Spectre: An Improved Phantom Edge Finder

Both edge finders use the topological sum to remove noise from second difference responses, so that zero-crossings in these responses represent reliable boundaries. Spectre applies the same techniques to a combination of first and third differences, producing a new map that reliably distinguishes real boundaries from spurious boundaries appearing in staircase intensity patterns. Similar processing is used to mark response regions not associated with zero-crossings (including roof edges) and to compute boundary strengths.