Re: gEDA-user: next PCB release - 1.99za vs 4.0

[DJ Delorie <dj@xxxxxxxxxxx>]

> Alright, we'll allow a "top and bottom" layer group.

With flex cable, "top" and "bottom" aren't limited to one layer each.

> > > I think that if we want components on multiple layers (or all
> > > layers), that should be a property of the component, not a layer
> > > group/physical layer thing.
> > 
> > What's the difference?  It has to be implemented somehow.
> >
> 
> Well, I'm trying to implement this, so "somehow" matters to me ;).

Either we have one loop that filters layers by need, or a nested pair
of loops.  Same complexity.  How do you handle layers that aren't
assigned to physical layers?  Like documentation layers?  What about
layers that apply to more than one physical layer, like outers or
inners, or all?  A tree structure ties you down to a specific physical
structure, a list does not.  What about buried vias?  Do they go on a
special layer that spans physical layers?

> I disagree. If physical layers can contain multiple drawing layers (and
> they must, to keep silk and copper layers separate), it makes sense to
> have a rigid structure. Then we can display the layer selector in a tree-
> like display and it is clear to everyone what is what.

Internal structure and what we present to the user are two different
problems.  Yes, we need to keep track of the physical stackup.  No,
that does not need to dictate our internal structure.

> http://download.wpsoftware.net/code/pcb/layer.h

I would add a LayerPos to each Layer, and store the Layers separately.

>   1. At the top level, each PCB has a layer stack. This is the ordered
>      list of physical layers, plus one "internal" layer that is never
>      saved out or edited. (For now, this will contain the ratsnest layer
>      and nothing else.)

Why one?  Why not save it?  We already save rats to file.  What about
layers that don't correspond to a physical layer?

>   3. Each drawing layer has a type associated with it - one of COPPER
>      (conductive), SILK (non-conductive) or INTERNAL (not saved out:
>      DRC highlights, etc, will be on such a layer).

paste mask keepout fab drill notes

Why not save DRC highlights?

>   4. TODO: There is no way to do keepouts. I have heard arguments for
>      allowing global keepouts, per-layer keepouts, per-function-block
>      keepouts, etc, and I don't know how it should be structured.

keepouts are just another layer.  Let DRC worry about what to do with
it.


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