How to define the symbology used in QGIS print composer without using separate styles for screen and print

Question

I use scale-dependent symbols for mapping (rendering Openstreetmap vectors as it happens). For instance paths show up when zoomed closer than 1:50000, but only roads show when zoomed out.

The rules for rendering are defined to make mapping usable on a monitor screen.

When printing my needs are different - the paper is bigger than the screen so it doesn't follow that the same rules should be used to define the rendering.

In another application I use it is possible to instruct the print layout to render the map using the rules normal for a particular scale even though actually physically printing at a different scale. For example I may want to render the map using the rules normal for a 1:50000 map, but because I want to squeeze the map onto a smaller piece of paper I need to actually print it to be actually at 1:100,000.

At the moment every time I change map extent or scale everything remains connected (changing map extent changes scale changes rendering style etc etc)... so the only option I see is to re-write the complex qml files I use to define rendering styles, so that I have one set of styles for printing and one for on-screen use. Am I missing something? This will be an extremely inefficient way to work.

Answer 1

Here's an approach which should work. In your rules you are creating for your rule based rendering you can put a secondary criteria which matches whether the map is being rendered in a composition or to the main map canvas - this is done using the '$map' variable. $map will be 'canvas' for the main window, or a map item's id if it's being rendered in a composition.

So, in the image below I've set up orange scaled dependent lines for the map canvas and purple scale dependent lines for composer maps:

Answer 2

Given what you said, I'll assume you set rule-based rendering, using the zoom as a criterion. Just use two rules for each layer - one to set styles on how you want them on 'normal working scale' for viewing (say anything below 1:100000), and another for a the zoom range in which you build your print maps.

In print composer, then insert a map, manually set zoom level to the 1:100,000 you want, and set the dropdown box at the top to 'render' rather then cache. Granted, you'll have to adjust the rendering rules a bit, which is a bit of work initially, but that would be the case in any application.

Due to you forcing the print composer to fully re-render the image, on a scale level you set for the print, you should force the rendering rules to be enforced for the different scale then your normal 'viewing' in QGis would do.