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Date: Fri, 11 Sep 2015 12:07:45 -0400
Message-Id: <201509111607.t8BG7jio032558@envy.delorie.com>
From: DJ Delorie <dj AT delorie DOT com>
To: geda-user AT delorie DOT com
In-reply-to: <msu9bk$u37$1@ger.gmane.org> (message from Kai-Martin Knaak on
	Fri, 11 Sep 2015 12:10:59 +0200)
Subject: Re: [geda-user] Notice to developers: layers have now a type.
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> >   "copper",       /* LT_COPPER */
> >   "silk",         /* LT_SILK */
> (...)
> 
> These are exclusive, are they? 

If you mean you can't use silkscreen dye to make copper traces, yes :-)

The LT_* are layer types, enums.  Not bitmask flags.

> >   "invisible",    /* LT_INVISIBLE */
> 
> What is the use case for invisible? What process should (not) see this 
> layer?

This might be a hold-over from the internal "invisible" type of layer.
For example, if you're looking at the top silk, the bottom silk is
drawn as "invisible" as are bottom elements.  Some of the exporters
need to know this to export the board correctly.

> I imagine, this there is a potential for increased speed, if common
> layer selections were calculated and cached in advance.

If you have thousands of layers, sure.  But we currently have at most
16.  Iterating over 16 things is not going to be a bottleneck, and
certainly not worth making the code more complicated (and less
maintainable).

Most of the CPU performance is in moving layer data between the
internal representation and the graphics card.