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From: geda AT psjt DOT org (Stephan =?utf-8?Q?B=C3=B6ttcher?=)
To: "Nicklas Karlsson \(nicklas.karlsson17\@gmail.com\) \[via geda-user\@delorie.com\]" <geda-user AT delorie DOT com>
Subject: Re: [geda-user] The nature of gEDA layers
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Date: Tue, 02 Feb 2016 13:54:12 +0100
In-Reply-To: <20160202102831.87be1281db69271eaf892fc9@gmail.com> (Nicklas
	Karlsson's message of "Tue, 2 Feb 2016 10:28:31 +0100")
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"Nicklas Karlsson (nicklas DOT karlsson17 AT gmail DOT com) [via
geda-user AT delorie DOT com]" <geda-user AT delorie DOT com> writes:

>> > Currently, the PCB objects come with a lot of implicit aura
>> > attached, polygon clearance, soldermask clearance, solderpad,
>> > thermals, ...
>> > 
>> > A generic geometry may need to be more explicit.
>> > 
>> > Everything is drawn at a depth.  There are positive and negative
>> > depths. Deeper stuff is drawn before shallower stuff.  Negative
>> > depth draw holes and clerances.
>> > 
>> I feel like this is the kind of magic that looks elegant and generic 
>> on inception but tends to turn into an obstacle further down the 
>> evolution of the application. Specifically:
>
> Depth I consider a little bit confusing for use in pcb/gpcb.

Gerber used the term LEVEL, and the polarity toggles in adjacent levels.
The levels are implicitly defined in a Gerber file:  Whenever the
drawing polarity changes, a new level is entered.

Using the sign of a level/depth property as a polarity designator may be
a bit confusing, but less so than a lot we have right now in the data
format.

>> * In addition to positive and negative, there may arise the need for a 
>> third mode of drawing. E.g. XOR-drawing. (No, I don't know what XOR-
>> draw would be good for in the context of EDA -- just brain storming)
>
> There is something similar as xor in gerber file format, inverting
> polarity I think. 

The term XOR do not appear in the Gerber spec.  The polarity is either
DARK or CLEAR.

> To draw material and cut out would however be useful. Clearance in
> polygon in sort of make cut out but it is not possible to draw cutout
> manually. For layer via/board it would make sense with other forms of
> cut out than round for drilled holes.
>
>> * The depth parameter kind of enforces the way how to render the 
>> design. It would be more flexible and potentially easier to extend to 
>> mark-up the intended meaning.

That is a question of syntax details.  Following the Gerber terminology
may be a good idea: "level", not "depth", and larger levels draw later.

> Layers are one pixel "thick" and stack up is used for order. It would
> be possible with more than on bit thick or more colors for different
> materials, it may add complications or solve overlapping problem which
> may occure if using two layers for one mechanical position.

Layers are 2D and have a meaning in Z only by interpretation of
Attributes by a HID.  

Levels/depths are the drawing order of graphics items within a single
layer.  What item shall be cleared by a line clearance, and what item
shall persist?

 1st draw a DARK polygon
 2nd draw a CLEAR circle areound a via hole
 3rd draw a DARK circle and a DARK line wowards that circle

The CLEAR circle must not clear the via pad nor the line, but it must
clear the polygon.  The standard library will assign level numbers to
certain classes of graphical items, with gaps for special needs.
 

>> ---<)kaimartin(>---
>
>
> Nicklas Karlsson

-- 
Stephan