Script syntax

The R scripts for the plugin use extension .rsx. It is classic R script with several metadata lines (starting with ##) that define the interaction between QGIS and R. Metadata amongst other things specify how UI of the tool will look like.

The script loads necessary packages by itself. Combination of sp and rgdal or sf for vector data and sp and rgdal or raster for raster data. Any other packages need to be directly loaded using library() command in the script body.

Metadata

##script_name=name script_name is the name of the script; under this name, it will be listed in processing toolbox.

##group_name=group group_name is the name of the group of the script, which allows sorting of scripts into groups in processing toolbox.

Script behaviour

Several metadata lines define the general behaviour of the script.

##output_plots_to_html (older version of this metadata keyword is ##showplots) defines that there will be graphical output from the script that will be presented as an HTML page with images.

##load_raster_using_rgdal (legacy alias ##dontuserasterpackage) specifies that raster data should not be pased to R using raster package as RasterLayer or RasterBrick object. Raster data are instead read using rgdal into object SpatialGridDataFrame from sp.

##load_vector_using_rgdal specifies that vector data should not be pased to R using sf package as sf data.frame. Instead data are passed as Spatial?DataFrame (where ? is the type of vector object) from sp package.

##pass_filenames (legacy alias ##passfilenames) specifies that data are not passed directly. Instead only their file names are passed.

Inputs

The inputs to R script are specified as: variable_name=variable_type [default_value/from_variable]. This metadata line also specifies how tool UI will look in QGIS, as inputs are one section of the tool UI. In this specification variable_name is the name of the variable used in R script, variable_type is a type of input variable from possible input types (vector, raster, table, number, string, boolean, Field).

The default_value is applicable to number, string and boolean inputs.

The from variable_ applies to Field and must point to variable_name of vector input.

So the inputs can look like this:

##Layer=vector specifies that there will be variable Layer that will be a vector.

##Size=number 10 specifies that there will be variable Size that will be numeric, and a default value for Size will be 10.

##X=Field Layer specifies that variable X will be field name taken from Layer.

Outputs

The outputs of R script are specified as ##variable_name=output output_type. This line also specifies how tool UI will look in QGIS, as outputs are one section of the tool UI. In this specification variable_name specifies variable from the script that will be exported back to QGIS, output_type is one of the allowed types that can be returned from R script (layer, raster, folder, HTML, number, string, table).

So the inputs can look like this:

##New_layer=output vector specifies that the variable New_layer will be imported to QGIS as a vector layer.

##New_raster=output raster specifies that variable New_raster will be imported to QGIS as a raster layer.

Printing from R to tool log

If any output of any line in R script should be outputted to tool log, it needs to be preceded by >. So, for example, the following code will print the number of rows in Layer.

>nrow(Layer)