It would be 'rather simple'. If enabled, a scan around the cursor position is made within an adjustable region. No fall off, just a diameter. Then, if anything else than white is encountered the cursor is moved there. No analysis, no fuzzy stuff, just a threshold/edge detection. One can determine to which side of the line to snap himself. It would be nice to snap to the middle of the line in question, but that, I agree, will open Pandora's box as all aspects of averaging, pattern recognition and so on are coming up. But a simple trigger should not be impossible. It would take the tedious strin out of tracing a plan. If that proves to be as useful as I imagine, color values and ranges could make that a bit more flexible in terms of a more soft selection. But a simple black and white, and snap to the nearest edge in the focus would do a FANTASTIC job.
Lemo