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Date:	Sun, 27 Nov 2011 14:15:19 -0800
From:	Andrew Poelstra <asp11 AT sfu DOT ca>
To:	geda-user AT delorie DOT com
Subject:	Re: [geda-user] Mathematics of the arc element in pcb.
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On Sun, Nov 27, 2011 at 10:36:03PM +0100, myken AT iae DOT nl wrote:
> Hello all,
> 
> I'm working on some functionality in pcb and I have a question.
> 
> The question is about the start angle and the delta of an Arc element in pcb.
> If I draw an Arc in pcb then I will get the following entry in my pcb file:
> Arc [X Y Width Height Thickness Clearance StartAngle DeltaAngle SFlags]
> (no not literally but I hope you know what I mean ;-).
> 
> If the Width and the Height are the same the start angle (45 degrees) and
> the delta (90 degrees) work as expected,
> Arc[200000 150000 20000 20000 100 200 45 90 "clearline"]
> 
> But if the Width and the Height are not the same, it looks like the start
> angle and the delta are totally ignored.
> Arc[200000 150000 30000 20000 100 200 45 90 "clearline"]
> 
> Can anyone explain to me the mathematics behind the Arc element or point
> me to the source code that does the mathematics?
>

As DJ said, width and height are applied as stretching, after a circular
arc has been drawn with the given angles. StartAngle 0 means the arc
starts at the left (opposite of traditional maths in which angles usually
start at the right), and increase counter-clockwise.

The HID's all draw stretched arcs in this way, except Peter Clifton's
GL code, which still does not support them. It draws circular arcs using
Width as the radius.

Further, any "real" code which looks for intersections, connectivity, etc,
only uses the width/height to calculate the endpoints. For the body of the
arc, almost all of it just assumes a circular arc (using Width as the
radius). So when using stretched arcs, you'll have all sorts of DRC and
connectivity troubles.

Also rotations don't work properly.

Officially, pcb does not support stretched arcs for these reasons.



If you're willing to donate some time and numerical analysis to get this
stuff working well, that'd be wonderful. Peter C. has suggested just
getting rid of arcs and using bezier curves for everything, but you can't
do elliptical arcs with bezier curves :(



-- 
Andrew Poelstra
Email: asp11 at sfu.ca OR apoelstra at wpsoftware.net
Web:   http://www.wpsoftware.net/andrew
"I don't understand. Are you saying dualism is always good, or always bad?"

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