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

[DJ Delorie <dj@xxxxxxxxxxx>]

> When you say "just exist", you mean they are there by default on new
> boards, not that they're magic layers, right?

There by default on new boards - yes

not magic - well, I still think "intended purpose" is something that
PCB needs to know about, to do its job well.  We can allow for
exceptions by having an "other" type, but I think pcb really does want
to know about magic layers like silkscreen, soldermask, etc.  I don't
mean that there has to be exactly two and they're treated differently,
you can have three silkscreens all on inner layers if you want, but
for efficiency and usefulness, a fast way to determine the "type" of a
layer - conductor, silk, paste, mask, DRC, etc - is needed.  To me,
this means every layer gets a "type" and "location" value.

> There are some cases where you might want to have inner layers for
> footprint. In that case, the user would get the 'mapping' dialog when
> importing the component. (This mapping will need to be editable after
> the fact, of course).

Right.  Common is easy, uncommon is possible.

> However, by /default/, we will only have top and bottom layers when
> creating a new footprint.

I can see default footprints using "inner" as well.  Sometimes you'd
want a part to have a keep-out for inner layers for EMI reasons.

> Actually, we might need the mapping dialog in all cases, just for
> the case when the user wants to flip the component so the top is on
> bottom, bottom on top.

That happens so often that it's a special key in pcb already - 'b'

> Well, a both-sides silkscreen layer makes little sense. If a user
> wanted that, he could duplicate the top silkscreen to get the bottom
> one. I don't think that would be common enough to require special
> code.

Outlines for bolts, cutouts, mechanical clamps, etc.

But it's just a special case of support we need elsewhere - for
example, a both-sides soldermask is very common, so much so that most
FABs charge you less if both masks are the same.  In that case, any
customization to the mask would need to be on a both-sides layer.

> 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.

> From a development stance, how do we want to structure layers and
> layer groups? It looks to me that we should have layer groups map
> to physical layers. Within each layer group, we can have as many
> drawing layers (of whatever types) as we want.

I suspect the internal structure is... we have a list of layers.
Anything that needs a layer group, really just wants a subset of
layers, and we need to make sure that filtering layers is fast.  I see
no reason to have any more structure than that, it just complicates
the design.


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