In Gimp, I could do this by
"Layer/Transform/180 rotation" and then
"Layer/Transform/Flip vertically" -- and then with some mouse, putting things together but it is slow. I am now looking for simpler solution, particularly interested in Inkscape but also interested in other FOSS tools such as Gimp and Blender.
My goal is to save time by this
"reflection in line" i.e. by reusing. I want to do just half of the work and then get the other half easily. Similarly, suppose I do a four-times symmetric shape, I want to reuse the one piece to get the 360 degrees' shape.
How to do mathematical operations such as reflection in line?