How to rotate a geometry created by the geometry builder in QGIS

Question

This topic goes into making polygons based on point locations; Using QGIS Geometry Generator to get rectangle from point? Is there a way to apply a rotation to the geometry generated? I have a field that contains a 0-360 degree rotation value and would like to rotate my built geometries based on that value.

Answer 1

I would suggest using shapely which is provided as a python package. You would interested in the rotate function:

shapely.affinity.rotate(geom, angle, origin='center', use_radians=False)¶
Returns a rotated geometry on a 2D plane.  
The angle of rotation can be specified in either degrees (default) or   radians by setting use_radians=True. Positive angles are counter-clockwise and   negative are clockwise rotations.  
The point of origin can be a keyword ‘center’ for the bounding box center   (default), ‘centroid’ for the geometry’s centroid, a Point object or a   coordinate tuple (x0, y0).  
The affine transformation matrix for 2D rotation is:  
/ cos(r) -sin(r) xoff | sin(r) cos(r) yoff | 0 0 1 /
where the offsets are calculated from the origin Point(x0, y0)

This thread provides an example possible setup

Also, this is a C++ solution you can check out as well.

Answer 2

• if you don't know Python, you can directly use the Affine Transformations or the Vector Transformation plugins (and others as Scaling and rotating vector layers in qgis using qgsAffine for example)

• if you know Python, before using Shapely, you can use the rotate function of a QgsGeometry in PyQGIS (Rotación de archivos vectoriales con método ‘rotate’ de QgsGeometry en PyQGIS, in Spanish but the scripts are universal) or the Affine Python module.