rigorously investigated. Anyway, the story is great

The Paper Menagerie by Ken Liu (a Chinese-American man)

from the Mar/Apr 2011 issue of Fantasy & Science Fiction (available as a free story from Del Rey’s website and as an audiobook from Podcastle)

This is a realy, really sad and beautiful story. Definitely one of the best in 2011 – if this keeps up, the hard problem will be deciding which Ken Liu story to nominate for the Hugos!

Also, finally a story that deals with immigration. Even people who belong to a non-WASP ethnic group tend to write about “the sourceland” as Deepa D put it, and not the immigrant or the ethnic-minority experience… at least in spec fic. (The trend is probably reversed when it comes to contemporary non-genre literature!)

Some of it reminded me of the scene in Michael Ende’s Momo where the kids play with the new toys. Also very melancholy.

You can say The Paper Menagerie is fantasy, or you can say it’s magical realism, it’s going to find its home anywhere. Go read it now.

_________

Note: a big thank you to España Sheriff who linked the audiobook version on hugo_recommend – I would’ve missed the podcast release altogether if not for this notice.

I hate GIMP with a passion. (Yes, I do use it, I only have Linux on my laptop and I can’t live without a bitmap image editor – this is not a case of “I hate GIMP because it’s not Photoshop and I choose to ignore it”, I hate it as a regular user. ) I have to admit that for a long while I did not even try Inkscape because I’ve heard recommendations from people that went like “Inkscape is essentially GIMP with vectors”. It’s not, and thank G-d for that. (Here have a few more essentialist explanations!)

Anyway, so far I really enjoy Inkscape, which has to count for something given my dislike of vector art in general… I thought it’d be interesting to point out a few things that seem to be missing from the usual documentation. (There is a lot of useful documentation, be sure to check out the free Inkscape book!)

You can actually use it as a freehand drawing program. At this point I think it’s the best there is for line-art sketching (especially at the great price point of zero ): your sketches can scale up indefinitely and they do not take up a lot of space in your hard disk because of the vector format. The downside is that erasing is not as easy as with a bitmap program.

The built-in calligraphy tool looks completely useless for serious drawing at first glance, but if you play around with the settings, you can come up with something interesting.

Here are my current tool presets:

Width: 4-6
Pressure sensitivity varies width but not opacity
Thinning: 0.10
Angle: 30 degrees
No angle sensitivity (my portable device does not support this and I haven’t tried it on my desktop)
Fixation: 0
Caps: 2.50
Tremor: 0
Wiggle: 0
Mass: 0.02

Here is a nice tutorial that does a good job at explaining the options, but doesn’t really provide sample presets. I thought I had some of my presets from a different tutorial, but I can’t locate it anywhere (pointers would be appreciated, I’d like to offer some credit). Most of the presets people recommend – and indeed the defaults – I don’t really like and IMO they are not very well-suited to sketching. Give mine a spin if you are a predominantly bitmap artist like me

I have to thank WildGica whose enthusiasm for Inkscape proved sufficiently infectious

1. My friend Sarina has recently co-founded a band, Legionaire Now, and they have a fan interviews feature on their website. (Sign up for their newsletter to become eligible!) They picked me this week and I did my best to answer their questions. Read the interview here! It includes, among other things, autism, pizza and David Bowie

2. This also has to do with Sarina in a sense. I was just about to respond to her comment to my previous entry with “sure, my English might be ‘correct’ in the technical sense, but I always have this nagging feeling it’s not poetic enough”… I logged into my email… and I was very surprised to see an announcement from Astropoetica that they would like to run my poem I sent them (IIRC) sometime in December. It’s my first – and to date, the only -  English-language poem I submitted to any magazine, so this makes me really happy. The poem will probably be featured in the 2010 summer issue.

So I think I need to reevaluate my writing goals! In Hungarian I have been published in professional venues, and now that I’ve managed to sell multiple pieces to English-language semi-pro markets, I should really set my sights higher and submit to pro markets. My last excuse was “my English is not poetic enough”, which has just been thoroughly demolished. If it’s poetic enough to sell an actual poem, it’s definitely poetic enough for prose!

But, butbutbut, I have a brand new excuse at hand namely that I don’t have time to write. Unfortunately this is more or less true… I didn’t write any fiction or poetry in February, and in March I only had time to do scripting for Amesirun, my upcoming video game. (FYI – I moved the release date to April because of the holidays. Right now I think I could sleep for a week straight.)

I have to write more, and submit what I write. I also have two stories I want to revise, and a nonfiction piece on diversity in a recent spec fic video game (you’ll see which) that I need to finish ASAP.

I’ll also have to update this site more often, since it has started to get much more traffic lately than usual, mostly due to 1. Amesirun 2. my Linux posts. So, more Amesirun or more Linux?

Here is a strange symbol! (I turned off floating images because the image size is quite big this time.)

If you click on it, you can see a cryptic inscription in the top right corner. If you remember the previous teaser, that one also had similar elements… if you wait patiently, I might tell you where they come from and what they mean.

I finally had some time to try out ArtRage 3… I bought the new version with the upgrade discount when it came out, but I was just too busy to install it. Most of the time I draw stuff on my tablet PC, but I run Linux on it and that means I have to mess around with Wine to get ArtRage running (it’s worth it if you ask me). 2.5 is very stable right now, which is why I didn’t feel pressed to install 3.0 right away, even though I bought it and it was a bit more expensive than expected! I mention this because I just tried it out on my Windows desktop (easier install) and used the new watercolor emulation to produce the vertical lines seen in the image. So that was a new feature, and one I’ll be using excessively from now on, especially if I can get 3.0 working under Wine!

In other news, the game is progressing nicely, more than half of the graphics and the script has been made (I think I should also post a story teaser…). This means 40k of script so far, so there will be about 80k of script altogether. You can look forward to lots of branches, points-based scoring and more endings than you’d expect I hope I’ll manage to finish producing the assets and testing everything before Pesach!

Why Linux

When I bought an X60T, I thought I wouldn’t bother with Linux, everything was nicely preloaded with Windows XP and I saw no particular need to install Linux and suffer with the tablet features. However, I soon found that many of the drivers were really unstable and most of the extra features were really annoying. (Do you really want your screen to rotate based on motion sensor data?! I know I don’t!)

A few months later, I experienced a series of hardware malfunctions (strangely, beginning with the fingerprint reader). Among a bunch of other things, my hard drive also needed to be replaced.  I turned in the laptop for a warranty replacement… and waiting some more… and more… now since I used this computer for work, I found this annoying to say the least. I called the service center and it turned out the new hard disk had arrived, but they hadn’t preloaded it with the OS yet because the new version had to be shipped from the UK [?!]. I told them “just leave it blank, I’ll put Linux on it”. I don’t think they believed me, but I had my X60T back the next day and with nothing on the hard drive. Yay!

So, Debian Linux it was.

I originally intended to write a really long howto which amassed information from several other howtos and my own experimentation, but I was short on time, and the notes I made were sparse to say the least. So this post will only include small bits and pieces of unsorted information. I still use Debian on this tablet, so you can ask me about my current configuration…

Setup

Make sure the hardware actually functions. This is not trivial. I was told by the service center that they had lots of problems with this tablet, it was produced in small quantities, etc. When I set out to install Linux, there was a testing CD available from Lenovo… in theory. In practice the disk image was completely broken. If they fixed it since then, great. If not, just get any rescue boot CD and run the usual tests.

The laptop can boot via LAN, you have to set the BIOS to “Diagnostic mode” to get to the PXE interface (I did not find it at first). But I opted to install through USB using a memory stick so I did not actually test this. After you booted from the stick, net install is very easy, basically plug and play if your LAN uses DHCP.

Video card works out of the box, but make sure it uses the correct driver (i810). See /etc/X11/xorg.conf. Sound card also works right away. Don’t forget to install appropriate mp3 etc. codecs and the Gnome ALSA mixer.

Tablet stylus

If memory serves right, I used this howto adapted to Debian to set up the pressure sensitive stylus. I do have the same lines in my xorg.conf.

Pressure sensitivity worked right away. IIRC, in GIMP you need to enable it  manually.

ArtRage 2.5 is a great little piece of software for drawing on your tablet PC, the Windows version runs under Wine. (Pressure sensitivity works in Wine out of the box if you’ve done the above.) There is a free demo available, but I bought the full version for $20 after trying. Note that it has an MSI installer.

Xournal is a journal program for Linux, also recommended for your tablet.

I don’t use handwriting recognition so sorry, no info on that.

Screen rotation

Add the following line to /etc/X11/xorg.conf under Driver “i810″:
Option “RandRRotation”

Make sure xbase-clients and libxrandr2 are installed. I use Gnome, but I couldn’t get gnome-randr-applet to work, so you don’t need to install that. Also, none of the screen rotate scripts I found online worked for me (using the methods delineated there), so you’ll have to make your own, but it’s easy once you know the following:

These commands rotate the screen AND the cursor to vertical position:
xrandr –output LVDS –rotate right
xsetwacom set stylus rotate 1

These commands rotate to horizontal position:
xrandr –output LVDS –rotate normal
xsetwacom set stylus rotate 0

(Note that despite the “left, right”, etc. these are not relative rotations, ie. if you repeatedly –rotate right, nothing will change instead of the screen rotating around all the way.)

I do not recommend using other rotations. You should be very careful with –rotate left, because that way if you put the laptop in your lap in slate mode, the ventilation ducts are covered and your machine can overheat. I made this mistake under Windows and I was careful to set up Linux so that it doesn’t even try rotating that way. It doesn’t cause any serious problems – when it happened to me, the laptop simply shut down upon detecting excessive temperature after a while – but it definitely doesn’t improve your computer’s health.

A very easy solution is to make two executables out of these, put them in /usr/local/bin (or wherever you desire as long as it’s in your path) and link them on the Gnome toolbar as a ‘horizontal’ and a ‘vertical’ button. You can also combine the two scripts so that the resultant script detects the current rotation and always rotates to the other position. And you can map it to your favorite keypress or the ‘rotate’ button.

The motion sensor can also be used to detect rotation and rotate the screen accordingly, I don’t like that feature so I haven’t set it up for myself. Just make sure it never rotates to the ‘left’ position (see above).

There is also a possibility to rotate the screen when you close the lid. It is detected as a keypress, so you just need to detect the keypress and make your script execute upon that. I haven’t set it up yet and I probably won’t because I prefer the manual rotate. I think it’s also possible to detect stylus ejection??

CPU speedstepping and/or throttling

I haven’t tried CPU throttling, and I don’t think you should, either, unless you are feeling adventurous and/or have some money to burn I think Windows does some throttling in addition to speedstepping, but the settings aren’t public. Experiment at your own risk.

As for speedstepping – the default settings by cpufreqd are that it should always run at max speed, which is bad because that’s really overkill. Be sure to enable “ondemand” everywhere and uncomment its lines in cpufreqd.conf!! Yeah, I know, my bad… but it took me an hour to notice they were still commented. Also, I think there is a typo in the howto, what you add to /etc/modules should be “cpufreq_ondemand” instead of “cpufreqd_ondemand”. For some obscure reason, cpufreqd.conf didn’t work for me with percentages (which is the default so I’m puzzled)… so you might wish to change those to actual speeds and also say echo 1000000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq.

[note: I think the link to the howto is broken. I wrote this paragraph two years ago or so.]

Hard disk shock protection

I had a lot of trouble getting hdapsd to work, unfortunately I did not make detailed notes, so all I have is this:

You’ll need hdapsd and you’ll also need to compile thinkpad-source. To test if the laptop detects movement, install and run hdaps-gl (it’s in hdaps-utils). To check if it really suspends the HDD, simply run dmesg.

Finally my brother came upon a solution, I’ll have to ask him if he still remembers it if anyone else has the same problem.

Trust me, you need the shock protection. I’ve seen howtos where it was treated like an unnecessary feature. That’s NOT true, the HDD is vulnerable, and if you boot under XP (Vista, whatever), you can see that the HDD is very often suspended by the protection system. Because you’ll use this laptop in slate mode a lot, it will run for prolonged times at odd orientations, you’ll often turn it around to turn the lid to slate mode, etc. My first HDD died in months even when I used Windows with the factory settings, so there is a lot of wear and tear… (now that it’s working under Debian, I haven’t had any problems in years B”H).

Missing

A lot, I will try to add more as I chance upon it (if that happens…), feel free to ask questions… I think the only thing I did not set up on it myself was hdapsd (I admit I gave up in utter exasperation after three days… then it took about the same amount of time for my brother to get it to work but he actually succeeded).

If you are a grad student, you probably have Here is a very easy solution that beats putting your average pdf reader on screen and suffering around with the scrollbars! (Scrollbars on an unfamiliar laptop can annoy the heck out of even the most patient presenter. Trust me on that.)

I talked about making complicated zooming presentations with Impress!ve in the previous entry, but this versatile little piece of open-source software can be used for this purpose too, and you don’t need to do anything taxing.

Just install Impress!ve (it should run on Windows, Linux and Mac OS X, but I only tried the first two) and run it on the article pdf! Absolutely no configuration necessary, though you might want to launch it with the -e parameter if zoomed-in pages are low resolution (depending on your video card and drivers).

That’s it! You immediately have the article on your screen and converted into a nice presentation. Useful commands:

* Left/right arrow to move forward/backward

* Tab to create an overview (choose between pages)

* Enter to create a spotlight (really spiffy! You don’t need that laser pointer)

* Z to zoom in on the page (it zooms where the cursor is placed)

Have fun!

However, I was faced with an obstacle. None of the major zooming presentation editors like Prezi (no relation to me despite the name! Though I worked together with one of their founders on a research project, Hungary is a small place…) or Ahead offer easy printing. Maybe there’s one which does, but I was in a hurry and didn’t find anything. Also, my laptop only has Linux, so Windows and (especially) Mac solutions like Keynote were also out.

I came up with a solution that worked quite well for my purposes, though it probably wouldn’t work for you if you expect full Prezi-like functionality like the rotating virtual camera, etc., and it only offers three levels of zoom, so I don’t claim it’s perfect. But since several people have been asking about it, here is the howto beyond the cut! Note that it assumes that you can make a regular presentation, and you have some computer proficiency. (If you have specific questions, comment away and I’ll do my best to answer!)

1. Make a slide background that is easily tiled

Since I needed to talk about four areas of interest in my presentation, I drew a petal-like shape and flipped it horizontally and vertically until I ended up with a “flower” composed of four separate slides fitting together. I filled the background with a uniform color.

2. Create your slides with OpenOffice Impress

Alternately, you can use any editor which outputs a PDF file with slides on separate pages. (LaTeX Beamer, etc.) Maybe you can even use Powerpoint – I confess have no idea whether Powerpoint can export to PDF (if you buy me a copy I can check, but it would be quite pointless to buy me a copy just for that, since I prefer Impress).

3. Download and set up Impress!ve (includes editing the source)

Impress!ve (formerly known as KeyJnote) is a  ~150kb Python script created by Martin J. Fiedler, we will be using this script to create the presentation. Its website explains how to download it and get it to run. I had some minor trouble with it – I ran into a bug where files exported from OpenOffice (but not from LaTeX) would crash it upon drawing the slide titles, I solved it simply by removing the reference to slide titles from the source code. Since the error message told me exactly which line was the culprit, it took just a few seconds to fix it. (If anyone is interested, I can try to replicate the bug, but it’s not too relevant here and strangely it only occurred on my laptop and nowhere else. I just mention it here for historical accuracy )

Now you will need to edit the source code (impressive.py). Don’t run away! It will not hurt!

…Seriously. You just need a text editor, any text editor will do, because the settings you will have to change are right at the top of the file. Open impressive.py. Right under the title and license information, there are a bunch of variables prefaced by this comment: # You may change the following lines to modify the default settings

Let OverviewBorder = 0 and OverviewLogoBorder = 0. (Maybe it is possible to change these settings using a command line parameter, but I didn’t find it in the documentation and just changing the Python script seemed easier.) Save, quit.

4. Configure your presentation

Configuration is very straightforward, but first you need to understand how the two zoom levels are set up. What we just did above means that the Impress!ve overview page (which is a feature that the script offers, it shows thumbnail images of each slide) does not have black borders any longer. So you can make a reasonably seamless image out of four (or more!) slides which fit together (which you made in sections 1-2 above). In my case I had four “petals” of a strange flowerlike thing for the four areas I intended to cover in my presentation. (If I end up having a lot of time at some unspecified point in the future, I might make a video and upload it to Youtube to make this explanation clearer.)

Of course there are slides you don’t want to see on the top-level zoom result, breaking your carefully crafted layout! You probably have a title slide, a bunch of extra slides and whatnot. To solve this, launch Impress!ve, go through your stack of slides, and use the “o” button on your keyboard to toggle “visible on overview”. In my case, each petal had several slides explaining the topic, and only the first slide of each topic was set to display on the overview. When you’re finished, press “s” to save your configuration! It will be loaded automatically when you start the presentation (unless you moved it to another directory).

So by now you should have the top-level zoom set up. You can trigger it with the Tab key during your presentation. Your medium-level zoom is the default, it just displays one slide on the screen.

What to do about the bottom-level zoom? This is also very straightforward. Impress!ve offers a zooming tool by pressing the “z” key. It zooms to where the cursor is positioned on the slide that is currently displayed. Also, if you press Tab while zoomed in, you will be moved to the top level in two steps, automatically and very smoothly. A further charming feature is triggered with Enter, “this is left as an exercise to the reader”

Make sure you don’t make the text overly small (resist the temptation of the zoom!), because after all, the whole purpose of this is to have slides which can be easily printed!

5. Print out the PDF

See, that was easy!

***

To be honest, when I first tried this, I thought I would mix up Tab, Z, Enter and all that during my live presentation and it would go horribly wrong. But I did not do more test runs than with my usual plain vanilla OpenOffice presentations, and everything went without hitches.

***

Ein großes Dankeschön to Martin J. Fiedler who made Impress!ve. Keep up the great work!

Read the scary story including meowing hard disks, laptops breaking in half, software named after huge trees, and actual useful tips on system rescue – click on the cut!

So let’s take it one step at a time… in my desktop computer, one of the hard disks started meowing – honest! I’ve never heard such a peculiar sound coming from a hard disk before – and spinning down at random but invariably very inconvenient intervals, refusing to spin up again.

This was an old IDE hard disk, so I bought a SATA HDD and promptly ran out of SATA ports. I had to reorganize all of my data to fit on my hard disks, swap disks with my brother, etc. This is not what you want to do in your spare time, trust me.

As for my laptop, it simply broke in half. Seriously – the screen hinge just snapped, and now nothing holds up the screen. (I didn’t do anything! I was sitting and typing!) I will need to take it back for warranty replacements – it seems like a simple repair, just replace the hinge and that’s it, but warranty on my laptop still holds so I don’t want to tamper with it.

Random things which might be useful for you (they were certainly useful for me!):

* Gnome Baobab, a Linux tool which shows you how much space your stuff takes up in a nice visual way,  is extremely useful for reorganizing your data…. unless some of it is on NTFS partitions. Then its performance becomes erratic at best. (To be honest, this is a known issue and a fix is on its way.)

* System Rescue CD can save your life your sanity. Always keep a copy around. Always. Go and burn one right now. You know you want to!

* If you have more than one hard disk and more than one operating system, keep them on separate disks. If a disk decides to croak, you’ll hopefully still have a working OS… and hopefully it will be Linux and not the other way round! ;] I wouldn’t even have put this here because I thought it was common sense, but someone just expressed complete puzzlement over me organizing my data this way.

* If there is a good solution to burning Blu-ray disks under Linux which does not involve trial and error and ruining expensive media in the process, I haven’t found it yet. I haven’t ruined any disks, but I was extra cautious, and for good reason… most burners – like k3b – do not recognize the drive or the media correctly, Nero does but it also crashes often and refuses to burn at speeds over 1x.  This is a pity because I would actually have bought Nero if it functioned properly, I was so surprised to see a Linux version! (Burning works under Windows – so it’s not the drive at fault – but it’s hard to do that, having just been deprived of your Windows partition. Whoops!)

So, is there a nice utility with a nice GUI which burns you Blu-ray disks under Linux and just does what it’s told? Somehow I don’t feel like burning stuff from the command line. (I know, I know, I could make an ISO and… but… but…)

Also, I find these new spiffy 1-TB hard disks disquieting… hard disks die so often, and if one of these dies, that’s a lot of data! …so what I’m doing right now is a mix of mirroring and backing stuff up to Blu-ray. I would love to have my data just floating around in “the Cloud”, but I work with a lot of image data and video which take up a lot of space, so it’s not feasible to upload everything somewhere and download as needed. Yet.

BTW I bought this HDD (found one for a great price in a local shop) and it’s surprisingly quiet. Much more quiet than the meowing disk which it replaced… even when it wasn’t meowing!

This entry is about using luvcview under Linux for some very simplistic video recording for your eye tracker, or any other purpose. I discovered these things while I was trying to set up the visible spectrum Starburst algorithm, but after trying out several algorithms I decided to use a different one…  so I don’t know if these will be of any use to anyone! Still, here goes.

Once you’ve bought a webcam (which will be the subject of a different post) you can test it out using luvcview, a very simple video app. (Remember, you want to test with something simple, and if it works, you can move on to something more complicated.) This little Linux app is even suited to recording two concurrent video streams for your eyetracker, try this:

luvcview -d /dev/video1 -i 30 -o videoA.avi
luvcview -d /dev/video0 -i 30 -o videoB.avi

luvcview saves config with F1, loads with F2! To say this is underdocumented is an understatement – it’s not in the help or any of the obvious places, it took me half an hour to find it.

Here is a sample config file. I confess I have no idea where I found it – Google only brings me up the source code for luvcview.

id value # luvcview control settings configuration file
9963776 128 # name:Brightness type:1 min:0 max:255 step:1 def:128
9963777 32 # name:Contrast type:1 min:0 max:255 step:1 def:32
9963778 32 # name:Saturation type:1 min:0 max:255 step:1 def:32
9963788 1 # name:White Balance Temperature, Auto type:2 min:0 max:1 step:1 def:1
9963795 255 # name:Gain type:1 min:0 max:255 step:1 def:0

Unfortunately there is no way (that I could find) to save exposure settings in luvcview. This is really annoying, because one of the webcams I used required the exposure to be set to a nonstandard setting to capture at 30 fps.

I tried and tried and tried to get two streams to start recording at the very same time (either with luvcview or something more complicated) and I couldn’t find a good solution – I wasted a whole day with this, but then I realized Joel Clemens who made the Windows version of Starburst couldn’t find one either:

- While recording your eye tracking videos, be sure to flash the user with a camera flash or equivalent strobe to allow a spot where you can see where to sync the videos. (this is because it is not reasonable to assume that we can start recording from the 2 cameras at exactly the same time)

So I was a bit relieved His solution worked very well for me in practice. I simply used my cell phone camera to produce a flash, and then found the first frame and cut both video streams there. (I haven’t tried his Windows port of Starburst because I only have Matlab for Linux – but I have to say his documentation was very useful even for the Linux version!)

Simply using the camera flash method and luvcview to record two streams produces two nice streams which can be input to Starburst… but Starburst will be the subject of a separate post, IY”H.

What to do if you experience framedropping, glitches, etc. while recording from two USB webcams at the same time which are not present when you’re only recording from one of them? 1. Don’t panic 2. read this quick howto!
Basically you have two webcams taking images at a resolution of at least 640 x 480 at 30 frames per second. If you’re unlucky like me, then you end up in a situation where both cams work fine when used individually, but when both are plugged in, the video seems to stutter and drop frames, even though you have USB 2.0 slots. Unfortunately this situation was not covered on any of the low-cost eyetracking websites I came across. I was really stumped, at first I thought it was a driver issue with my Logitech webcams, but it arose both under Linux and under Windows, so eventually had to I rule that out.

The real reason is that on some laptops (maybe even desktop computers…), all of the USB slots are assigned to the same USB bus. An USB bus is inside your computer, so normally you can’t tell by inspecting your laptop visually. Even slots on different sides of your laptop can be assigned to the same bus!

This is how to tell:

1. Unplug all of your USB devices.
2. Run the command “lsusb” under Linux. This shows you all of your USB buses.
3. Plug one of your devices into one of the slots.
4. Run lsusb again. The device should appear on the list, assigned to a specific bus.
5. Unplug the device.
6. Repeat 3-5 until you’ve found out which USB slots on your laptop connect to which buses. I have 3 slots, all of which belong to the same bus! (There might even be internal buses you can’t access without taking the laptop apart – in my laptop there is a built-in fingerprint reader which has its own bus all to itself, as you’ll see in the sample image below.)
7. Connect both cams to USB slots which belong to different buses.
8. Record!

This is an example of what you should see when you’re finished – both cameras plugged in, each on a different bus (click to enlarge):

If you’re unlucky, all of the available slots belong to the same bus. Or maybe you have slots where your device does not fit (some laptops have slots in really unwieldy locations) or you just don’t want to unplug something. What to do now?

The solution is to buy a PCMCIA to USB converter. These are reasonably cheap and most laptops have a PCMCIA slot where you can put them (or even two). Put the PCMCIA card into your laptop, boot up, plug one of your cams into a free USB slot and the other into the card. This way the data flow from both cameras won’t be directed through the same USB bus and you will hopefully experience smooth video on both.

Disclaimer:
This worked for me and is a cheap solution to a persistent problem I had when building my eye tracker. Still, no guarantees! It might not work for you. Try it at your own risk.

Unfortunately there is a nasty followup to this, namely that the bandwidth required is quite a lot – take a look at device bandwidths on Wikipedia and you can see that USB ranks quite high on the list. So if you want to make your head-mounted rig wireless, then Bluetooth is probably not an option, having much less bandwidth than USB (though I haven’t tried yet, but it looks futile). The ideal solution would be WUSB or the new third-generation Bluetooth protocol, but these are really “emerging technology” in the worst sense of the word – expensive, easy to break, hard to find, etc. Still, in a few years you will probably be able to do that in a relatively cheap and painless way, so keep this in mind!

For the time being, if you really need to make your recording rig portable, you can mount your laptop on your back… I have an ultraportable, so this works for me! One might think Eee PCs or similar netbooks would also be suitable, but unfortunately video recording is quite processor-intensive, so you want a powerful computer (or some custom-built setup). And powerful ultraportables are really expensive (I didn’t buy mine for eye tracking either…). Anyway, most eye tracking these days is aimed at people sitting in front of a screen, not wandering around and interacting with random objects, so it’s probably not a problem if your rig is not wireless (it’s quite portable anyway). I just think it would be cool.

Also, lots of eye trackers only use one camera, but I don’t like those setups – compensating for head movements is much easier if you have a scene camera in addition to the eye camera. Of course, I’m doing offline measurements… a lot depends on exactly what you want to do, and there is a tradeoff between accuracy and online processing.