Redraw a Point in the lower left corner (QGIS)

Question

I want to draw a point in the lower left corner of the QGIS map window (canvas). I tried the geometry genarator in the layer attributes:

point_n(@map_extent, 1)

This works, but I want the point really to be in the corner and not just the symbol. I need this, because I want to be able to klick on the point to select it.

I tried to write the map extend coordinates in an virtual attribut field (x and y) to catch them with a Virtual Layer and this query:

Select id, make_point(x, y, 31256) as geometry FROM Koordinaten

Unfortunately this doesn't work. Has anyone an idea on how to solve this?

Answer 1

You can create a point geometry at the bottom left of the current map canvas that dynamically changes by first creating a virtual field with the coordinates of this point in any of your vector layers. Then you can access these coordinates a virtual layer query to create the point.

1. Inside field calculator (to create coordinates), the variable @map_extent from QGIS expressions is not available. So you have to create a custom function for the task to create the map canvas extent, see here how: https://gis.stackexchange.com/a/411313/88814 (thanks @katagena for the hint in the comment to this post).

   Then create two fields canvx for the x-coordinate with x_min(currentExtent()) and do the same for canvy with y_min(currentExtent()).

2. New create a virtual layer with this query and replace my_layer with the name of the layer that contains the attributes canvx and canvy:

    SELECT MakePoint (p.x , p.y) FROM color as p where p.id=1;
   

   The where p.id=1 part is optional, but is to avoid creating one and the same point at the bottom left separately for each feature in your layer. Change the id=1 part so that it fits your data.