I have to google those. Thanks guys.
But here is another problem, and maybe a bug (?)
The arch, which is a displaced sphere, doesn't cast a proper shadow. I realized that, when I compared the images with the imported one with the actual ones.
I did a test with this arch alone (see image below) and tried a lot of things to make the shadow correct. After I searched the forum for answers I found something about negative displacements and offsets. So I made sure to have no negative ones and all offsets are set to 0. No luck.
Then I changed the "rendering method": as you can see "force displacement" messes up the faces, but creates at least the middle shadow part. Anyway it looks weird, so I set the arch's rendering method back to default, but it stays messed up.
I also unchecked "raytrace objects" and "Do raytraced shadows" (not that I really knew, what I was doing...
), but nothing helped.
In case someone wants to test it on his own, the preset pack containing the rock arch is available here:
https://planetside.co.uk/free-downloads/terragen-presets-pack-volume1/First, is there any way to make the displaced internal object cast correct shadows, and why isn't the object's displacement method resettable?