> postprocess the rendered depth map somehow, look for sharp dropoffs This is fine, if you want a simple ~solid stroke. A simple edge detection kernel filter (

> postprocess the rendered depth map somehow, look for sharp dropoffs This is fine, if you want a simple ~solid stroke. A simple edge detection kernel filter (e.g. Sobel) on the depth and/or normal map is the basis of most outline-like things you see (in games at least) today. This is useful, but it's not an accurate representation of the occluding contour, and it's in an unhelpful form for further processing (kind of like a raster vs. vector image).