@gk fotografie reminded me today of a technique I used years ago to create a line drawing by sandwiching a positive and negative film. Started me thinking about how to do it in the digital world. I found that it actually wasn't that difficult by simply duplicating your image layer. Setting the mode to Color Dodge, then inverting, and finally applying a Gaussian blur set to around 18. Further refining it with a hue/saturation layer to desaturate the image, then refining the lines with a curves layer. The only thing is that the lines don't have the crispness I remember from the darkroom method. Like all things new, it could use some more refining.