I'm drawing rivers and roads as linestrings with width set in map units from an attribute.
When zooming out, of course they vanish quickly, because 10, 20 or 100m is less than a pixel at 1:100,000
How do I tell QGIS to always draw that line, maybe by changing from map scale to millimeters or by setting a scale-dependent minimum width?
For comparison, Google Maps always draws major roads (highways, etc.) even when the scale becomes such that they're the width of cities. I'm looking for that kind of effect.