Placing perpendicular labels in QGIS?

Question

I'm trying to obtain the same label style than Perpendicular Placement Orientation from ArcGIS in QGIS. As QGIS doesn't have this feature as default, the option would be using custom placement for this.

An ArcGIS example:

A workaround in QGIS

Settings:

Data defined coordinate X (label in the middle of he line):

$x_at( num_points( $geometry)/2)

Data defined coordinate Y (same as X's location):

$y_at( num_points( $geometry)/2)

Data defined Rotation:

angle_at_vertex( $geometry , num_points( $geometry)/2)

For only for 2/3 labels, the placement is acceptable. For lines with a lot of breaks with different angles, it's more difficult to obtain a nice label.

ArcGIS seems to search the line segment with fewer breaks or a straight segment. Is there a way to obtain coordinates or geometry of the straightest line segment using Expression builder for a better label placement?

Answer 1

Actual workaround

Suppose this kind of geometries:

I tried to find a good placement. I'm working with atlas also, so I used a 3% distance of atlas feature from the mid part of the interpolation line per feature's vertices to place labels:

Something like this:

And I create an arrow from the feature's middle vertex to the label:

---

Settings:

Arrow:

Geometry generator

make_line(make_point($x_at( num_points( $geometry)/2) , $y_at( num_points( $geometry)/2)),
  project( line_interpolate_point(make_line(make_point($x_at(1),$y_at(1)),make_point($x_at(-1),$y_at(-1))),
  length(make_line(make_point($x_at(1),$y_at(1)),make_point($x_at(-1),$y_at(-1))))/2),length(@atlas_geometry)*0.03,
   (radians(line_interpolate_angle(make_line(make_point($x_at(1),$y_at(1)),make_point($x_at(-1),$y_at(-1))),
  0.0001)))+ pi() *0.5))

Label

Data define X

x(project( line_interpolate_point(make_line(make_point($x_at(1),
  $y_at(1)),make_point($x_at(-1),$y_at(-1))),
  length(make_line(make_point($x_at(1),$y_at(1)),
  make_point($x_at(-1),$y_at(-1))))/2),length(@atlas_geometry)*0.03,
   (radians(line_interpolate_angle(make_line(make_point($x_at(1),
  $y_at(1)),make_point($x_at(-1),$y_at(-1))),0.0001)))+ pi() *0.5))

Data define Y

y(project( line_interpolate_point(make_line(make_point($x_at(1),
  $y_at(1)),make_point($x_at(-1),$y_at(-1))),
  length(make_line(make_point($x_at(1),$y_at(1)),
  make_point($x_at(-1),$y_at(-1))))/2),length(@atlas_geometry)*0.03,
   (radians(line_interpolate_angle(make_line(make_point($x_at(1),
  $y_at(1)),make_point($x_at(-1),$y_at(-1))),0.0001)))+ pi() *0.5))

Maybe this could be helpful for someone.

If there is a better approach, glad to receive it.