Label callout lines to the closest point on the label

Question

I am trying to improve callout lines on maps I'm making. I'm currently using the method in this link to make my callout lines. That works fine, bu I'd like the lines to always go to the closest point of the label like this:

I think it would work if I used code down the line of this in the expression editor for the geometry generator.

make_line(
  make_point($x, $y),
    make_point((CASE
      WHEN "auxiliary_storage_labeling_positionx" > $x
        THEN "auxiliary_storage_labeling_positionx"
      WHEN ("auxiliary_storage_labeling_positionx"  + "labelwidth") > $x  
            AND ("auxiliary_storage_labeling_positionx") < $x
        THEN ("auxiliary_storage_labeling_positionx" + "labelwidth")
      ELSE $x
      END )
    ,(CASE
      WHEN "auxiliary_storage_labeling_positiony" < $y
        THEN "auxiliary_storage_labeling_positiony"
      WHEN ("auxiliary_storage_labeling_positiony"  + "labelHeight") < $y
            AND ("auxiliary_storage_labeling_positiony") > $y
        THEN ("auxiliary_storage_labeling_positiony"  + "labelHeight")
      ELSE $y
      END )
  )
)

I'm not sure if I have all the < and > correct but that's beside the point. The issue with that is that I can't seem to figure out how to get the width and the height of the label.

I am not very good with the expression editor, and have never done anything with the function editor.

Answer 1

you can also use the data defined 'Alignment' to achieve it and use only a simple line within the geometry generator without any AND conditions:

make_line($geometry,make_point( 
"auxiliary_storage_labeling_positionx" , 
"auxiliary_storage_labeling_positiony" ))

and an expression like this for the horizontal:

if(x(closest_point($geometry,
 make_point( "auxiliary_storage_labeling_positionx" ,
"auxiliary_storage_labeling_positiony" ))) 
 > x(make_point( 
"auxiliary_storage_labeling_positionx" , 
"auxiliary_storage_labeling_positiony"
)),'Right','Left')

and something similar for the vertical alignment:

if(y(closest_point($geometry,
make_point( 
"auxiliary_storage_labeling_positionx" , 
"auxiliary_storage_labeling_positiony" ))) < 
y(make_point( 
"auxiliary_storage_labeling_positionx" , 
"auxiliary_storage_labeling_positiony" 
)),'Bottom','Top')

the 'closest_point' statement you'll only need if the geometry is polygon or line.

for the Centered position you can change the expression and add a variable d for a treshold (e.g. in meters) in which the label will be centered:

with_variable ('d', 200,
CASE
WHEN 
(x(closest_point($geometry,
 make_point( "auxiliary_storage_labeling_positionx" ,
"auxiliary_storage_labeling_positiony" ))) 
< x(make_point( 
"auxiliary_storage_labeling_positionx" , 
"auxiliary_storage_labeling_positiony"
))-@d)
THEN 'Left' 
WHEN
(x(closest_point($geometry,
 make_point( "auxiliary_storage_labeling_positionx" ,
"auxiliary_storage_labeling_positiony" ))) 
> @d+x(make_point( 
"auxiliary_storage_labeling_positionx" , 
"auxiliary_storage_labeling_positiony"
))) THEN 'Right'
ELSE 'Center' END)

Answer 2

QGIS 3.10 (not released at the time the question was asked) now supports callout lines out of the box so it will make the whole process much more seamless. Lots of options for controlling gaps between feature/label and leader line, and alignment is adjusted automatically I believe.

https://qgis.org/en/site/forusers/visualchangelog310/#feature-label-callouts