Valgrind is a nifty too to do code instrumentation – tell you how much time gets spent in different parts of your code tree. KCachegrind is a visualisation tool for valgrind outputs. They are two separate tools that together can provide useful insight into where you code is slow or doing too much work. To set them up on ubuntu is simple:

sudo apt-get install kcachegrind valgrind

After installation, you can lauch QGIS under valgrind like this:

cd /tmp
valgrind --tool=callgrind --dump-instr=yes --simulate-cache=yes \
 --collect-jumps=yes /usr/local/bin/qgis

Note that everything you do in QGIS will take 10x longer because of the overhead valgrind places on it. Now load up a project or do something that you feel is taking longer that it should in QGIS. Then exit QGIS. You will be left with a nice big unintelligible xml file in your /tmp directory. Not to worry though, kcachegrind can open it an make sense of it for you. The file will be called something like:

callgrind.out.18364

So to open it with kcachegrind you can simply do:

kcachegrind callgrind.out.18364

KCacheGrind will take a few moments to digest it and then show you something like the screenshot below:

Application call graph for QGIS in KCacheGrind (click for larger image)

The callgraph can be displayed in a number of different ways and provides extremely useful information if you are trying to identify bottlenecks. In the image above, you can see that when QGIS starts, almost 50% of startup time is spent loading python plugins. There are also views that show how many times a given function is called which can provide some helpful hints as to what to optimise.

Valgrind can also be used to detect memory leaks, but unfortunately for GUI lovers, there isn’t a nice GUI environment to explore the leak report data. Don’t let that put you off though – you can get extremely useful feedback from the tool. Here is the syntax to do a leak check on QGIS:

valgrind  --tool=memcheck -v --error-limit=no --leak-check=full /usr/local/bin/qgis

When it finishes running QGIS you will get an extremely verbose report (printed to stdout) that lists all the potential issues. Here is a short snippet of what you might see:

==3144== 627 errors in context 151 of 997:
==3144== Invalid read of size 1
==3144==    at 0x4C2A06A: __GI_strcmp (mc_replace_strmem.c:540)
==3144==    by 0xA3080BA: _nl_load_locale_from_archive (loadarchive.c:161)
==3144==    by 0xA30750D: _nl_find_locale (findlocale.c:107)
==3144==    by 0xA306B38: setlocale (setlocale.c:379)
==3144==    by 0x680F13F: QgsCoordinateReferenceSystem::setProj4String(QString) (qgscoordinatereferencesystem.cpp:753)
==3144==    by 0x680C262: QgsCoordinateReferenceSystem::loadFromDb(QString, QString, QString) (qgscoordinatereferencesystem.cpp:271)
==3144==    by 0x680B9B3: QgsCoordinateReferenceSystem::createFromSrsId(long) (qgscoordinatereferencesystem.cpp:213)
==3144==    by 0x6776E2B: QgsDistanceArea::setSourceCrs(long) (qgsdistancearea.cpp:64)
==3144==    by 0x6776CF9: QgsDistanceArea::QgsDistanceArea() (qgsdistancearea.cpp:45)
==3144==    by 0x67C06DE: QgsMapRenderer::QgsMapRenderer() (qgsmaprenderer.cpp:46)
==3144==    by 0x71B8D7B: QgsMapCanvas::QgsMapCanvas(QWidget*, char const*) (qgsmapcanvas.cpp:115)
==3144==    by 0x5314EC: QgisApp::QgisApp(QSplashScreen*, bool, QWidget*, QFlags<Qt::WindowType>) (qgisapp.cpp:419)
==3144==  Address 0x144ed1f1 is 1 bytes inside a block of size 12 free'd
==3144==    at 0x4C282ED: free (vg_replace_malloc.c:366)
==3144==    by 0xA306BC8: setlocale (setlocale.c:173)
==3144==    by 0x680F0B0: QgsCoordinateReferenceSystem::setProj4String(QString) (qgscoordinatereferencesystem.cpp:743)
==3144==    by 0x680C262: QgsCoordinateReferenceSystem::loadFromDb(QString, QString, QString) (qgscoordinatereferencesystem.cpp:271)
==3144==    by 0x680B9B3: QgsCoordinateReferenceSystem::createFromSrsId(long) (qgscoordinatereferencesystem.cpp:213)
==3144==    by 0x6776E2B: QgsDistanceArea::setSourceCrs(long) (qgsdistancearea.cpp:64)
==3144==    by 0x6776CF9: QgsDistanceArea::QgsDistanceArea() (qgsdistancearea.cpp:45)
==3144==    by 0x67C06DE: QgsMapRenderer::QgsMapRenderer() (qgsmaprenderer.cpp:46)
==3144==    by 0x71B8D7B: QgsMapCanvas::QgsMapCanvas(QWidget*, char const*) (qgsmapcanvas.cpp:115)
==3144==    by 0x5314EC: QgisApp::QgisApp(QSplashScreen*, bool, QWidget*, QFlags<Qt::WindowType>) (qgisapp.cpp:419)
==3144==    by 0x52DFBE: main (main.cpp:607)

Just from starting QGIS and then immediately shutting it down, you will see a rather large list of errors – some of which originate from QGIS itself, and others which originate from some of the underlying libraries we use. The leak detection tool is a hand way to debug when you are getting application crashes and when it seems to be using more memory than you think reasonable. One of these days lets hope that it returns a big fat nothing when we run QGIS on it

Here is a handy tip if you are building QGIS under windows using our official INSTALL documentation. The document recommends that you open MSVC Express and then right click the BUILD_ALL and INSTALL targets. I find using a graphical user interface for this kind of thing tedious and so I looked around for a way to carry out the build from the command line. Here is how to do it:

cmake --build . --config RelWithDebInfo
cmake --build . --target install --config RelWithDebInfo

You should run those commands from within your build directory. I made a little batch file called INSTALL.bat containing the above two lines which I just call when I want to build in windows now.

QGIS got another useful feature a few days ago, courtesy of Alexander Bruy. Here’s the commit.

What this lets you do is to find out what projection your rasters are in. It then exports this to a .WLD file. It also allows you to create .PRJ files.

If you have GDAL Tools installed (and activated), you’ll find this new tool under Raster > Projections > Extract Projection. It gives you this efficient-looking dialog to choose your options in:

The "extract projection" dialog.

The batch- and recursive modes are a great feature as well. You could easily find yourself in a situation where you have a whole project full of rasters, with undocumented projections, in countless subfolders, that got handed to you without comment. Whereas previously that would be a hassle, now you can just point this tool to the right directory and set it loose. Voila, instant projection information! And in your format of choice, no less. Ladies and gentlemen… a round of applause!

QGIS Server (the ‘map’ part of the name has been dropped) is an awesome way to publish maps and you can make really good looking cartographic products with QGIS. I have been building out a good looking online map for a client and I suddenly realised that we don’t have any way to do road shields. This weekend I hacked together basic support for label shields. It integrates with the labelling-NG tool in QGIS master. Currently it provides only basic options – distance from label at which to start the shield and the shield colour. Only rectangular (with slightly rounded corners) shields are currently supported. In the future we could do lots of other fancy shield types and allow setting outline colour, use a gradient for the background, different shapes and so on. In it’s current rather basic configuration though it suits my immediate needs and probably I will add to it as the need arises.

Shield options (click for larger view)

Yes the screenshot is taken in windows – something for a future blog post Here is a little sample of what you can do with shields  (click for larger image):

Some contrived examples of using the label shields

If you want to try it out, it is in my github clone here:

git remote add timlinux git://github.com/timlinux/Quantum-GIS.git
git fetch timlinux
git branch --track shield-labels timlinux/shield-labels

This week Marco Hugentobler (http://sourcepole.ch) committed a nice new feature to QGIS master branch: Line pattern fills. The default selection of hatches that can be used to fill polygons in QGIS is quite limited. Of course you can always use SVG symbols to extend that, but Marco has added another option. With the line pattern fill symbol style, you can create multiply symbol layers of pattern fills with different rotations, allowing you to create a wide range of fill styles. Rather than try to describe this, its easier to demonstrate with a few images.

Of course these new layer styles can be combined with existing layer styles (SVG Fill, Simple Fill etc.) to extend the already huge range of possibilities for symbolization in QGIS. Thanks Marco for another great feature!