Getting x-coordinate of midpoint from line layer

Question

I Need to get a midpoint from a line layer. Property layer -> Style -> Simple symbol -> Generator geometry (line)

I have code #1:

x(line_interpolate_point($geometry, $length / 2)

this code return NULL

I have code #2:

point = $geometry.interpolate($geometry.length() / 2).asPoint()
make_line(make_point(point.x(), point.y()),
make_point( "auxiliary_storage_labeling_positionx", "auxiliary_storage_labeling_positiony"))

this code return NULL

What is my mistake? I am using QGIS 3.8.3

Answer eurojam: Edit code result:

 make_line(
 make_point(x(transform(line_interpolate_point(transform($geometry,'EPSG: my geographic','EPSG: my project')),'EPSG: my project','EPSG: my geographic')), y(transform(line_interpolate_point(transform($geometry,'EPSG: my geographic','EPSG: my project')),'EPSG: my project','EPSG: my geographic'))),
 make_point( "auxiliary_storage_labeling_positionx", 
 "auxiliary_storage_labeling_positiony")
 )

Answer 1

I suppose that your data CRS (linelayer) is 4326 (WGS84), so $length will result something in Meters and line_interpolate expects something in lat/lon values because your $geometry object is in WGS84. You can use a transform within your statement:

line_interpolate_point(transform($geometry,'EPSG:4326', 'EPSG:3857'),$length/2)

This will transform your line geometry first in something cartesic, so the line_interpolation will result in a valid point. At the end you have to transform your point back into your original CRS:

    transform(line_interpolate_point(transform($geometry,'EPSG:4326', 'EPSG:3857'),$length/2), 'EPSG:3857', 'EPSG:4326')