Labeling line twice based on direction in QGIS

Question

TL;DR: I would like to put two labels on the same line as on the picture below.

I have three layers:

• Points: `"Point_ID""
• Lines: `"Line_ID""
• Label table (non-spatial): "ID", "Line_ID", "Start_point_ID", "End_point_ID", "Label"

Some of the lines are referenced twice in the Label table but with different directionality. I would like to label my lines based on the label direction in the label table, just like on the picture below.

My question is how could I achieve this?

I know I could duplicate the line feature and add an extra attribute based on the joined Label layer with the values like "forward", "backward" but I would like to stay with one line layer.

Is it maybe possible to use geometry generator tool in QGIS? I have the Point_ID-s if I join the Label layer. (I don't seek complete solution to my problem. I just need some guidance.)

Or is there a way more easier way I don't know about? A plugin for example?

Answer 1

Instead of single labels, use rule-based labeling and create two rules: one for the label this way, the second for the label that way.

For the second one, in placement tab, activate Geometry generator with this expression to change the line direction: reverse($geometry) and be sure to have checked the box Line orientation dependent postion (screenshot 1).

On the placement tab, activate Show upside-down labels by selctign always (screenshot 2).

Answer 2

A way would be something like this (not tested with real data/layers):

CREATE TABLE IF NOT EXISTS Lines_with_label AS
SELECT
lns.geometry, lbl.Label
FROM Label_table AS lbl
JOIN Lines_layer AS lns
ON lns.Line_ID = lbl.Line_ID
WHERE
ST_StartPoint(lns.geometry) = 
    (SELECT Points.geometry from Points WHERE Points.Point_ID = lbl.Start_point_ID LIMIT 1)
AND
ST_EndPoint(lns.geometry) = 
    (SELECT Points.geometry from Points WHERE Points.Point_ID = lbl.End_point_ID LIMIT 1)
;

which creates a new layer only with lines which have a label associated, aond only if labels are in the correct direction (i.e. Start_point_ID is the first point of the corresponding line, and End_point_ID the last). That should work also with polygons, BTW.