How do I mirror a vector layer along a line in QGIS?

Question

ArcGIS (Map & Pro) has a nifty utility that allows you to mirror a vector layer along a line segment you give it. Before I spend time trying to write something from scratch, I wanted to see if there was a workflow where I could accomplish the same in QGIS. I need actual features here, not just displayed graphics. Here's an example of some dots and the line along which I'd like to mirror them:

I've played around with the affine and v.transform (grass) tools in QGIS 3.14. Neither appeared to allow me to issue the tool a line segment to use as the reference. I installed Vector Bender as well, but it doesn't seem to work for this use case (though the documentation is too slender to tell).

Answer 1

You can mirror your points along the line using QGIS expressions. You have two possibilities, 1: using geometry generator for visualization only, without creating actual geometries, but it is dynamic, so every change in the points is immediately reflected in a mirrored point, or 2:creating the points as actual geometries in a new, static layer you can use for further processing.

For both solutions, the expression remains the same (see below). In the expression, you only have to replace in line 6 'line' (red box on the screenshot) with the name of your line layer.

On the following screenshot, you see version 1 with the original points (red) and the mirrored (blue) points on the same layer, but as an additional symbol layer.

1. Create an additional symbol layer on your point layer, set it to geometry generator, geometry type: point and paste the following expression.

2. Menu Processing / Toolbox / Geometry by by expression. Set the point layer as your Input layer and set Output geometry type to points, then paste the following expression.

   end_point( 
       with_variable ( 
           'closest', 
           closest_point (   
               geometry ( 
                   get_feature_by_id ( 'line' , 1 ) 
               ) , 
               $geometry
           ),
           extend (
               make_line ( $geometry, @closest), 
               0, 
               distance ( $geometry, @closest )
           )
       )
   )