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Date: Sat, 28 Jun 2014 22:17:55 -0400
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From: DJ Delorie <dj AT delorie DOT com>
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In-reply-to: <1404004663.18463.42.camel@pcjc2lap> (message from Peter Clifton
	on Sun, 29 Jun 2014 02:17:43 +0100)
Subject: Re: [geda-user] pcb: Patch for arcs with different radii for x and
 y on screen
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> > I've used both types of tools, and IMHO it's better to have a "canned"
> > geometry that you can reference, as well as making exceptions.
> > Sketchup has a good implementation of this, but lacks a way of finding
> > all features that are exceptions.
> 
> Is this in favour of, or against indirecting through reference to a
> separately defined padstack? (We could still choose to copy-on-write, or
> edit-all when modifying vias / pads).

In general, in favor of.  The trick is handling the exceptions
properly.

I'm also in favor of storing footprints that way, so you could (for
example) update all your 0603 footprints at once.

> Would a pad-stack definition be explicit for the board stack-up in use,
> or would it more usefully read something like:
> 
> TOP PASTEMASK LAYER      : Round 2.0mm
> TOP SOLDERMASK LAYER     : Round 2.2mm
> TOP (or START?) LAYER    : Round 2.0mm Clear Round 2.4mm 
> INTERMEDIATE LAYER       : Round 2.1mm Clear Round 2.5mm 
> BOTTOM (or END?) LAYER   : Round 2.0mm Clear Round 2.4mm 
> BOTTOM SOLDERMASK LAYER  : Round 2.2mm
> BOTTOM PASTEMASK LAYER   : Round 2.0mm

I think pad stacks, and footprints in general, need a more "semantic"
stackup (like your example) that can be merged to any actual layer
stack.

> The additive combination of multiple underlying layers is a nuisance
> from a code point of view.. it means we need to combine these various
> data-sources to check for connectivity, and it means some operations can
> be slowed - as the spatial indexes are kept per-layer, not per group.

I think we need additive and subtractive layers, though.  Managing
connectivity is no harder than what we have now, if we design suitable
iterators.  The only trick will be managing polygons - we'd need to
cache an "effective polygon set" for connectivity purposes.

> IMO, although the history pre-dates my involvement, PCB's layer groups
> only really exist as a substitute for being able to tag objects by
> class, or property.

And pretty colors :-)

> > Photo mode does this too, and OSH Park interprets PCB layouts this
> > way.
> 
> Does photo mode use anything  other than the outer-layers though?

Yes.  If you do a four-layer board, you can see shadows of the inner
layers through the outer layers, just like a real board.