OSL screen node

Hi, I have been using Blender for about a year now, however I am new to Blender Artists, and I have no experience with OSL (or coding in general for that matter), but, I could get my head round the OSL introduction on Blender Cookie, and hope that my query is fairly simple. I was wondering how it would be possible to implement an OSL Screen Shader (similar to an Add/Mix shader): in order to create a human skin shader similar to the one outlined by Ben Simmonds for BI.

I believe the equation for screen blending is something like 1-((1-A)*(1-B)), but I don’t know how to use the formula in an OSL shader.

Thank you in advance for any help you may be able to offer.

That’s not possible because OSL does not support multiplication of shaders.

Oh well, thanks for clearing that up

You can do it with colours. See Jimmys page: