Refraction <1 gives black render??

[Brrrt]

Hi,

I was rendering some landscape that I made and it came out with a completely black outputpic.
I suspected that it was a mistake in the watershader.
The refractionindex was <1 (0.9971), which was made possible by just clicking the leftarrow.
Now that I set it back to =1 the render comes out normal.
Is this a normal 'feature' or did I do something that TG doesn't allow?
I had enough free memory and all that and the other settings were within normal parameters.

I'm on Win 8.1 x64 with 4GB RAM (750 MB or so free during rendering), TG 3.1 free.
Let me know if you need more systeminfo.

[Dune]

Yes, seems very likely. A refraction index of less than 1 is not logical (I don't know exactly, but I think light is then just 'trapped' under the water surface), and I had the same when experimenting.

[Oshyan]

Yes, Terragen lets you set unrealistic or impossible values for many settings. Sometimes it produces interesting results, sometimes nothing. Generally the sliders show you what are "sensible" or recommended values; in this case 1 is the bottom of the slider, although yes the increment buttons do go below 1.

- Oshyan

[Matt]

An index of refraction < 1 is allowed in nature(*) but Terragen currently doesn't support it.

*For example, in the real world, if you are underwater (which has an IOR of 1.33) and you look at an object with an IOR less than 1.33, the effective IOR of the object will be less than 1. It will be the ratio of the object's IOR to the water's IOR. This example is easy to construct - just make an air bubble underwater.