Refraction / Fog Colour / Intersecting Geometry

[hammyboi]

Hello,

 

I'm trying to set up shaders for tea pouring into a teacup. I got to a stage where everything was exactly what i needed, with the exception of one issue. Because i'm using a realflow mesh, it doesn't intersect with the cup. Without intersection, the tea is wrong, because there is no meniscus... in fact, the opposite where the edges curve round concave rather than convex. It also meant that the light was travelling through air, then tea, then air, then hitting the cup. Whereas it should be air > tea > cup. This gave me some incorrect results. So to fix this, i've moved the cup up to intersect the water. However, this has given me another issue. If you take a look at the attached images, you can see the strange black line i'm getting round the edges. After doing a lot of investigation, it seems that this happens if the fog colour changes from anything other than pure white. Even a 0.999 value gives me an identical result. It seems that there are some strange maths going on as a result of this intersection.

 

Has anybody had this error before?