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27.6. L’interface de traitements par lots

27.6.1. Introduction

Tous les algorithmes (y compris les modèles) peuvent être exécutés en tant que processus par lots. Autrement dit, ils peuvent être exécutés en utilisant non seulement un seul ensemble d’entrées, mais également plusieurs, en exécutant l’algorithme autant de fois que nécessaire. Ceci est utile lors du traitement de grandes quantités de données, car il n’est pas nécessaire de lancer l’algorithme plusieurs fois à partir de la boîte à outils.

Pour exécuter un algorithme en traitement par lots, cliquez avec le bouton droit sur son nom dans la boîte à outis et sélectionnez l’option Exécution par lots dans le menu contextuel qui apparaît.


Fig. 27.32 Clic-droit pour ouvrir l’interface de Traitements par lot

Si vous avez déjà affiché la boîte de dialogue d’exécution de l’algorithme, vous pouvez également lancer l’interface de traitement par lots directement, en cliquant sur le bouton Éxécuter comme processus de lot….


Fig. 27.33 Traitement par lot à partir de la boîte de dialogue de l’algorithme

27.6.2. La table des paramètres

Executing a batch process is similar to performing a single execution of an algorithm. Parameter values have to be defined, but in this case we need not just a single value for each parameter, but a set of them instead, one for each time the algorithm has to be executed. Values are introduced using a table like the one shown next, where each row is an iteration and columns are the parameters of the algorithm.


Fig. 27.34 Traitement par lot

From the top toolbar, you can:

  • processingAlgorithm Toggle advanced mode: Available only when the algorithm has parameters that are marked as advanced, this button allows to show or hide such parameters in the batch dialog.

  • symbologyAdd Add row: adds a new processing entry for configuration

  • symbologyRemove Remove row(s): remove selected rows from the table. Row selection is done by clicking the number at the left and allows keyboard combination for multi selection.

  • fileOpen Open a batch processing configuration file

  • fileSave Save the batch processing configuration to a .JSON file that can be run afterwards

By default, the table contains just two rows:

  • The first row displays in each cell an Autofill… ► drop-down menu with options to quickly fill the cells below. Available options depend on the parameter type.

  • The second row (as well as each subsequent one) represents a single execution of the algorithm, and each cell contains the value of one of the parameters. It is similar to the parameters dialog that you see when executing an algorithm from the toolbox, but with a different arrangement.

At the bottom of the table, you can set whether to checkbox Load layers on completion.

Une fois le nombre de lignes souhaitées atteint, vous pouvez remplir les paramètres avec les valeurs correspondantes.

27.6.3. Remplir la table de paramètres

For most parameters, setting the value is trivial. The appropriate widget, same as in the single process dialog, is provided, allowing to just type the value, or select it from a list of possible values, depending on the parameter type. This also includes data-define widget, when compatible.

To automate the batch process definition and avoid filling the table cell by cell, you may want to press down the Autofill… menu of a parameter and select any of the following options to replace values in the column:

  • Fill Down will take the input for the first process and enter it for all other processes.

  • calculateField Calculate by Expression… will allow you to create a new QGIS expression to use to update all existing values within that column. Existing parameter values (including those from other columns) are available for use inside the expression via variables. E.g. setting the number of segments based on the buffer distance of each layer:

  • Add Values by Expression… will add new rows using the values from an expression which returns an array (as opposed to Calculate by Expression…, which works only on existing rows). The intended use case is to allow populating the batch dialog using complex numeric series. For example adding rows for a batch buffer using the expression generate_series(100, 1000, 50) for distance parameter results in new rows with values 100, 150, 200, …. 1000.

  • When setting a file or layer parameter, more options are provided:

    • Add Files by Pattern…: adds new rows to the table for files matching a File pattern in a folder to Look in. E.g. *.shp will add to the list all the SHP files in the folder. Check checkbox Search recursively to also browse sub-folders.

    • Select Files… individually on disk

    • Add All Files from a Directory…

    • Select from Open Layers… in the active project

Output data parameter exposes the same capabilities as when executing the algorithm as a single process. Depending on the algorithm, the output can be:

  • skipped, if the cell is left empty

  • saved as a temporary layer: fill the cell with TEMPORARY_OUTPUT and remember to tick the checkbox Load layers on completion checkbox.

  • saved as a plain file (.SHP, .GPKG, .XML, .PDF, .JPG,…) whose path could be set with the Autofill options exposed beforehand. E.g. use Calculate by Expression… to set output file names to complex expressions like:

    '/home/me/stuff/buffer_' || left(@INPUT, 30) || '_' || @DISTANCE || '.shp'

    You can also type the file path directly or use the file chooser dialog that appears when clicking on the accompanying button. Once you select the file, a new dialog is shown to allow for auto-completion of other cells in the same column (same parameter).


    Fig. 27.35 Remplissage automatique des paramètres de traitement par lot

    If the default value (Do not autofill) is selected, it will just put the selected filename in the selected cell from the parameters table. If any of the other options is selected, all the cells below the selected one will be automatically filled based on a defined criteria:

    • Fill with numbers: incrementally appends a number to the file name

    • Fill with parameter values: you can select a parameter whose value in the same row is appended to the file name. This is particularly useful for naming output data objects according to input ones.

  • saved as a layer within a database container:

    # Indicate a layer within a GeoPackage file
    ogr:dbname='C:/Path/To/Geopackage.gpkg' table="New_Table" (geom)
    # Use the "Calculate By Expression" to output to different layers in a GeoPackage
    'ogr:dbname=\'' || @project_folder || '/Buffers.gpkg\' table="' || @INPUT || '_' || @DISTANCE || '" (geom)'

27.6.4. Exécuter le traitement par lots

To execute the batch process once you have introduced all the necessary values, just click on Run. The Log panel is activated and displays details and steps of the execution process. Progress of the global batch task will be shown in the progress bar in the lower part of the dialog.