Applying conditional label opacity in QGIS

conditionalexpressionlabelingqgistransparency-opacity

I'm working with QGIS 3.16 Hannover.

I've got one layer with all Spanish provinces (provincias) and, on top of it, several layers with the regions (AND, ARA, AST…). Each province pertains to one region and each region may contain one or more provinces.

I want to label the provincias layer with different opacity, depending on whether their region layer is visible or not. Here is where I try to set the differential label opacity

And this is the result I intend to achieve (layer AND, visible)

And I've tried the following code, without success:

case
   when (overlay_within('AND') and is_layer_visible('AND')) then 50 
   when (overlay_within('ARA') and is_layer_visible('ARA')) then 50
   [...]
   else 20
end

It only works with the first region, and this is what happens when I make visible the next one (ARA)

What's wrong?

Best Answer

Click data driven override for label opacity and select Assistant. Add the following expression, set values from/to to 0 and 1, and Opacity from / to to the opacity values you like (see screenshot at the bottom).

The expression works like this:

The idea is to first get an array of all layers with @layer_ids (line 3)
Then for each of these layer (using array_foreach, line 2) check if (line 4) it is visible (line 5) and if so, return it's name (line 6), otherwise an empty string (line 7).
Use array_contains (line 1) to check if the resulting array from step 2 contains a region's layer name that corresponds to the value of the attribute field "region" (2nd last line) in the provinces layer. In this case, the result is 1, otherwise 0.

The final expression looks like:

array_contains(
    array_foreach (
        @layer_ids,
        if (
            is_layer_visible( @element),
            layer_property( @element, 'name'),
            ''
        )
    ), 
    "region"
)

Replace region in the second last line with the name you use as fieldname in the provinces layer. Be sure these values correspond to the names of the layers (as can be seen on the screenshot).

By the way: it is always a good idea to provide the data + project file you work with, at least dummy data. This saves time for you as well as the one answering (time to set up a testing project and time adapting the answer to the values and names you use in your project).

Related Solutions

Displacing polygon labels along perimeter using QGIS

You can use a Label placement defined by Geoemtry Generator. Use it to define a polygon where placements are allowed: inside a buffer around the outer boundary of your polygons (bufferA), but not inside a buffer around a polygon from another layer, the "label blocking" polygon. Let's call the buffer around this "label blocking" polygon bufferB. Now you have to clip (difference) from bufferA the area of bufferB.

To achieve this, use this expression for label placement, where polygon2 is the name of the layer containing the label blocking polygons and the buffer size of 8000 and 1000 can be changed to suit your needs:

difference (
    single_sided_buffer (boundary($geometry), -8000),
    collect_geometries (
        overlay_nearest (
            'polygon2', 
            buffer ($geometry, 1000),
            limit:=10
)))

Screenshot: yellow polygons are labeled in a way to avoid labels where the red polygon from another layer overlaps. The blue polygon is just for visualization purpose and shows the geomtry cretated using Geometry generator with the above expression: it defines the areas where labels are allowed:

Applying labelling with obstacles in QGIS

Why obstacles do not work

The setting Features act as obstacles only works for features of the same layer.

Use Geometry generator to define label placement

Go to Label / Placement tab and use Geometry generator to define the area inside the polygon where the label is allowed to be placed. Use an expression that excludes the area around the points: create a buffer around the points and subtract that area from the polygon you want to label.

Screenshot:

Area hached in dark blue is the area created by the geometry generator: the area where label placement is allowed:

How to create the area where the label is allowed

To get the point geometries from the layer point on your polygon layer, use aggregate('point', 'collect', $geometry), create a buffer around $geometry and merge (dissolve) the single buffers creating again a buffer around the result with distance of 0. Then use difference to subtract this from your polygon geometries ($geometry). The whole expression looks like (replace 5000 on line 7 with a buffer size that fits your needs):

difference (
    $geometry,
    buffer (
        aggregate( 
            'point', 
            'collect', 
            buffer ($geometry,5000)
        ),
        0
))

For better results, you can try setting placement mode to Horizontal (instead of Around Centroid) - this is especially useful with many "forbidden" areas where only small areas left for the label to be placed:

Best Answer

Related Solutions

Displacing polygon labels along perimeter using QGIS

Applying labelling with obstacles in QGIS

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