Didn't have to use soft shadows. It looks quite good without. But I had to restart rendering the sequence after I found that there were some things to improve that weren't visible in low resolution. For example some PFs used as colour function in the rock textures are very small, and since the terrain is collapsing the PF stays in place which looks like the PF moves up. This produces some weird flickering where the sand layer doesn't cover the rock. This is happening with the PFs too that have a larger scale, but it's almost not noticeable. So I removed the small PFs in the textures. Besides I set higher quality settings for the cloud layers. I render the sequence now with 1280X720px which is not the same frame ratio, but I increased the overall quality settings a bit, so I think it will be OK even at a lower resolution. Rendertimes are about ten minutes per frame now.