How to use a Processing Algorithm in a expression function used in a Geometry Generator (Points Along Lines "on the fly")

Question

I want to use the Processing Algorithm, pointsalonglines, in the expression function used in Geometry Generator.

This is because I want to create points "on the fly" along my line and in a equal distance.

The final reason why, is because I want to show the distance of a track every X meters or kilometers.

All is explained in this two links:

Hatching features in layer using QGIS

Anita Graser help me in this link:

https://anitagraser.com/movement-data-in-gis/#comment-21044

Answer 1

You can use the following expression to create the points along the line every n distance (here 5000 considering the layer use meters)

collect_geometries(array_foreach(
generate_series(0, length($geometry),step:=5000),
make_point_m(
  x(line_interpolate_point($geometry, @element)),
  y(line_interpolate_point($geometry, @element)),
  @element)
))

Alternative if you need to deal with layer projection (needed if your layer is not using meters in particular) considering EPSG:4326 is your layer projection and your local projection is EPSG:25831

collect_geometries(array_foreach(
  generate_series(0, length(transform($geometry,'EPSG:4326', 'EPSG:25831')),step:=5000),
  transform(make_point_m(
    x(line_interpolate_point(transform($geometry,'EPSG:4326', 'EPSG:25831'),
    @element)),
    y(line_interpolate_point(transform($geometry, 'EPSG:4326', 'EPSG:25831'), @element)),
    @element
  ), 'EPSG:25831', 'EPSG:4326')
))

Although it works for a part and I set the distance value in the M part of the geometry, I don't really see how I can then reuse the M in the labels part as you want in the second part of your requirements. Using Font marker can be another way e.g https://gis.stackexchange.com/a/314789/638 with expression to_string((@geometry_part_num - 1) * 5) || ' Km'

Edit note: comment from @RBenet made me change the second part of the answer