Referencing an output in an expression in process modeler

Question

In the process modeler, I'd like to reference an output of a previous step as one of the parameters of an algorithm. In this case, the previous step was clipping a large raster and I want to use the clipped raster data to shift some vector data, so as the parameter to the vector transform I've got the following:

raster_value(@Clip_raster_by_mask_layer_OUTPUT, 1, $geometry)

This doesn't work. However, if I output everything from the model and then do the same transform operation manually and use 'name_of_clipped_layer' instead of @Clip_raster_by_mask_layer_OUTPUT, then it works as I expect. Is it possible to get this model to do what I want?

Answer 1

I faced the same issue using QGIS 3.22, and I just gave up because nothing has worked for me. I just save the raster in some folder and then use it as input for the next step. If you work with a standard folder structure in which only the main folder will change, you can create a "file/folder" input, and then create some "Tmp" folder somewhere inside your main folder, and save your outputs there. Then you can use them as inputs for your next steps.

you can use a pre-calculated value expression to do so, it will be something like.

@projectfolder || "\\Subfolder_A\\Subfolder_Aa\\TMP\\" || "your_raster.tif".

Where @projectfolder is the variable to your "Project Folder" input, then you add your path, and finally, you add your raster (you can choose another type instead of tif.

So for your next algorithm, you use the same expression as a pre-calculated input value.

Answer 2

You have to update to at least QGIS version 3.24 to reference previous algorithm outputs using @layer_name_OUTPUT in the expression builder. In prior versions it does not work because of an unsolved issue.