pcb-rnd knowledge pool

Abstract: Better highlighting of short circuits using the minimal cut algorithm.

  The original code was highlighting only pins/pads when a short was found after rats nest optimization. This was not very helpful on a complex board. There had been a long discussion on the geda-user mailing list about the best solutions. There were a few very good ideas, including:

• manual tagging of objects (lines, polys, arcs, vias) with net and warn where two differently tagged net connects
• automatic tagging based on "where it was connected first", then the same warning mechanism as above
• trace history (using the undo buffer?) and go back until when it was not broken, check what exactly broke it
• history combined with tagging
• calculate minimal cut

I choose minimal cut for my patch because it doesn't require tracing the full history or any manual administration of nets vs. objects (which I would find inevitable even with manual tagging - directly or indirectly the user needs to be able to change net tags).

The minimal cut is the least amount of object whose removal would resolve the short. It is best demonstrated on an example:

Removing all the marked lines/polys/vias would surely resolve the short (sometimes leaving rat lines behind). Minimal cut is better than randomly removing objects, tho: it guarantees that the minimal amount of objects are to be removed. On a complex board, this place is likely to be close to the place where the problem really is - much closer than the pins/pads.

Since mincut can be expensive on large boards, the feature can be disabled or enabled, using the configuration system.

Before mincut got implemented, there were different ideas floating on the mailing list.