flat shading with normal bias causing artifacts

We are encountering serious problems with flat shaded geometry

Here you can see the hills and spheres shadow look great. On their faces though, there’s some artifacts caused by the normal bias being 0 (at least I think that’s the reason).

The documentation states, that the normal bias value prevents the effect called “shadow acne”. Increasing that value creates this:

As you can see, the shadow of the hill is sliced to pieces (I guess that’s caused by the indentation of the faces along their individual normals which moves them apart)

The shadow on the sphere is all good now.

Tweaking both, the bias and the normal bias never get me a result where both problems are solved.

Can someone please explain how to get flat and smooth shaded objects to work side by side properly?

Looks like the following values resulted in an acceptable result:

  • Bias at 0.08
  • Normal Bias at 0
  • Shadow Quality High (if lower, bias needs to be increased even further and results in peter panning)