Creating polygon covering the whole layer extent using QGIS expressions

Question

Using QGIS expressions, it's easy to create a polygon with the extent (bounding box) covering all geometries of a layer using layer_property(@layer,'extent').

However, how do I get a polygon covering the whole layer extent, not only the extent of the geometry?

See the screenshot: the expression generates the blue polygon. However, I look for an expression generating a polygon symbolized here with the pink frame:

Answer 1

You can define a custom function to get crs extent.

from qgis.core import *
from qgis.gui import *

@qgsfunction(args='auto', group='Custom')
def crs_extent(layer_name, feature, parent):
    layers = QgsProject.instance().mapLayersByName(layer_name)
    
    if layers:
        bounds = layers[0].crs().bounds()
    else:
        raise ValueError(f"Layer {layer_name} could not found")
        
    return QgsGeometry.fromRect(bounds)

---

Result for crs_extent(@layer_name):

Result for crs_extent('OSM'): (No empty space at the top/bottom)

Note: Layer's CRS is WGS84 (EPSG:4326). If you use projected CRS for Layer, you may not get an expected result. For example, UTM's unit is meter, but crs().bounds() returns bounds in degree.