Displaying vertex coordinates of polygon or line without creating new layer in QGIS

Question

I am wondering if there is a way to display coordinates for each vertex of a polygon by only using expressions, without creating a new layer. So this question is basically inspired by Displaying vertex coordinates of a polygon on a map in QGIS and enhancing it.

I thought about creating an array containing all vertices as geometry. So I have created this expression:

array_foreach( -- Go throuth every element of the following array
    string_to_array(geom_to_wkt(nodes_to_points($geometry)),','), -- Create the array of WKT information splitted by comma [ 'MultiPoint ((0.2974026 51.8987013)', '(0.00909091 51.50… ]
        make_point( -- Create a point geometry for every array content
            to_real(if(
                length(regexp_substr( -- Only create X coordinate if
                    regexp_substr(@element,'\\d.*\\d.[0-9]'),'([^ ]+)'))>0, -- everything between the Brackets of 'MultiPoint ((0.2974026 51.8987013)' before the whitespace '0.2974026' is not empty like ''
                    to_real(regexp_substr(regexp_substr(@element,'\\d.*\\d.[0-9]'),'([^ ]+)')), -- Extract the X coordinate of 'MultiPoint ((0.2974026 51.8987013)' --> 0.2974026
                    NULL -- If invalid coordinate use NULL, maybe replace with 0
            )),
            to_real(if(
                length(regexp_substr( -- Only create Y coordinate if
                    regexp_substr(@element,'\\d.*\\d.[0-9]'),'(?<=\\s).*'))>0, --  everything between the Brackets of 'MultiPoint ((0.2974026 51.8987013)' after the whitespace '51.8987013' is not empty like ''
                    to_real(regexp_substr(regexp_substr(@element,'\\d.*\\d.[0-9]'),'(?<=\\s).*')), -- Extract the Y coordinate of 'MultiPoint ((0.2974026 51.8987013)' --> 51.8987013
                    NULL -- If invalid coordinate use NULL, maybe replace with 0
            ))
        )
)

Which returns an array like [ <geometry: Point>, <geometry: Point>, <geometry: Point>, <geometry: Point>, ... ], which represents all the vertex of the polygon. One can easily extract X and Y coordinates out of it by adding x() or y() in front of make_point(), or by just adding another array_foreach() like array_foreach(*expression above*,x(@element)).

How could I use such an array to label every vertex with its coordinates? I thought of extracting them from the array, but cant iterate over it. Would this be possible? Or something similar? I am kind of stuck now...

Answer 1

There is a way to display the coordinates... though not as a label.

One can add style with geometry generator to extract the vertices, then add a font marker style that will be overridden to display the vertices coordinates..

1. add the vertices

Add a style of type geometry generator, geometry type point computed with the expression nodes_to_points( $geometry)

2. Style it as a Font Marker

3. Instruct to display the coordinates

Scroll down and click the box to the right of character(s) to edit the data-defined override. Enter the following expression

round(x( geometry_n( $geometry, @geometry_part_num )),4) || ' - ' || round(y( geometry_n( $geometry, @geometry_part_num )),4)

Let's break it down a bit:

geometry_n( $geometry, @geometry_part_num )) --> get the nth geometry from the one being rendered. From step 1, the geometry being rendered is an array of point (each vertices). Because we use @geometry_part_num, it is applied on every part (= on every vertex).
x(..) and y(...) -> the x/y coordinates of the point
round(..) --> let's not display 18 decimals :-)
|| ' - ' || --> Concatenate the X and Y coordinates, separate them with a -

Answer 2

There is still something to copy from OpenJUMP (and improve). In OpenJUMP vertex XY is available as a decoration. Unfortunately the font or the numbers is tiny and there is no user interface for changing it.

EDIT

I was reading the question too fast and used wrong decorations first.