Setting map extents in print layout with QGIS Geometry Generator

Question

I have a LineString layer that has geometries generated to draw boxes around them with an additional 10% of the line length added to each end.

I then want to use print layout and set the map extents as the bounds for geometries generated. Is there a way to do this?

Answer 1

Easy option

There is a Margin around features function in the atlas settings that you could use.

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Solution based on Geometry generator

If you indeed want to use the extent of a layer resulting by geometry expression, proceed as follows:

1. In the layer containing the lines, create a virtual field using field calculator to create a field named xmin with this expression: x_min (bounds ( [your_geometry_expression] ))

   • Be sure to replace [your_geometry_expression] with the expression that creates the extended lines.

   • If you create virtual fields, you can add, delete, modify the lines and still get correct results.

2. Repeat step 1 for xmax, ymin and ymax.

3. Go to the Layour manager -> Item Properties of the map/atlas -> Extents and use the fields created in steps 1 and 2 as input for data driven override.

Screenshot

Extents -> Data driven override (ɛ icon) in red box; below, highlighted in yellow: setting for Margin around feature (here set to 0). As you see, the extent from the extended line (red) is used, not the extent of the initial line (black):