23.3. Gli Strumenti di Processing

Processing è il componente principale della GUI di Processing, e quello che probabilmente tu usi nel tuo lavoro quotidiano. Mostra l’elenco di tutti gli algoritmi disponibili raggruppati in diversi blocchi chiamati Sorgenti dati, Modelli e Script personalizzati che puoi aggiungere per estendere l’insieme degli strumenti. Quindi Processing è il punto di accesso per eseguirli, sia come singolo processo che come un processo batch che coinvolge diverse esecuzioni dello stesso algoritmo su diversi insiemi in igresso.


Fig. 23.6 Strumenti di Processing

Le Sorgenti dati possono essere (de)attivate in :Processing settings dialog. Per impostazione predefinita, solo le sorgenti dati che non si basano su applicazioni di terze parti (cioè quelli che richiedono solo strumenti QGIS per essere eseguiti) sono attivi. Gli algoritmi che richiedono applicazioni esterne potrebbero richiedere una configurazione aggiuntiva. La configurazione dei sorgenti esterni provider è spiegata in un later chapter di questo manuale.

Nella parte superiore della finestra di dialogo degli strumenti di processing, troverai una serie di strumenti per:

  • lavorare con processingModel Modelli: Crea Nuovo Modello…, Apri Modello Esostente… e Aggiungi Modello agli Strumenti…;

  • lavorare con pythonFile Scripts: Crea Nuovo Script…, Create Nuovo Script da Modello…, Apri Script Esistente… e Aggiungi Script agli Strumenti…;

  • aprire il pannello processingHistory Storico;

  • aprire il pannello processingResult Visualizzatore Risultati;

  • toggle the toolbox to the in-place modification mode using the processSelected Edit Features In-Place button: only the algorithms that are suitable to be executed on the active layer without outputting a new layer are displayed;

  • open the options Options dialog.

Below this toolbar is a search Search… box to help you easily find the tools you need. You can enter any word or phrase on the text box. Notice that, as you type, the number of algorithms, models or scripts in the toolbox is reduced to just those that contain the text you have entered in their names or keywords.


At the top of the list of algorithms are displayed the most recent used tools; handy if you want to reexecute any.


Fig. 23.7 Processing Toolbox showing search results

To execute a tool, just double-click on its name in the toolbox.

23.3.1. La finestra di dialogo configurazione di un algoritmo

Once you double-click on the name of the algorithm that you want to execute, a dialog similar to that in the Fig. 23.8 below is shown (in this case, the dialog corresponds to the Centroids algorithm).


Fig. 23.8 La finestra di dialogo Algoritmo - Parametri

The dialog shows two tabs (Parameters and Log) on the left part, the algorithm description on the right, and a set of buttons at the bottom.

The Parameters tab is used to set the input values that the algorithm needs to be executed. It shows a list of input values and configuration parameters to be set. It of course has a different content, depending on the requirements of the algorithm to be executed, and is created automatically based on those requirements.

Anche se il numero e tipo dei parametri dipende dal tipo di algoritmo, la struttura di base è simile per tutti. I parametri della tabella possono essere uno dei seguenti tipi.

  • A raster layer, to select from a list of all such layers available (currently opened) in QGIS. The selector contains as well a button on its right-hand side, to let you select filenames that represent layers currently not loaded in QGIS.

  • A vector layer, to select from a list of all vector layers available in QGIS. Layers not loaded in QGIS can be selected as well, as in the case of raster layers, but only if the algorithm does not require a table field selected from the attributes table of the layer. In that case, only opened layers can be selected, since they need to be open so as to retrieve the list of field names available.

    You will see an iterator button by each vector layer selector, as shown in the figure below.


    Fig. 23.9 Vector iterator button

    Se l’algoritmo ne contiene molti, potrai selezionarne anche solamente uno. Se il pulsante corrispondente a un vettore in input è attivo, allora l’algoritmo verrà eseguito iterativamente su tutte le sue geometrie, invece di una sola volta per tutto il vettore. Il numero di output dipende da quante volte eseguirai l’algoritmo. Questo ti permette di automatizzare il processo quando tutte le geometrie in un vettore devono essere elaborate separatamente.


By default, the parameters dialog will show a description of the CRS of each layer along with its name. If you do not want to see this additional information, you can disable this functionality in the Processing Settings dialog, unchecking the General ► Show layer CRS definition in selection boxes option.

  • A table, to select from a list of all available in QGIS. Non-spatial tables are loaded into QGIS like vector layers, and in fact they are treated as such by the program. Currently, the list of available tables that you will see when executing an algorithm that needs one of them is restricted to tables coming from files in dBase (.dbf) or Comma-Separated Values (.csv) formats.

  • An option, to choose from a selection list of possible options.

  • A numerical value, to be introduced in a spin box. In some contexts (when the parameter applies at the feature level and not at the layer’s), you will find a dataDefined Data-defined override button by its side, allowing you to open the expression builder and enter a mathematical expression to generate variable values for the parameter. Some useful variables related to data loaded into QGIS can be added to your expression, so you can select a value derived from any of these variables, such as the cell size of a layer or the northernmost coordinate of another one.


Fig. 23.10 Expression based input

  • A range, with min and max values to be introduced in two text boxes.

  • A text string, to be introduced in a text box.

  • A field, to choose from the attributes table of a vector layer or a single table selected in another parameter.

  • A coordinate reference system. You can select it among the recently used ones from the drop-down list or from the CRS selection dialog that appears when you click on the button on the right-hand side.

  • An extent, a text box defining a rectangle through its corners coordinate in the format xmin, xmax, ymin, ymax. Clicking on the button on the right-hand side of the value selector, a pop-up menu will appear, giving you options to:

    • Calculate from layer: fills the text box with the coordinates of the bounding box of a layer to select among the loaded ones

    • Use map canvas extent

    • Draw on canvas: the parameters window will hide itself, so you can click and drag onto the canvas. Once you have defined the extent rectangle, the dialog will reappear, containing the values in the extent text box.


    Fig. 23.11 Extent selector

  • A list of elements (whether raster or vector layers, tables, fields) to select from. Click on the button at the left of the option to see a dialog like the following one. Multiple selection is allowed and when the dialog is closed, number of selected items is displayed in the parameter text box widget.


    Fig. 23.12 Multiple Selection

  • A small table to be edited by the user. These are used to define parameters like lookup tables or convolution kernels, among others.

    Cliccate sul bottone sul lato destro per vedere la tabella e aggiornare i suoi valori.


    Fig. 23.13 Fixed Table

    A seconda dell’algoritmo, potrai modificare il numero delle righe, usando i pulsanti sul lato destro della finestra.


Some algorithms require many parameters to run, e.g. in the Calculatore raster you have to specify manually the cell size, the extent and the CRS. You can avoid to choose all the parameters manually when the algorithm has the Reference layers parameter. With this parameter you can choose the reference layer and all its properties (cell size, extent, CRS) will be used.

Along with the Parameters tab, there is another tab named Log (see Fig. 23.14 below). Information provided by the algorithm during its execution is written in this tab, and allow you to track the execution and be aware and have more details about the algorithm as it runs. Information on algorithm execution is also output in the View ► Panels ► Log Messages Panel.

Notice that not all algorithms write information to the Log tab, and many of them might run silently without producing any output other than the final files. Check the Log Messages Panel in that case.


Fig. 23.14 Algorithm Dialog - Log

At the bottom of the Log tab you will find buttons to fileSave Save Log to File, editCopy Copy Log to Clipboard and clearConsole Clear Log. These are particularly handy when you have checked the Keep dialog open after running algorithm in the General part of the Processing options.

On the right hand side of the dialog you will find a short description of the algorithm, which will help you understand its purpose and its basic ideas. If such a description is not available, the description panel will not be shown.

For a more detailed help file, which might include description of every parameter it uses, or examples, you will find a Help button at the bottom of the dialog bringing you to the Processing algorithms documentation or to the provider documentation (for some third-party providers).

The Run as batch process button triggers the batch processing mode allowing to configure and run multiple instances of the algorithm with a variety of parameters. Nota sulle proiezioni

Processing algorithm execution are always performed in the input layer coordinate reference system (CRS). Due to QGIS’s on-the-fly reprojecting capabilities, although two layers might seem to overlap and match, that might not be true if their original coordinates are used without reprojecting them onto a common coordinate system. Whenever you use more than one layer as input to a QGIS native algorithm, whether vector or raster, the layers will all be reprojected to match the coordinate reference system of the first input layer.

This is however less true for most of the external applications whose algorithms are exposed through the processing framework as they assume that all of the layers are already in a common coordinate system and ready to be analyzed.

By default, the parameters dialog will show a description of the CRS of each layer along with its name, making it easy to select layers that share the same CRS to be used as input layers. If you do not want to see this additional information, you can disable this functionality in the Processing settings dialog, unchecking the Show layer CRS definition in selection boxes option.

If you try to execute an algorithm using as input two or more layers with unmatching CRSs, a warning dialog will be shown. This occurs thanks to the Warn before executing if layer CRS’s do not match option.

Potrai comunque eseguire l’algoritmo, ma sappi che nella maggior parte dei casi ciò produrrà cattivi risultati, come ad esempio layer di uscita inconsistenti, proprio perché questi non sono sovrapposti.


Use Processing algorithms to do intermediate reprojection

When an algorithm can not successfully perform on multiple input layers due to unmatching CRSs, use QGIS internal algorithm such as Reproject layer to perform layers” reprojection to the same CRS before executing the algorithm using these outputs.

23.3.2. Dati generati dagli algoritmi

I dati generati da un algoritmo possono appartere a una delle seguenti tipologie:

  • Raster

  • Un vettore

  • Tabella

  • File HTML (usato per risultati testuali e grafici)

These are all saved to disk, and the parameters table will contain a text box corresponding to each one of these outputs, where you can type the output channel to use for saving it. An output channel contains the information needed to save the resulting object somewhere. In the most usual case, you will save it to a file, but in the case of vector layers, and when they are generated by native algorithms (algorithms not using external applications) you can also save to a PostGIS, GeoPackage or SpatiaLite database, or a memory layer.

To select an output channel, just click on the button on the right side of the text box, and you will see a small context menu with the available options.

In the most usual case, you will select saving to a file. If you select that option, you will be prompted with a save file dialog, where you can select the desired file path. Supported file extensions are shown in the file format selector of the dialog, depending on the kind of output and the algorithm.

The format of the output is defined by the filename extension. The supported formats depend on what is supported by the algorithm itself. To select a format, just select the corresponding file extension (or add it, if you are directly typing the file path instead). If the extension of the file path you entered does not match any of the supported formats, a default extension will be appended to the file path, and the file format corresponding to that extension will be used to save the layer or table. Default extensions are .dbf for tables, .tif for raster layers and .gpkg for vector layers. These can be modified in the setting dialog, selecting any other of the formats supported by QGIS.

If you do not enter any filename in the output text box (or select the corresponding option in the context menu), the result will be saved as a temporary file in the corresponding default file format, and it will be deleted once you exit QGIS (take care with that, in case you save your project and it contains temporary layers).

You can set a default folder for output data objects. Go to the settings dialog (you can open it from the Settings ► Options ► Processing menu), and in the General group, you will find a parameter named Output folder. This output folder is used as the default path in case you type just a filename with no path (i.e., myfile.shp) when executing an algorithm.

Durante l’esecuzione di un algoritmo che usa un vettore in modo iterativo, il percorso del file inserito è usato come percorso di base per tutti i file generati, i quali sono nominati usando il nome del vettore e aggiungendo poi un numero che rappresenta l’indice di iterazione. L’estensione del file (e il formato) viene usata per tutti i file generati.

Apart from raster layers and tables, algorithms also generate graphics and text as HTML files. These results are shown at the end of the algorithm execution in a new dialog. This dialog will keep the results produced by any algorithm during the current session, and can be shown at any time by selecting Processing ► Results Viewer from the QGIS main menu.

Some external applications might have files (with no particular extension restrictions) as output, but they do not belong to any of the categories above. Those output files will not be processed by QGIS (opened or included into the current QGIS project), since most of the time they correspond to file formats or elements not supported by QGIS. This is, for instance, the case with LAS files used for LiDAR data. The files get created, but you won’t see anything new in your QGIS working session.

Per tutti gli altri tipi di output, troverai una casella di controllo che potrai usare per indicare se caricare o meno il file una volta che è stato generato dall’algoritmo. Come impostazione predefinita, tutti i file vengono aperti.

Optional outputs are not supported. That is, all outputs are created. However, you can uncheck the corresponding checkbox if you are not interested in a given output, which essentially makes it behave like an optional output (in other words, the layer is created anyway, but if you leave the text box empty, it will be saved to a temporary file and deleted once you exit QGIS).