Window multi sub object weird distortion problem? [MAX-VRAY]

[retrostarskywalker]

Hi,

 

I have an unusal problem. I am doing an interior which is looking out of a window. for the glass I made a rectangle box about 4mm thick as I usually do, applied my glass material and it all works fine...

 

..However, I wanted to make the glass slightly dirty on the outside so I made a multi sub object with 2 mats. 1, the original glass and 2, a dirty version using a vray blend. Then changed the id channel of the poly face on the outside (see image). http://postimg.org/image/4n9whk8br/

 

The dirt shows up nicely but now the outside modelling/background (what is seen out of the window) is distorted in such a way that everything is shifted to left considerably and slightly enlarged I think too. Almost like looking through a really thick piece of glass or something.

 

From trying to figure it out I realised it has something to do with 'copying' from one slot to another in the multisub object. Lets say I used a multisub object with 2 materials but they were instances of each other (so pointless to use a multisub object), this works and I get no distortion.

If I however make the other material a straight 'copy' of the original then I get distortion problem, even though its just a copy, no vray blend or anything.

 

Thanks in advance for any help.