Stroke

I would like to apply a stroke effect to this filter sample - like the stroke in Photoshop that applies a line around an image (I really only need to retain the gray line and everything else transparent). The way I did it here with the Min and Max works in about a second here but I am adding this a more complex filter and it grinds to a halt which ThreeDee explained is is a result of those components rasterization http://filterforge.com/forum/read.php...&PAGEN_1=2.

I tried several ways -like Blur one, then Blend with Multiply mode, and Threshold but failed to get a well defined stroked border. I am wondering if someone has any other 'lite' ways of generating a border like this?

street 2.ffxml

Thank you ThreeDee. I really appreciate all of your help.