This is a tutorial answering a question I received after the trees post. It's about creating depth in your background using blur and transparency. It's a joined Inkscape and Gimp tutorial.
We start out with a simple 4 layer background scene. A gradient sky and three planes of rolling hills with a few trees. The Inkscape file to this tutorial can be found here. When creating your own scene make sure you change the colours slightly to make the planes stand out and be distinguishable.
You can create the blur right in Inkscape by using 'Filters/ blur' and then e.g. a motion blur. In the filter properties you can set the effect parameters. Setting them both to the same value turns the motion blur into a even blur.
I personally prefer the 'cleaner' approach and keep the vectors non-blurred and do the effects in a bitmap program for better control.
Open Gimp or the bitmap editor of your choice (as long as it allows layers) to stack up the bitmap layers.
When using more layers like in the purple example the furthest layers in the background will have the highest blur values and the lightest colours (you can even work with transparency here and make the blend into the sky/ background).
This workflow does work for all sorts of layered/ stacked backgrounds - not just trees and hills. You might want to try it with skyline, houses or an industrial complex with lots of pipes. It should make for a cool effect.