Using QGIS Geometry Generator to create labels between associated features in different layers

Question

This question arose from a comment on this answer to Calculating distance between multiple points within buffer using QGIS.

I have followed the steps there, which use a one-to-many spatial join to generate separate features for each point that is within a buffer polygon in another layer (called buffers). Points that fall within multiple buffers are duplicated. The result is a layer called joined. The points layer has a field called buffer_id containing the id of the joined buffer polygon.

The lines connecting the points with their associated buffer centroid can be visualised using a Geometry Generator symbol layer like so:

collect_geometries(
    array_foreach(
        overlay_within(
            'buffers',
            @geometry
        ),
        make_line(@geometry, centroid(@element))
    )
)

How can I create the labels for the lines as shown in the screenshot?

Answer 1

Prerequisite

For this solution to work, the point layer must have a field (in this example called buffer_id) that contains the id of the joined buffer polygon. The solution was implemented in QGIS 3.30.3.

Label values

The values for the labels are generated with this expression:

to_int(                                           -- optional cast to integer
    length(
        make_line(
            $geometry,      
            centroid(
                geometry(
                    get_feature(                  -- get the centroid of the buffer that matches the buffer_id of the joined point. 
                        layer:='buffers', 
                        attribute:='id',   -- 'id' in single quotes is the `id` field name in the buffers layer
                        value:="buffer_id"        -- "buffer_id" in double quotes is the field value of the **current point** in the joined layer
                    )
                )
            )                                                                        
        )
    )
)

The expression finds the feature in the buffers layer where the buffer_id matches the joined field in the points layer, makes a line from the buffer centroid to the current point and gets the length of that line.

The expression is entered by clicking the button next to the Value text edit. The mode is changed to Offset from point and the lower center quadrant is selected.

Label placement

The labels are placed at the midpoint of the connecting lines using this expression:

centroid(                                 -- get the centroid (mid-point) of the line joining the point with the centroid of its associated buffer
    make_line(
        $geometry,      
        centroid(
            geometry(
                get_feature(                  
                    layer:='buffers', 
                    attribute:='id',      -- 'id' in single quotes is the `id` field name in the buffers layer
                    value:="buffer_id"
                )
            )
        )                                                                                
    )
)

The expression is entered by checking Geometry Generator and clicking on the ... button next to the text edit area. Point / MultiPoint is chosen as the geometry type.

Label rotation

The labels are rotated parallel to the line using this expression:

main_angle(                              -- get the angle of the line joining the point with the centroid of its associated buffer
    make_line(
        $geometry,      
        centroid(
            geometry(
                get_feature(                  
                    layer:='buffers', 
                    attribute:='id',     -- 'id' in single quotes is the `id` field name in the buffers layer
                    value:="buffer_id"
                )
            )
        )                                                                                
    )
) + 90                                 -- add 90 to make the label text parallel to the line

The expression is entered as a Data defined override by clicking the button next to Rotation in the Data defined section