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From:	geda AT psjt DOT org (Stephan =?utf-8?Q?B=C3=B6ttcher?=)
To:	"Peter Clifton \(petercjclifton\@googlemail.com\) \[via geda-user\@delorie.com\]" <geda-user AT delorie DOT com>
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Date:	Sun, 27 Dec 2015 16:15:21 +0100
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	(Peter Clifton's message of "Sat, 26 Dec 2015 15:22:04 -0600")
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"Peter Clifton (petercjclifton AT googlemail DOT com) [via
geda-user AT delorie DOT com]" <geda-user AT delorie DOT com> writes:

> Focus on within-layer clearances for now.
>
> Any more complex design rules (such as dielectric breakdown through
> the layer stack) requires more data than we have in PCB's data-model,
> and may actually belong in a separate calculation utility that would
> calculate out a geometric design rule that can be applied in PCB.

I am playing with the idea to build a DRC tool that works on the gerber
output, using libgerbv and the clipper library (C++)

 http://angusj.com/delphi/clipper.php

I have made Python bindings for importing gerbers into clipper polygon
data structures, perform bloating/shrinking and clipping operations.

Design rules would be specified as boolean operations on suitably bloated
layer data, and anything left in the end is a violation.

An example that I have working now:

 from pyclipper import Clipper, gerbv, OffsetPolygons
 proj = gerbv.project()
 proj.load("v01.front.gbr")
 proj.load("v01.plated-drill.cnc")
 scale=10000
 raw     = proj[0].polygons(scale)
 drill   = proj[1].polygons(scale)
 clip = Clipper()
 clip.clear() 
 clip.addsubject(raw)
 # clean up, and remove keyholes
 routing = OffsetPolygons(clip.union(),1)
 # require 12 mil annuklar rings
 annular = OffsetPolygons(drill, 120)
 clip.clear()
 clip.addsubject(annular)
 clip.addclip(routing)
 annular = clip.difference()

Any polygon left in annular is a violation.

For net specific rules I'd need to add some input that gives for each
net at least one coordinate (per relevant layer).  And some code to find
the polygon at the given coordinate.  Some rules, like clearance checks,
require iterations over the individual ploygons in a layer.  At that
point they could be tagged with a netname by the given coordinates.  And
then there could be overrides of bloat values for specifiv nets.

I plot the ploygons with gnuplot right now.  A gerber export may be
usefull to visualize the violations in gerbv.  Or some gui.

-- 
Stephan

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