Just wanted to share my method for making 2D trees using Blender.
Start Blender and ensure the Sapling addon is enabled. Add a new tree and adjust the parameters to suit your needs.
Enable leaves and populate the tree with enough leaves to cover the tree.
Add a leaf texture to the leaves. The leaves are all automatically UV wrapped, the only thing left to do is to assign a material to them and add a leaf texture. The same with the bark.
Change to Cycles rendering system, and under "film" select transparent to prevent the background from rendering. Also add in all the components necessary for lighting such as the sun and/or HDRI. Experiment a bit to achieve some effect which you like. The camera is set to orthographic and positioned.
Then finally, render. The result is usually very nice.
The last thing which does not do much benefit to the rendering at the moment within the sim is the tree shader and lighting within Flightgear. Potentially when compositor comes it will be very easy to implement a new foliage shader. In my opinion it is best to place them as a single size and slightly larger than real life. Currently under some conditions trees are rendered which are hardly visible and only weigh down the performance.
Also ps. I am color blind, so maybe the colors look a bit odd in the final render, again, things like this are very easily adjusted using Blender.
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