How do I get rid of unwanted shadows from decals?

Attached is a jpg showing the Enscape window with nice snow decal (as I thought it would be) in the top left corner, and in the bottom left corner the rendering at ultra quality – with unwanted square black shadows. The decal is placed around with Skatters "render only" option. All the views rendered like that.

The material dialogue is shown on top of the SU window to show how i made the decal.

How can I get the result I see in the enscape window? Why do the decals throw shadows when rendered?

I showed these illustrations to a client, and they liked the idea very much and want to use them so I need to figure this out quickly.

Do you be any chance have any tips for making snow with Enscape/Sketchup?

I have never done it before so this is a first try.

Thank you in advance for any useful answers.

Done. Gets worse.

OK. From that post I understand that my problem is a result of indirect ligting beeing calculated for the decals geometry, not the opacity mask.

How I solved this was to (in Illustrator) cut out the decal to fit the opacity mask.

This kind of worked, but the complex piece of geometry made the file much slower to work with.

(For some unknown reason, I also lost my street lights, so my evening renders are not very nice anymore)

Thank you , MatthiasB