Updating symbol size units from mm to map units in rule-based symbology in QGIS

Question

I have a quite complex rule-based symbology that is currently using millimetres as size units.

All third levels look the same, they just have different parents.

Is there a way to change symbol size units from millimetres to map units for every rule entry so I don't have to click each entry and switch (which would be a pain in the * for a few dozen entries)?

When I have a categorized approach, I have the general symbology, that I can easily change and then applies to all categories, but how do I achieve that with a rule-based symbology?

The same goes for other aspects, like symbol size. Can I change that size for all symbols at once? e. g. from 15 mu to 18 mu.

Answer 1

Use the following script in QGIS Code Editor. It also works on nested rules. Select the layer before running the script.

layer = iface.activeLayer()
renderer = layer.renderer().clone()

def set_output_unit(rules):
    for rule in rules.children():
        rule.symbol().setOutputUnit(1) # 1: MapUnits    
        if rule.children():
            set_output_unit(rule)

rules = renderer.rootRule()
set_output_unit(rules)

layer.setRenderer(renderer)
layer.triggerRepaint()

Answer 2

Quick and dirty would be:

• export Style as *.qml
• open it in Notepad++ for example
• search for: <Option value="MM" type="QString" name="line_width_unit"/>
• replace with: <Option value="MapUnit" type="QString" name="line_width_unit"/>
• import your style again