Arduino Based Solar Panel Power Monitor

My dad has a solar thermal collector on his roof (2x20 vacuum tube collectors).   His commercial controller gives total kWh collected, but I want an instantaneous kW indication.   I am developing this using Arduino (see Microcontrollers Revisited).

Version 1 used a 7 segment LED display:

But I have now received an LCD display off the slow boat from China courtesy of Ebay, so wanted to update it to give more information on the display.

The updated board is shown below:

I made a little mistake when I was modifying the circuit board, and when I added the LCD contrast potentiometer, I accidentally left it one of the digital output pins connected to ground if you turned the potentiometer too far.   The Arduino board suddenly stopped working altogeher, so I thought I had fried it.  I was pleasantly surprised when I powered it from a 9V battery rather than from the USB socket, because it came back to life and appeared to work normally.
We installed it on the panel, and started to get sensible readings off it this morning (see above picture), but it was reported dead this afternoon, with the Pin 13 LED flashing every now and then (which is odd because my software does not use Pin 13)....It starts to work if you power it down for a few minutes, but within 10 mins it crashes again with Pin 13 LED flashing.....So I think it is dead - will de-solder the nano and put another one in instead....

Well, replaced Arduino nano on the PCB with a new one.   It worked fine....for a while (10-15 mins), then as I was packing up, I noticed that the display was very dim, but the LED back light still worked ok, which made me think the battery was ok.   5 minutes later, it was doing the same behaviour as the previous one - no LCD display, and the Pin 13 LED flashing.   This time though, the new Aruino Nano has a working voltage regulator on the USB port.  When I plugged the board into the computer via USB, the board reset and is working fine again.....I must put a current meter on the 9V battery to make sure it is not pulling a ridiculous current or something.  If I am lucky it was just a dead battery, but I am still surprised that the LED backlight worked ok, when the arduino could not boot...I'll have to think about this a bit more...

There was definitely a dead battery involved - the 9V battery was only giving 5.9V, so I can see why the 5V voltage regulator may have been struggling.  The thing I need to find out though, is why is the battery dead?  This should be a nice low power circuit, but I will have to get a new one and check the current it is drawing.

Microcontrollers Revisited

Quite a few years ago (probably 10), I started getting interested in using microcontrollers to do small computing tasks that required input-output, and low power consumption.   I did not get very far with them because every time I wanted to do something, I would have to write software from scratch (to talk to a display or sensor etc.).  Also there was a lot of soldering involved to put the boards together, with crytstals, capacitors etc. to support the microcontroller.

I recently discovered Arduino (http://arduino.cc), which is a simple microcontroller with a standard PCB board layout, where assembled boards are sold cheaply.  The Arduino Uno seems like a good one to use for prototypes, as all the I/O pins are taken out to headers that you can attach jumper wires to easily.   For 'production'  versions though, the Arduino Nano seems like a better option, as it is much smaller, and you can solder connections directly onto the board rather than using jumpers etc.  I bought a few of these (or at least clones of them) very cheap (~£11 each) off Ebay.

You can download a simple development environment where you can write the code for the boards in C/C++, compile it, and load it onto the board via USB - seems to work very well.

By far the best feature of Arduino though is the user contributed libraries - there are libraries for accessing one-wire devices, LCD displays etc., so you do not have to start from scratch for each project, which makes development much, much quicker.

So, I am starting to think of all of those 'I could make one of those, but it is a bit of a waste to use a full-blown computer for it' projects.   The ones I am starting on are:

• Solar Thermal Monitor (Power Meter for water heater solar panel) - I have a first version working - see the solThMon directory in my Github repository.  A bit more description is provided in the github wiki.
• Alternative Weather Station Receiver - the idea is to use a simple 433MHz radio receiver to read the signals from our weather station, so we do not need the big LCD display that came with it (no progress yet, but I have the hardware for it...).

To give an idea of what these things look like, here is a picture:

Making Videos from Images

My daughter collected a nice series of images of the demolition of the old Steetley chimney in Hartlepool:

There are 25 images, so we thought it would be nice to make a little movie of them.

First they need a bit of processing, because the horizontal is not level.   We rotated the images, and trimmed off the edges to straighten them up using ImageMagick using:

mogrify -resize '700' -rotate 4 -shave '50x60' -resize '640' *.JPG 

This re-sizes the images to 700 pixels wide, rotates clockwise by 4 deg, shaves 50 pixels of the top and bottom, and 60 pixels off the sides.  It then re-sizes to 640 pixels wide. We then turned it into an mp4 avi movie file using mplayer's mencoder using:

mencoder "mf://*.JPG" -mf fps=4 -o Steetley.avi -ovc lavc -lavcopts vcodec=msmpeg4v2:vbitrate=800 -vf scale=640:480

This plays nicely using mplayer, and uploads to flickr too.

Final movie is here:

Or on Flickr here

Webcam using NLSU2 and OpenWRT

I am trying to set up a simple web camera to take some photos and videos of birds on our bird table.
I have had an edimax ip camera for a long time, but have had some trouble with using and configuring it from Linux.  So, I am having an attempt with a different approach from bits of hardware I have around.   There is a Linksys NLSLU2, which is made as a network storage device, and I just picked up a logitech usb web camera.   I decided to use OpenWRT as the operating system, because I used that on a previous project (bifferboard weather station).
This post is a few notes of where I have got to, and the problems encountered.

Operating System

• Download OpenWRT:   To start with I used a tarball of OpenWRT 'backfire' 10.03, but it problems with packages not being compatible.   Rather than understand why, I went for checking out the latest source code from the 'backfire' branch of the OpenWRT SVN repository.
• Build OpenWRT:  This is as simple as changing into the source directory and doing 'make menuconfig', selecting the target system as 'IXP4xx', then the profile as NSLU2, exiting and typing 'make'.   This creates a flash disk image as bin/ixp4xx/openwrt-nslu2-squashfs.bin.
• Put the NSLU2 into upgrade mode by powering off, pressing and holding the reset button with a paper clip then powering on.  Wait for ~10 sec until the top LED changes colour (very difficult for me to tell as I am colour blind...), then release the reset button.  The top LED now flashes two slightly different colours.
• Connect the computer with the OpenWRT image on it to the same ethernet network as the NSLU2 - Wireless connections do not work!.
• Flash the new image onto the NSLU2 using 'sudo upslug2 -i openwrt-nslu2-squashfs.bin'.
• Once the flashing is complete, the NSLU2 reboots and you should be able to telnet into it - default ip address is 192.168.1.1, which is a pain if that is the same one as your router...

Configuring OpenWRT

The basic OpenWRT build does not do much, so it needs to be configured and the extra required packages adding:

• Default IP Address - this can be set by editing package/base-files/files/etc/config/network [there is probably a better way to do this, but this works...].
• Add the drivers for the USB video cameras - using menuconfig select them in the kernel config section - make sure they are selected to be built-in [*] rather than modules [M], because otherwise they are not included in the flash drive image.
• Add some software to do something with the video source - add the mjpg-streamer and motion packages.
• Re-build OpenWRT and re-flash it.
• You should now be able to look at :8080?action=stream and see the image from the camera from mjpeg-stream

Making it Do Something Useful

• mjpg-streamer seems to work nicely and produces a jpeg stream that can be viewed on a browser.  No sound though - will have to look into how to deal with that.
• motion also seems to work - need to modify /etc/motion.conf to tell it what to do.
• I would really like to to record video clips when motion is detected, so am currently trying to see if I can build ffmpeg to link that to motion.....
• I have run out of time this weekend.  What I have done is just used motion and removed mjpeg-streamer, as motion provides a mjpeg stream too.  The basic set-up is:
• Logitec usb web cam.
• Motion records images to /tmp/cam1/// - separated into different directories to stop the numbers of files getting out of hand.
• A little php script /cgi-bin/browse.php is used to browse through the directories and view the images.
• I could not get ffmpeg to compile, so no videos for now.
• This is going to be installed at my sisters, so will have to talk her through setting up her router so I can ssh into it remotely to fix it when it does not work.....
• My version, using the same motion and php script set up, but using the edimax ip camera can be seen at http://maps.webhop.net/webcam.

Trouble with Ubuntu 12.04 LTS

Over the last few days I have upgraded my family's computers from Ubuntu 11.10 to 12.04.   I had been getting a bit over-confident at how good Ubuntu was, and so was not really worried about things going wrong during the upgrade, as I would have been in the past - it had been getting to the state that it 'just worked'.   So I made the mistake of doing the upgrade over a weekend when we were particularly busy, which meant it was a particular pain when things went wrong.   The issues we have had are:

1. Our PackardBell OneTwo was un-useable - booted to a black screen - didn't have chance to work out why so I just saved some logs to report the bug later, and re-installed 11.10.
2. My home server (my old Fujitsu Amilio laptop) appeared to upgrade without problems...but every time I tried to access it, it was dead.   I think it was suspending, even though I had set the power settings to never suspend.   Have tried leaving the lid open, with me logged in, with the screen blanked - seemed to be working when I left - need to work out what was going on there....

More on Samsung CLP-325w on Ubuntu

I have been pretty pleased with the Samsung CLP-325w - working nicely from my daughter's Ubuntu PC.
Have just had to re-install Ubuntu 11.10 on that (see separate post), so need to get the printer working again.
I had received a question about why I did not use an open source driver, and used the Samsung one last time.   The main reason was that the 'Recommended' driver when I installed the printer only worked in monochrome, not colour.
Just tried again, and used the 'recommended' driver, which was a generic PCL6 one.  Again this printed monochrome, until I realised that under the 'Printing' options it defaults to 'Normal (monochrome)' print quality.  Changing that to 'Normal' got it printing in colour.
Currently using it plugged into the USB port on the computer, because having a bit of trouble with network printing - job for next weekend.

Ubuntu 12.04 on Packard Bell OneTwo

Some of my previous posts have described my experiences of running Ubuntu Linux on my daughter's Packard Bell OneTwo 'all in one' touch screen computer.

Ubuntu 11.10 was working fine, but updated it to 12.04 last night.   No errors during update, and it was late when it finished so switched it off and went to bed.   This afternoon she reported that she had to resort to booting Windows because Ubuntu does not work - she got a black screen that she left waiting for 5 min, before giving up and booting Windows.

From a quick fiddle booting in 'recovery mode' and trying the safe graphics option, I think it is something to do with the graphics hardware not working, but am going to have to investigate more....just lent her my laptop to do her homework for now....

Went through a few options that I found on the internet, but none seemed to be quite my problem.  Also tried running 12.04 from a memory stick in case the upgrade had gone wrong, but it failed to work that way too....So I gave up - saved a selection of files from /var/log/ and re-installed Ubuntu 11.10.

Android Development on 64 Bit Ubuntu

I have been getting back into Android development over the last few days (more on that in future post).  Because I was travelling I initially set up the development environment (Eclipse) on my work laptop running Windows XP.   All worked fine.
Then I tried to continue work on my Lenovo laptop running Ubuntu 11.10, 64 bit edition.   It refused to build R.java, which contains all the resources such as screen layouts.  No errors to say why......

After much grumbling, searching on google and reinstalling eclipse and the android sdks without success, I discovered that to get it to work on a 64 bit machine you need to install a package called ia32-libs.   (http://developer.android.com/sdk/installing.html#troubleshooting).  Don't know why, but installing it has got it building android packages again - phew!   Should have read all the way to the bottom of the page, rather than just scanning.....

Ubuntu 11.10 on a Lenovo G570 Laptop

I have just bought myself a new laptop.  I got a Lenovo G570 from PC World for just under £450.  It seems pretty good Intel i5 processor with 4GB Ram.

Installed the 64 bit version of Ubuntu 11.10.  Installation went without hitch.  Initial boot hang for some reason, but switching it off and on again solved that, and it has worked fine since.   Basically everything I have tried worked out of the box - screen, track pad mouse, web cam, USB.   Therefore absolutely no issues - a very good advert for Lenovo and Ubuntu.

I was really surprised at how powerful it is - I just installed a simple OSM tile server on it using the instructions at http://wiki.openstreetmap.org/wiki/Ubuntu_tile_server.   Loading the british isles OSM data extract took less than 2 hours.   My home server (ex laptop) - an intel dual core processor and 2GB Ram running 32 bit Ubuntu takes almost 24 hours.

I am thinking I should have bought 2 of them and use one as the server - it only uses 20 Watts when running at low load, which is probably less than my existing one.....

Using Blender 2.5

My daughter and I have been trying to learn to use Blender 2.5 to crate 3D models for use in our simple games for Benjamin.   It has proved harder than I had expected!   The main issue seems to be that most tutorials are written for Blender 2.4, and there are lots of changes to the user interfaces in 2.5.  Plus the official documentation is still for 2.4 and documentation for 2.6 is in preparation...but not 2.5.

This is a few reminders for us for how to do things.   I will add more as I work them out.

Adding an armature to pose a model:
See http://www.youtube.com/watch?v=nNTyE9SGRYw for details of linking bones using inverse kinematics.  The author has a web site with a lot of additional information here.

1. Add a bone in Object Mode
2. Go to edit mode and position the bone within the model mesh.
3. Extrude the bone to add a joint (e.g. elbow)
4. In Object mode select the model mesh then shift-right click to select the bone.   Do control-P to bring up the 'parenting' menu and select Armature Deform with Automatic Weights.
5. Set the model view to solid (or texture), and in the armature properties menu, select X-Ray so that you can see the bones.
6. You can now enter pose mode and move the armature to pose the model.
7. To do a complete skeleton you should set up the back bone to be the parents of all the other bones so you can just grab the back bone to move the model.

Simple Games for Cheap Android Tablet

My Autistic son has developed quite a liking for hand held electronic devices - he likes our daughter's Nintendo DS with an animated dog on it, and apparently plays with an iPad at school.
I am not a fan of 'i' anythings because of the difficulty developing software for them, so I thought I would get him a cheap android based tablet to try.   Just need some software for it now.

I have got a Eken M009S from Amazon - it was just under £70.   I was hoping to develop some simple games using javascript to run in the web browser, which would mean they would work on any device, but the processor on the device is not up to it - there are some nice javascript game demos at http://kevs3d.co.uk/dev/ which work very well on my laptop, but are so slow on the tablet that they are not useable.

This is a shame as it would have been nice to develop something that would work on any platform - I think I will have to remember how to write 'native' (or at least java) android programs....

Access Web Server on VirtualBox Guest

I have a Windows computer where I use Oracle VirtualBox to allow me to run Ubuntu Linux.
I just found out how to access the web server on the virtual machine from the Windows Host.
You need to use the vboxmanage command that comes with virtualbox:
First do:

vboxmanage list vms

to list the names all of the virtual machines on the computer, then do:

vboxmanage modifyvm "[virtual machine name]" --natpf1 "http,tcp,,8080,,80"

You can then point your web browser on the windows host to http://localhost:8080 to see the web server on the virtual machine.

I don't know the syntax - see the VirtualBox Manual, Chapter 6 for details.

Ubuntu Linux on a Packard Bell OneTwo - Update

The Packard Bell OneTwo touch screen computer has not been used for quite a while, so I decided to start using it rather than my daughter's PC, because it uses a lot less power and is quieter.

The main issues from when I tried it before were:

• Screen flicker every now and then
• Touch screen calibration issues.

We installed Ubuntu 11.10 on it as a clean install.  Went pretty well.  The issues were:

1. Used a normal screen shape rather than wide screen.   Solved this by going to the settings / display menu and changing to a 16:10 aspect ratio screen resolution.
2. Touch Screen Calibration:  This took a while to solve because when you pressed the screen the mouse pointer appeared at a different position.  Eventually I traced it to the display settings that I had altered - Although there is only one monitor on the computer, the software detects two displays (there must be tv-out hardware in the box, but no connector outside).  I had un-clicked the 'mirror displays' option and this had resulted in the two screens being shown side by side.  I didn't think anything of it as there is no monitor connected to the second, but it confused the touch screen calibration - switching the un-used display to 'off' solved it and the touch screen calibration is fine now.
3. Multi-Touch:  The touch screen works just like a mouse.  I think it should be able to cope with multiple touches so you can do 'pinch' gestures etc. to re-size windows.  This is not working.   I tried using the mtview utility as described in the Ubuntu wiki, and it seems like multi touch is working - you can draw with two fingers at the same time, but the window manager does not seem to be using them.

Therefore, Ubuntu 11.10 worked much better 'out of the box' than the previous version I had tried - the main missing thing is multi-touch support for the touch screen.  This is an important one to fix for me because our son is VERY short sighted, and looks very closely at the screen, often pressing his forehead against it.  Once he has done that, the touch screen does not work because it only detects one press at a time!   I am going to have to learn how x windows input devices work to debug this I think.....

Breaking out of a Frozen SSH Terminal Session

For a long time I have found that if the internet connection is broken during a ssh terminal session that the terminal hangs and the keyboard is un-responsive - CTRL-C, CTRL-D etc. do not do anything.  I have always just cursed and closed the terminal window.

I just found that ~. will break out of the frozen session, giving you access back to the terminal.   ~? gives a list of other escape commands, but I am not sure what I would use those for.

Using Android Phone as a Modem

For quite a while I had been able to use my Samsung Galaxy Apollo phone as a modem - I just plugged it into my work laptop (running Windows XP) and it appeared as a modem, which I could use to connect to the internet nicely.

Then I tried (and failed) to update my phone using Samsung Kies and it stopped working.   I remembered as part of trying to get Kies to work I found an obscure code to type on the keypad to switch the phone between 'PDA' mode and 'Modem' mode.   I have just managed to switch it back to modem mode, and this has got my modem working again!

To do this I had to type *#7284# on the phone dialer keypad.  This brought up a menu where I selected the USB option to be 'Modem'

Now plugging it into my computer shows it up as a Samsung Mobile Modem.   I have the phone number set to *99***1#, which seems to work ok.  

I disabled the internal modem in the laptop because sometimes windows tried to dial that one instead, which is not connected to anything.

Samsung Colour Laser Printer and Ubuntu Linux

Note:  You might find a more recent post more useful.

I have got sufficiently sick of my Epson inkjet printer suffering from clogged print heads, and noticed how cheap colour laser printers have become, so decided to give one a try.

I settled for a Samsung CLP-325W (w=wireless) colour laser printer.   The reason for this is that Samsung provide linux drivers for it, and I had an old Samsung laser printer a few years ago that worked nicely with linux.

To start with, the CD that came with the printer would not mount on my daughter's Ubuntu 11.04 machine - no idea why, so we downloaded the Samsung unified driver for linux from the Samsung UK support web site.

We installed the driver by running install.sh as root, plugged in the printer and it detected ok and seems to work.  We had to select the "Samsung CLP-300 Series (SPL-C)" driver, because the recommended "CLP-325 foomatic...." one only printed black and white.

So far, so good!

The fun came when I tried to set up wireless printing so I could print to it from my laptop without my daughter's computer being switched on.   The first attempt was to repeat what I had done for the other computer, and install the Samsung unified driver on my laptop.  Unfortunately it refused to detect the printer, even when it was plugged into the USB port on the laptop.   I never found out why and tried a different approach.

I plugged the printer into my ethernet network and re-started it.  I found out that to print the diagnostics you hold the 'cancel' button (with the triangle symbol on it) for a few seconds.  This printed  out a network diagnostic page that said the network type was 'dhcp' and the ip address was 192.0.0.192.   This is odd because my router, which would act as dhcp server should give out addresses 192.168.1.*, so I assumed that it was lying about using dhcp and had a static default address.
Re-configured laptop to use the 192.0.0.* subnet and tried looking for the printer - tried a web browser, ping and nmap, and no sign of it.

Had a look at my router's dhcp list, and realised that the printer had been assigned an ip address by the router, which was not the one on the network diagnostics!!!!!
Re-printed the network diagnostics and got the correct ip address - don't know what happened there.

Pointed a web browser at the printer's IP address and got a nice status web page, with a 'login' option at the top right hand corner of the screen (once I had scrolled across because I couldn't see it on my laptop screen for some reason....).   Had to search on the internet for the default login credentials - found out that it is user name -  "admin", password - "sec00000".

Once logged in, I could set up the network to use a static IP address (so I always know where it is, and connect it to my wireless network).

Disconnected the lan cable from the printer, and sure enough, I can see the printer from my laptop.  Used the Samsung printer configuration tool to set it up as a network printer on the laptop and....it works!!!

phew!

CodeIgniter / Bonfire

I have been progressing an automated way of generating maps from openstreetmap data, allowing layers to be selected, including contours and hill shading etc. (https://github.com/jones139/disrend).

I want a web front end to allow people to generate maps using this renderer.   When I did this for my townguide program I just built a very simple php based web interface.
Ideally I would like things like user authentication though, which would mean writing a lot of code to go with it.
The CodeIgniter framework goes some way to this by providing an interface to sessions etc, but there is still quite a bit of boilerplate code to write to produce the database interface.
The CodeIgniter / Bonfire application seems to solve this by providing:

1. Roles based authentication.
2. Modular structure (this was an issue I had with codeigniter - you ended up with code scattered in different places).
3. A really neat module builder function that produces a basic module with database interface for you to build on.

I am just trying to get the hang of using bonfire now - I have had a few issues with installing modules based on the module builder code - will write up how to do this once I have it working...

Building Debian Packages

I worked out how to make simple debian packages by hand as described in a previous post.
Trying to build other software into packages is still rather difficult...
I found a really good, simple tutorial on how to use the debian tools to do this more easily at:
http://www.debian.org/doc/packaging-manuals/packaging-tutorial/packaging-tutorial.pdf.

Ubuntu packages and repositories

Creating Ubuntu (debian) packages always seems a bit fiddly....At the moment I can do it, so thought I'd better write down quickly how I did it.

Packages

• Create a directory and put the files you want to install into a directory tree under it (e.g. if you create a directory called [pkgname], files that you want in /usr/local/bin go in [pkgname]/usr/local/bin.
• Create a directory called [pkgname]/DEBIAN and put a file in it called 'control'.  This can be as simple as:

Package: grahamstestdeb
Version: 001
Architecture: i386
Depends:
Maintainer: Graham Jones (grahamjones139@gmail.com)
Description: Test Debian Package

• Create a file called 'postinst' in DEBIAN.  This will be executed when the package is installed. This can be

• #!/bin/sh
• echo "postinst works!!!!"

• Create the .deb package by doing

dpkg-deb --build [pkgname]

•  Copy your new [pkgname].deb file into a new folder called repository.
• Run 'sudo dpkg-scanpackages . /dev/null | gzip -9c > Packages.gz' in the repostitory directory.
• Upload the repository directory to your web server:

ncftpput -v -R -u maps3.org.uk -p ftp.maps3.org.uk public_html/apt/ *

• Add the following to /etc/apt/sources.lst 

deb http://apt.maps3.org.uk / 

• Run sudo apt-get update
• Run sudo apt-cache search [pkgname] - should find your package.
• Run sudo apt-get install [pkgname] - should run your postinst script.

For better instructions see: http://odzangba.wordpress.com/2006/10/13/how-to-build-local-apt-repositories/#comment-7304.

Rendering OpenStreetMap Data using Carto and Mapnik

Mapnik is often used to render OpenStreetMap (OSM) data.   The map style (line colours and sizes, icons etc.) are defined by a style file.  There is a very detailed one provided with OpenStreetMap's mapnik tools.   Unfortunately that level of detail means that the style definition is very complicated and it is hard to follow how it works.
A tool called carto is available which is a pre-processor that will take a simpler format of style file, and convert it into the mapnik file format.   I have been experimenting with trying to render OSM data using styles written for carto.   I had been hoping to use a nice graphical editor with map preview capability called tilemill, but tilemill does not work with postgresql datasources, which are necessary for OSM rendering.   Instead, I installed tilemill and used the version of carto that is included with it, but did not use the tilemill editor.

Carto Layer Definitions
Rendering data with carto needs you to do two things - define the data you want to plot on your map ('Layers'), and define how you want the data displayed ('Styles').

The datasource definitions go in a .mml file that looks something like this:

{
"_center":{"lat":36.870832154494,"lon":-113.79638671427,"zoom":5},
"_format":"png",
"srs":"+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs",
"Stylesheet":["style2.mss",
"areas2.mss",
"roads2.mss",
"labels2.mss",
"natural2.mss",
"contours2.mss",
"POIs2.mss"],

"Layer":[
  {"id":"coastline",
   "name":"coastline",
   "srs":"+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs +over",
   "class":"",
   "geometry":"polygon",
   "Datasource":{"file":"/home/graham/OSM/data/mapdata/world_boundaries/processed_p.shp","type":"shape"}},

 {"id":"landuse",
  "name":"landuse",
  "srs":"+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs",
  "class":"",
  "geometry":"area",
  "Datasource": {
               "type":"postgis",
      "dbname":"kefalonia",
      "user":"www",
      "password":"1234",
      "host":"localhost",
      "port":"",
      "table":"(select way,landuse,name,\"name:en\" from planet_osm_polygon where landuse is not null) as landuse"
               }
  }
]
}

The first few lines define the projection of the output map, and the stylesheets that are to be used to define how to render the data.   The bulk of the file is layer definitions.   In this example two layers are defined.  One is a shapefile containing the coastlines (the same one that is used by the normal OSM style).   The second layer extracts all areas tagged with 'landuse=xxx'.

We have not told mapnik how to plot the data though - this is done in the style files.  The ones relevant to this example are "style2.mss" and "areas2.mss".  Style2.mss contains:

@land: #e0e0c0;
@water: #C0E0F8;
@waterline: #8CE;
Map {
  background-color:@water;
}
#coastline::outline {
  line-color:@waterline;
  line-width:1.6;
}
#coastline::fill {
  polygon-fill:@land;
  polygon-gamma:0.75;
}

Here the things starting '@' are variables which allow you to define colours etc.  at the top of the file, then use them several times throughout it, making future maintenance easier.   In this example we define the background colour for the map, fill it in blue to represent water.   We then draw a line around the costline, and fill in the land with a separate colour.

The areas2.mss style is slightly more complicated:

#landuse {
   [landuse="residential"]
   {
      polygon-fill: #b0b0b0;
   }
   [landuse="industrial"]
   {
      polygon-fill: #a0a0a0;
   }
   [landuse="forest"]
   {
      polygon-fill: #a0d0a0;
   }
}

In this example the [] expressions are filters, which allows us to define different fill colours depending on the landuse tag.

These are just extracts - the actual styles I am using are getting progressively more and more complicated.   You can see the files, and a modified version of the generate_image.py script provided by OSM, which takes a carto .mml file as input in my svn repository (soon to be copied to github..)

Using these styles, I have updated my kefalonia map to give this:

and this:

These styles are nowhere near as clever as the standard OSM ones.  In particular I have not tried to devine different styles fo rdifferent zoom levels  instead I have two sets of stylesheets for two different zoom levels - I think in the end I will have there - one for a 'country level', another for a 'town level', and a third for higher zoom levels.

There are also a few issues - the line styles do not look as nice as the OSM ones - I do not know why...and there is a problem with the road names where sometimes other roads crossing over each other obliterate the labels.   THe reason is that I draw the road and the label at the same time - it will probably look better if I do the labels after all roads have been drawn.   Will hav eto work out how to do that!