Procedural Overhang Dirt

[Anonymous]

Hello all,

I'm looking for a solution to generate procedural overhang dirt. I mean leaky dirt that's only generated under extruded ledges like roof ledges, window ledges, etc. 
Attaching an image to demonstrate what I mean.

Basically, a directional seamless leak map that's masked by occlusion, but only in the areas facing downwards. 
I know I've seen it done before, but I can't figure out the node setup to get it working. Also, interested if someone has an OSL solution for this?

This seems to be the only information I can find on it, but it's outdated and using mental ray. I can't really get much useful info out of it. 
http://materialism-blog.blogspot.com/2012/05/procedural-dirt-update-downward-drips.html

If there's a way to get downward directional streaks under ledges procedurally in Arnold, I would love to know!

Thank you kindly!

[damaggio]

I dont know if you can do it with Arnold but Vray 5 has this handy feature, otherwise you can do it with traditional methods.

[Anonymous]

Interesting, 

yeah essentially like this feature from vray:

https://docs.chaosgroup.com/display/VMAX/VRayDirt#VRayDirt-bias

It must be possible in Arnold, at the very least there must be a way to do this using an OSL node?

[damaggio]

Sure , it would be useful to all Arnold users, someone will chime in I'm sure, I just use the old method in Vray.

https://docs.chaosgroup.com/display/VMAYA/VRayDirt

Arnold OSL from Master Zap

https://area.autodesk.com/tutorials/introduction-to-open-shading-language-osl/

[ntsimiklis]

I found this too, but their suggestions don't seem to work for me in Maya
https://answers.arnoldrenderer.com/questions/13399/directional-ambient-occlusion.html

Changing the Normal values on the AO node doesn't affect anything...