This post is about a small recipe to perform face detection using Nokia N900 phone. It's based on ROS and OpenCV and shows how these components are mixed together and configure. See this OpenCV wiki page about face detection to understand how it works behind the scene.

First of all, ROS needs to be installed on N900. I've built several ROS packages, including latest offical release, code name "C turtle". I assume you know how to ssh to your N900 and gain root access.

Disclaimer

Careful while tinkering with your N900, you may "brick" it (if you don't know what I mean, close this page). And I wouldn't be responsible if this would occur, or if anything would turn into something bad. You've been warned.

Setting up ros-n900 source

I've created a Google Code project dedicated to N900 ports and package developments. This project can be reached at http://code.google.com/p/ros-n900/. In download section, you'll find deb packages. You directly download them to your N900, or configure a new APT source pointing to this project:

$ echo "deb http://ros-n900.googlecode.com/files /" >> /etc/apt/source.list
$ apt-get update

Now install ROS with apt-get:

$ apt-get install ros-cturtle-base

This will install ROS on /opt partition (usually 2GB ext3 space), leaving rootfs untouched. ROS uses ~500MB to install. You can also install it on a (fast) SD-card, formatted using ext3 filesystem (don't use FAT32). You'd then need to create a symlink /opt/ros-cturtle-base pointing to your SD-card.

Accessing N900 webcams under ROS

brown-ros-pkg project hosts gscam, a very nice ROS packages used to access camera with GStreamer. Since N900 webcams are recognized as V4L2 devices, it's easy to setup a gstreamer pipeline. First install dependencies. On N900:

$ apt-get install gstreamer-tools

This example shows how to send videos to PC host using UDP. Device /dev/video0 is back camera (big one, high resolution), /dev/video1 is the front one (small resolution).

# On N900, assuming 192.168.2.14 is PC's IP address (usually true if following usbnet howto tutorial)
$ gst-launch v4l2src device=/dev/video1 ! videoscale ! video/x-raw-yuv,width=320,height=240 ! ffmpegcolorspace ! smokeenc ! udpsink host=192.168.2.14 port=1234

# On PC computer:
$ gst-launch-0.10 udpsrc port=1234 ! smokedec ! autovideosink

Here we are! A small window appears broadcasting N900 videos. You may even see your face. Install some more dependencies to build ROS package. On N900:

$ apt-get install libgstreamer0.10-dev

Now go to ROS stacks directory.

$ roscd
$ cd ../stacks

Install gscam package. Follow instructions here, download gscam archive in download section or install it from sources.

$ svn co -r 682 http://brown-ros-pkg.googlecode.com/svn/trunk/unstable/gscam gscam

Once installed, build it using rosmake:

$ roscd gscam
$ rosmake -i

(requires quite a lot of time, be patient...)

gscam requires environment variable GSCAM_CONFIG to be set. It stores gstreamer pipeline definition. I had lots of troubles running the correct pipeline, and finally got help from ros-users list. The trick is to convert YUV format (the only N900 cams seem to output) into RGB.

$ export GSCAM_CONFIG="v4l2src device=/dev/video0 ! videoscale ! video/x-raw-yuv,width=320,height=240 ! ffmpegcolorspace ! video/x-raw-rgb ! identity name=ros ! fakesink"

You can manually check it's working without ROS:

$ gst-launch-0.10 $GSCAM_CONFIG 

If it pauses, that's ok. Now run it using gscam node. It requires to run within "bin" directory.

$ roscd gscam
$ cd bin
$ rosrun gscam

At this time, gscam should say it's "processing..." (of course, a reachable roscore should run somewhere, for instance on PC host). Now back on PC, install n900-cam package.

$ roscd
$ cd ../stacks
$ svn co http://ros-n900.googlecode.com/svn/trunk/src/n900-cam
$ roscd n900-cam
$ rosmake -i

Now run n900-cam testcam.py node. It'll retrieve images from image_topic subscription, add a circle and display the result in a window (from ROS tutorials).

$ rosrun n900-cam testcam.py

So far so good. Let's face detect ourself ! This is closed to previous example, except now images are submitted to OpenCV for face detection. Code is coming from OpenCV samples and is glued here to work with a ROS node.

$ rosrun n900-cam facedetect.py

If you can see yourself with a red square around your face, that's good news. If not, either you're not human, either something is wrong with running configuration...

 

Having recently tested a lot this method, I've finally determine my "easy and cheap" way to build PCB using toner transfer method. I tried a lot of things, different papers. I've also tested the professional photoengraving  method, this one can be considered for the most accurate (but certainly not the cheapest).

So, what's the recipe ? This is closed to the last one, but now way faster, since only ~ 10 minutes are needed. Main difference is you don't have to be careful while peel off the paper, toner really sticks firmly, there's no risk damaging the tracks.

• I use glossy photo paper for laser printer (135g/m²)

• wash your PCB board with soap, then with window cleaner (or detergent with alcohol).
• sandpaper your board with ultra thin paper (600, one used for body car)
• wash again the board with soap, then with window cleaner. It must absolutely be dry.
  
• preheat the board,
  
• place the paper on the board. Be careful, it'll instantly stick to the board.
• iron the paper., first smoothly to help paper to sitck firmly, then with a lot of pressure (~1min)
  
• continue with the iron tip, and redraw all the circuit (black tracks will appear through the paper), for ~4/5min.
• Ironing time: ~4/5min
• then place the board on hot water, no soap. Wait for ~ 3/4 minutes, watching the paper beeing soaked. 
• peel-off the paper. Most can easily be removed, only the last layer, where the toner sticks, will mostly remain on the board.
• peel-off again the paper with a toothbrush.  Don't hesitate, it won't damage your PCB.
• once done, dry the board. Check if you've missed some paper.
• clean the board with window cleaner. While it won't remove toner, it'll help to remove paper residue and get an accurate board.
• you're done. You'll then need to etch the board. Once done, remove the toner using acetone (nail polish remover works well)

This video shows the whole recipe:




Now, as a conclusion, here's a comparison of different PCB creation techniques:

	Photoengraving	Toner transfer with transparency	Toner transfer with photo inkjet paper	Toner transfer with photo laser paper
Ease
Repeatable
Accuracy
Fast
Cheap
Total

[EDIT 2009-09-10] : here are some photos of the glossy paper I use, hoping it'll help choose the correct type.

• a new version of the mainboard: with an integrated i2c bus, better design
• a DC motor controller board: can be drive existing h-bridges, such as those found in RC toys. Extremely configurable and extensible, highly documented.

Jal slave i2c master (declared slave address)	Hardware i2c slave address
0x2E = 0b00101110	0x5C = 0b1011100

• I use glossy photo paper for laser printer (135g/m²)

• wash your PCB board with soap, then with window cleaner (or detergent with alcohol).
• sandpaper your board with ultra thin paper (600, one used for body car)
• wash again the board with soap, then with window cleaner. It must absolutely be dry.
  
• preheat the board, with your iron (max temperature, mine doesn't heat a lot)
  
• place the paper on the board. Be careful, it'll instantly stick to the board.
• iron the paper. Apply a lot of pressure, all over the board, for ~15min (yes a long time, but it may depend on your iron)
• continue with the iron tip, and redraw all the circuit (black tracks will appear through the paper), for ~5min.
• Ironing time: ~20min
• then place the board on hot water, no soap. Soak it for 30min.
• peel-off the paper. Most can easily be removed, only the last layer, where the toner sticks, will mostly remain on the board.
• soak it again for 1h
• peel-off again the paper. You may need to remove very small portion of paper, between the tracks. Leave the paper on  the tracks, it won't bother while etching since tracks remain accurate. This can be time-consuming, but be precise. Also don't be paranoid, the toner really stick well to the board and won't be removed easily
• once done, dry the board. Check if you've missed some paper.
• clean the board with window cleaner. While it won't remove toner, it'll help to remove paper residue and get an accurate board.
• you're done. You'll then need to etch the board. Once done, remove the toner using acetone (nail polish remover works well)

Here's some pictures:

Once all paper has been peeled-off, here's the global result: tracks are still covered with paper, but the whole seems accurate.	Zooming shows tracks are perfectly accurate, even holes in pads. Text is too fuzzy won't be rendered well if left as is. Using window cleaner can help removing extra paper residue.
Paper on tracks is clearly visible here. Important is the way limit between tracks and copper is clean, accurate.	After etching, here the result. Everything looks nice. There are two pen traces which I've added. The big trace on top of the board isn't consistent at all. Using a PCB pen is definitively not reliable...
Once cleaned... Great...	Even text is readable :)

	My first attempt was using standard paper. This was a on very small testing board, and results were amazing. Then I tried on a real PCB (photo). Several times. At least 4 times (maybe 5). And it always failed... Anyway, whatever the paper type is, the board has to be a little bit larger than the paper. Ironing will be easier and everywhere the same (hopefully).
	Again, whatever the paper, the board has to be cleaned and prepared using sandpaper. I use very thin sand paper, one used to sand body car. I clean it using soap and ultimately using window cleaner (or something with alcohol): it helps to get a dry board.
	The paper is then put down on the board, toner side on copper side. Pre-heat the board (using another paper, without any dust or the like). While you stick the paper to the board, be sure you're right because the toner will instantly grip the board.
	Iron the paper. Use a lot of pressure, everywhere. For this board, I've tested different ironing times: 5min to 12min, all attempts have failed...Iif you're not ironing enough, some tracks won't stick to board (I've observed). If you iron too much, tracks will get fuzzy (I've never observed it). I think ironing this board (10cm x 10cm) for at least 10min is ok.
	So far so good... One ironed, put the board on water. Hot, cold ? With ot without soap ? Some say putting the board a cold water help the toner to fix the board. I've experimented it: the toner also seems to diffuse on the paper, make it harder to remove. But remember, that's a standard paper, so it may be ok for other type of paper. Soap can also help to remove the paper. I tend to put the board on medium hot water (same temp as for dishes), with a little soap...
	After 15min, the board shows bubbles on its surface: every piece of paper without toner gets unstuck (remember, standard paper). This clearly shows how well the toner has fixed the board. That's promising... You may not observe those bubbles using glossy papers.
	You can then start to gently rub the paper. You should be able to easily remove most of the paper. Only the last layer will cause problems (and still cause problems...).
	You should not doing this, but who could resist... Just be sure not to damage any tracks.
	Finally, after 30-45min, you can get this type of results. Some paper is still stuck onto the board. And won't be removed, even after soaking it overnight. Now, 2 options: you used standard paper: the remaining paper makes tracks fuzzy, dirty. You can't remove it without damaging them. Like the paper, you're stuck... you used glossy paper for laser printer: there's also remaining paper (less, though), but tracks still look thin. That's ok, you can probably etch the board :)
	This picture shows tracks covered with paper, using standard paper. Some tracks are damaged, but the most important thing here is tracks are fuzzy, due irregular paper residues. If etched, you won't get a workable result... Now, may I iron the board too much ? Not enough ? Some tracks did not stick to the board. I tend to say "not enough". This is plausible as my iron doesn't heat a lot. Next time I'll try ironing it at least 20min...
	Another one... this time using glossy photo paper for laser. While there's still paper on tracks, those are accurate, well limited and quite consistent. Having paper residue is not a problem, it's the way it sticks that is important.
	Some tracks were damaged (still). I needed to double-check the board and correct those errors with a PCB pen. Note, on this pictures, one pen trace doesn't mean one error: I redraw some of the tracks too make them larger. There were maybe 3 errors for the whole board. I think µI didn't iron enough. Once ready, put the board into etchant. I continually move the board into the etchant (for 18min for this one), to be sure the board is etched everywhere, equally (hopefully).
	Tadaaaa ! Nothing to say except tracks are nice...
	Clean the remaining toner. People say it cannot be removed without acetone. I use nail cleaner without acetone, and it works perfectly.
	Tadaaa (again) ! Ready for soldering !
	A nice looking result. I can even read "SirBot Project   Mainboard" and the very small date "2008-04-08".