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Re: gEDA-user: Adding inner polygons to a plane

On Wed, 2011-05-11 at 19:05 +0100, Thomas Oldbury wrote:
> Sometimes, I want to add an inner polygon area to a plane in PCB. The
>    area might be a power supply which only has to cover a small area; e.g.
>    1.8V in a predominantly 3.3V area. However, if I just draw a polygon on
>    top of the plane, there is no cut-out formed and I get shorts. To do
>    what I want, I must cut a hole in the main polygon plane, then add my
>    smaller polygon into it. This is very time consuming and changing the
>    plane once created is very difficult. Is there a way to get PCB to
>    support nested polygons?

Not easily. For many power plane cases, these polygons won't actually be
truly nested, so it is impossible to infer which polygon the user wishes
to clip the other.

Perhaps it would be possible to support a flag on the "smaller",
clippiING polygon which makes it "bully" other polygons away from it,
but again - it is not clear what to do in the case where two polygons
with this flag touch each other. (Just short with each other I guess).

This class of object would not be so much a "pour", but behave more like
a line or arc segment. I'm guessing we would still need to retain
support for clipping it against pins, pads, lines and arcs.

Shouldn't take "that" much code to make it happen (I already implemented
a similar feature once before), I just need to hear that there is
general consensus that it is a sensible thing to do.

Often for the kind of inner layers you describe, you want some complex
boolean logic operation to produce the final shape from various sources.

Packages like Altium (which springs to mind), let you define a layer in
terms of its _negative_, so you split up a power plane by defining the
_boundary_ between the two (or more) regions.

Perhaps the way is to go more like mech-CAD systems in defining geometry
based upon a hierarchy of boolean operations.

Peter Clifton

Electrical Engineering Division,
Engineering Department,
University of Cambridge,
9, JJ Thomson Avenue,

Tel: +44 (0)7729 980173 - (No signal in the lab!)
Tel: +44 (0)1223 748328 - (Shared lab phone, ask for me)

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