Minimalia

I wanted to try Skia, the 2D graphic library from Google. The problem is: there's still no mature build script for linux and the samples are not compiled at all. So I patched some sources and wrote a simple build script for Waf, which in the end will build the skia library (as shared object) and the demo executable.

What is Waf? It is a python based framework that allows you to write "configure" like scripts, without the hassle of learning autotools and m4. I will show you the basics and what it takes to write from scratch a project build script. I assume you will be using Ubuntu 9.04.

So, first off: check out the skia sources, using the following command:

svn checkout http://skia.googlecode.com/svn/trunk/ skia-read-only -r 501

The revision 501 (on which I generated my patch file) will be downloaded.

Next, download this patch: skia.patch.gz and save somewhere on your system. It contains the modifications (hacks, actually) I did to have the demo compiled. After that, enter the directory skia-read-only and issue the command

zcat /path/to/skia.patch.gz | patch -p1

The patches will be applied and the waf build script, wscript, will be added. Open it and take a look. It starts like this:

Well, I just defined few lists. The first three: ports, opts and xml are the files I would like to be compiled in addition to all the other files sitting in few other subdirectories. With waf several approachs are possible: you can list all of your files to compile, or you can specify just the directories and let waf pick all the sources, or you can mix and match (as I did). Or you can put a wscript in each sub dir and create a hierarchy of wscript files, each one taking care of one piece of the build. This is what I like of waf: it does not enforce you doing only in a certain way - supposedly for your own sake - but let you decide. You may make a mess but we are adults and we don't like the maven approach. But I digress... So, take a look at the second part of the script:

In waf if you want to check for libraries, tools, headers and so, you define a function called configure. It takes one parameter, which is a sort of context constructed by waf, exposing some functions. You call them to have waf doing the checks for you. For example, at line 4, I check if the c++ compiler is installed. At line 7, I check for the freetype library writing

conf.check_cfg(package='freetype2',
               atleast_version='6.3',
               args='--cflags --libs',
               uselib_store='freetype',
               mandatory=1)

The check_cfg checks the libraries using the pkg-config tool, so basically it instructs waf to look into the PKG_CONFIG_PATH list of directories, searching in this case for freetype2.pc. Waf will check the version of the library to see if it equal or greater than 6.3. It stores the values for --cflags and --libs in the waf cache under the key 'freetype' (the key will allow you to reference the library somewhere else in the script) and, finally, waf will abort if the library is not found, because I've marked it as mandatory.

In the same way I check for other libraries at the lines 10, 13, 16. At line 19 and 22 I don't use the check_cfg but simply check because the libraries gif and jpeg are not shipped with a pkg-config file. Finally, at line 25 I just set up the compilation flags for both the skia and the executable. There are other ways to do so, and you can check the waf manual and its samples if you're curious.

For the build phase we have to define the function build. As for the configure fuction, the parameter passed by waf is the context. Now, in this function you should declare how your targets should be built. To do so you have to create a task generator. The task generator is able to generate the task (d'oh!) that will be performed by waf. In creating the task generator, you can specify everything about your target. Let's take a look

At line 6 I create the task generator using the shortcut offered by the context (in this case named bld). At line 7 I set the task generator to generate the target as shared library (cshlib) using c++ compiler to build it (cxx). At line 8 I want the library to be called "skia". At line 9 I specify the defines to use, in this case I use the list "defines" declared previously. At line 10 I set up the list of include dirs and at line 11 I instruct waf to export the same includes list. At line 12 I simply list the prerequisite libraries needed by skia. In listing the prerequisite I just use the keys chosen in the configure phase. The task generator is still not completed. At line 15 I set the generator to look into a set of directories for the sources. At line 18-21 I removed certain files from the set of sources. Finally, at line 24, 27, 30 I add other sources file to the list. Well, that's all. The generator will create a task able to compile the library.

For the demo executable, also within the build fuction, I create another task generator:

It works more or less like the first generator. At line 7 I use the uselib_local instead of uselib because skia is generated by the same script and not found on the system as external dependency. Again, I add all the sources found in the samplecode directory and then I adjust the list cherry-picking and removing few more files.

Time to run the script

To run the script you need the waf engine, which is a singular file named waf you have to store in your project directory. So, let's grab waf here

http://waf.googlecode.com/files/waf-1.5.11.tar.bz2

Explode the tar somewhere and fire the ./configure script. If all goes well the waf script is generated. So, copy it into the skia-read-only directory.

Now we are ready to configure and compile skia! Enter the skia-read-only dir and issue the command

./waf configure

The project will be configured. Probably you will get some warnings due to the missing requirements. Here's the command to install them in one shot:

sudo apt-get install libgl1-mesa-dev libfreetype6-dev libsdl1.2-dev libpng12-dev libgif-dev libjpeg62-dev ttf-mscorefonts-installer

Now, again ./waf configure and this time you should get something like that:

Time to compile skia library and the demo application. You just issue the following command

./waf

If you want, you can specify the number of parallel jobs to spawn. And you can enable the progress bar too:

./waf -j3 -p

You will see the compilation process rolling out.

If all goes well you will find in skia-read-only/build/default the libskia.so shared library and the demo executable.

To run the demo application, first set

export LD_LIBRARY_PATH=/path/to/skia-read-only/build/default


Then run the demo. Once it started you can switch from a view to the other using the right-arrow. Hereafter some samples

Some demos don't work perfectly, like the ones that should show a picture. The pictures are not there and the paths are hardcoded. But you can hack your way around and do some experiments. Enjoy!