Australian FOSS4G is here

So that came around quick.  Seems it is November already.

Tomorrow is the FOSS4G-Au a locally organized un-conference.

How do I register?

If you would still like to come – which you should – you can register here (it’s free!) but you must RSVP on the meetup page in order to get building access and internet access.

Where is it at? (Directions)

The un-conference is held at The Queensland Centre for Advanced Technologies in Brisbane. Kudos to CSIRO for allowing us to use their facilities.

More details on un-conference?

Jody Garnett has also done a post on the un-conference and what this kind of conference style is like.  You can find more details here: 

The venue


User defined expression functions for QGIS

Ever since I added expression based labels, including the new expression builder UI, something that I always wanted to add is the ability to define custom user defined functions in Python (or C++) and use them in an expression. The expression engine is used for labels, rule based rendering, layer actions, field calculator, and atlas composer tags.  Thanks to the all the awesome work on the expression engine by Martin all this cool stuff is now possible.

Today I pushed a commit into master that adds the ability to define a function in Python (or C++), register it in the expression engine, then use it anywhere expressions are used.

The good stuff

Lets take a use case from Ujaval Gandhi and his example of counting vertices for each feature.

First we need to import the new qgsfunction decorator function from qgis.utils. The qgsfunction decorator will take a normal Python function, wrap it up in the class used to define a function, and register it in the engine.

So what does an empty function look like:

from qgis.utils import qgsfunction
from qgis.core import QGis

@qgsfunction(0, "Python")
def vertices(values, feature, parent):

@qgsfunction(0, "Python") means we are defining a new vertices function that takes 0 args and lives in the “python” group in the expression builder UI. Any custom function must take (values, feature, parent) as python args. values is a list of QVariants passed into the function, feature is the current QgsFeature, and parent is expression engine node (you use this to raise errors).

Lets stick some more logic in there:

from qgis.utils import qgsfunction
from qgis.core import QGis

@qgsfunction(0, "Python")
def vertices(values, feature, parent):
		Returns the number of vertices for a features geometry
	count = None
	geom = feature.geometry()
	if geom is None: return None
	if geom.type() == QGis.Polygon:
		count = 0
		if geom.isMultipart():
		  polygons = geom.asMultiPolygon()
		  polygons = [ geom.asPolygon() ]
		for polygon in polygons:
		  for ring in polygon:
		    count += len(ring)
	return count

Pretty simple. Get the geometry from the feature, check if it’s a polygon, if it is then count the number of vertices and return that number.

Now that we have that all done we can save it into a file in our .qgis/python folder, lets call it (note you don’t have to save it here, anywhere that QGIS can find it will do.  Anywhere on PATH)

Lets open QGIS and run import

Importing functions from

Now open the label properties for the layer:

The new function shown in the expression builder

Nice! Notice also that the function doc string is used as the function help. How cool is that.  You can also see the $ sign in front of the function, this is because any functions that take no args are considered special and use the $ sign as a convention, this is all automatic when the function is registered.

And the result is:

The label using the new function

You can even use it in the rule based rendering:

Rule rendering using new function



  • You must unregister a function once you are finished with it using QgsExpression.unregisterFunction(name). This mainly applies to plugins where the user might unload your plugin and the code is no longer available. In the above example we could import userfunctions and never unregister because we plan on using it for the whole session.
  • You can’t override the built-in methods.

Slow opening of rasters in QGIS 1.7? Here is a fix.

Note: The following post only applies to QGIS 1.7.  QGIS 1.7.1 (upcoming patch) will not have this issue as the default behavior has changed. 

One thing I noticed when running QGIS 1.7 was my rasters were really slow to open and sometimes froze QGIS.  I thought maybe it was some bad plugin that I had installed, nope; maybe opening the raster of the network drive was causing it to be slow, nope. I tried a bunch of stuff, still nothing.  After a while a few other people posted to the mailing list saying they were having the same issue, turns out the solution was very simple.

In QGIS contrast enhancement for rasters is turned on by default, so each time a raster is opened QGIS had to calculate the stats (max and min for example) for the raster and then scale the contrast.  For a large raster this is pretty heavy and this was the cause of all those problems.

So what’s the solution?

Turns out QGIS will remember the contrast setting for all rasters if you want it to.

To fix the slow opening raster problem:

  1. Open a small raster (just any old picture will do)
  2. Double click the layer in the layer list
  3. Change Contrast Enhancement to No Stretch

    Change Contrast Enhancement to No Stretch
  4. Hit the little Save icon next to the drop down box
  5. Open your big raster.

That setting will now be remembered for each raster that is opened and improve the loading time.

On a similar note.  Tim over at has done some work to improve the raster performance even more with .  The improvements that  Tim has made are available in the nightly build of QGIS via qgis-dev in OSGeo4W, or by building the master line from


QGIS edit tools brainstorming or What edit tools should QGIS have?

One trend that come up a lot in the poll I recently ran (results will be out soon, just writing a summary blog post) is “Needs more powerful edit tools“.  And I agree.

Coming from a MapInfo background nothing much changes when you move to QGIS, MapInfo had the simple Add Point, Add Region, Add Line kind of tools and then you need to use plugins to do anything a bit more advanced (MapCAD).  QGIS has the start of a kind of MapCAD thing happening although not as complete.

Why not just let people write plugins?

However I think in order to make QGIS a more attractive package to a lot of people it needs to get some built in semi-advanced to advanced editing tools;  getting back to those poll results of “Needs more powerful edit tools”.

I thought to myself “What would I like QGIS to be able to do when it came to editing?” although I came up with a small set my editing needs are only relatively small.  I then thought the best way to find out what people need is to just create a blank canvas for people to throw their ideas around on and then go from there.

Why not just create a ticket?

My idea is to get a larger idea of what people need and want rather than just one-off tickets. Although the ideas will be at the edit tool idea level it will be easier to see how they should all fit together if they are all in one place and editable by other people.  You can then start asking questions like: Do we really need that UI there?  Can we merge these tools?  How should the output be handled?

The overall goal is to have a tight, thought out, group of edit tools rather then someone creating a plugin over here for one thing and some else creating something else over there.

So how do we get this going?

Well I have created a Google Document that anyone can edit and view in order to start brainstorming ideas. The link can be found here.

I have already created an example of two ideas that I would like to see.

So go ahead, throw some ideas up.  I’m interested to see where this can head.

Why use Google Docs and not the QGIS wiki?

Mainly because Google Docs makes it very easy to do frictionless editing of a document together.  No need for user names or passwords or overwriting someone else’s changes (Google Docs is all real time).

A QGIS user poll

I have just whipped up a small user poll for QGIS users. The poll is just to give me (and indirectly the team) some idea of people’s opinions about QGIS and what it could do better. The poll is only short (9 questions) and one of the main things is “What could QGIS do better?”.

The poll can be found here:

All the answers are anonymous so feel free to say what you like about anything. In the end if something isn’t working the way you think it should, write it down. It’s not going to hurt anyone’s feelings :)

If you are a partial QGIS users who mostly uses MapInfo or ESRI stuff I would also like to get your opinion.

Another cool open source project – OSGeo-Live

Another cool open source project that I have become a part of (as a QGIS packager and tester) is the OSGeo-Live project.  The OSGeo-Live project is a live DVD/USB/Virtual Machine built on xUbuntu(striped down Ubuntu linux) that has a lot of cool open source geo spatial programs all set up and ready to use.

The OSGeo-Live project contains:

  • Browser clients
  • A small sample of crisis management software
  • All the popular database engines (PostGIS, SpaitalLite etc)
  • Pretty much all the open source desktop GIS apps (QGIS, uDig etc)
  • Open Source GPS navigation apps and globes.
  • A collection of handy spatial tools
  • A ready to go web services ready to try in your browser or desktop GIS.
  • Some sample data to get started with for each project
  • And quick starts for each program.

The full list of software contained on the OSGeo-Live project can be found at

This is a good project if you want to get into the OSGeo tools and experiment but don’t want to install them on main machine until you know what you need.

As it is a live DVD/USB/Virtual Machine some apps will run slower than what they do on a native install but overall the speed is usable and good enough for testing.

Even better is that it was born in Australia :)

The project is also commercial supported by a Australian company

So give it a try if you are interested in the OSGeo movement, which you should be if you are reading my blog :)

Fresh off the press – QGIS 1.7 is released!

Tonight Tim Sutton officially made the release announcement for QGIS 1.7, so I’m guess I’m free to blog about the newest version now and its cool features.





What are you still doing here? Go and get it!

I am, as a heavy QGIS user and a guy-who-tries-to-write-features-and-patches-for-the-code, very happy with this release. I know a lot of people have put a lot of hard work and free time into working on features and bug fixes that keep making this free GIS system even better.

Some of the more notable new features in this release are…well there are just way to many for me to list here so go and check out the official list at

The QGIS team has shifted their source control system to using GIT, which I am very happy about as a lot of the guys on the #qgis IRC channel will know :). The bug tracer has also been moved to

Since the release of QGIS 1.6 there have been 1199 commits (using git to count: git log –pretty=oneline upstream/release-1_6_0..upstream/release-1_7_0 | wc -l). Not a bad effort if I may so myself.


If you are still reading this, I really hope it’s because you are waiting for QGIS 1.7 to install.