A basic knowledge of Python is needed to understand this recipe. Also, as it uses the Processing framework, you should be familiar with it before studying this recipe.

We are going to add a new process to filter the polygons of a layer, generating a new layer that just contains the ones with an area larger than a given value. Here's how to do this:

The script contains mainly two parts:

In our example, the first part looks like the following:

##Cookbook=group
##Filter polygons by size=name
##Vector_layer=vector
##Area=number 1
##Output=output vector

We are defining two inputs (the layer and the area value) and declaring one output (the filtered layer). These elements are defined using the Python comments with a double Python comment sign (#).

The second part includes the code itself and looks like the following:

layer = processing.getObject(Vector_layer)
provider = layer.dataProvider()
writer = processing.VectorWriter(Output, None,
  provider.fields(), provider.geometryType(), layer.crs())
for feature in processing.features(layer):
  print feature.geometry().area()
  if feature.geometry().area() > Area:
    writer.addFeature(feature)
del writer

The inputs that we defined in the first part will be available here, and we can use them. In the case of the area, we will have a variable named Area, containing a number. In the case of the vector layer, we will have a Layer variable, containing a string with the source of the selected layer.

Using these values, we use the PyQGIS API to perform the calculations and create a new layer. The layer is saved in the file path contained in the Output variable, which is the one that the user will select when running the algorithm.

Apart from using regular Python and the PyQGIS interface, Processing includes some classes and functions because this makes it easier to create scripts, and that wrap some of the most common functionality of QGIS.

In particular, the processing.features(layer) method is important. This provides an iterator over the features in a layer, but only considering the selected ones. If no selection exists, it iterates over all the features in the layer. This is the expected behavior of any Processing algorithm, so this method has to be used to provide a consistent behavior in your script.

Some of the core algorithms that are provided with Processing are actually scripts, such as the one we just created, but they do not appear in the scripts section. Instead, they appear in the QGIS algorithms section because they are a core part of Processing.

Other scripts are not part of processing itself but they can be installed easily from the toolbox using the Tools/Get scripts from on-line collection menu:

You will see a window like the following one:

Just select the scripts that you want to install and then click on OK. The selected scripts will now appear in the toolbox. You can use it as you use any other Processing algorithm.