gEDA-dev: [PATCH] PCB/gtk: Draw rotated arcs correctly

[Bernd Jendrissek]

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On Tue, Apr 17, 2007 at 12:32:55PM +0200, Peter TB Brett wrote:
> > > How about something along these lines:
> > >
> > >    angle = start_angle + 180 % 360;
> > >
> > >    gdk_draw_arc (gport->drawable, gport->u_gc, 0,
> > >                 DRAW_X (cx) - vrx, DRAW_Y (cy) - vry,
> > >                 vrx * 2, vry * 2, angle * 64, delta_angle * 64);
> > >
> > > This avoids a branch, and requires fewer instructions. OTOH, can
> > > start_angle be negative?

Cool, as long as my TO3's look right on the screen!  (I figured it was
the HID and not the data structure when I exported the borked footprint
to gerber and it was fine.)

While I'd be surprised if the '%' operator has any speed advantages, it
sure is more robust against out-of-range inputs.

In my quick search on gdk_draw_arc I didn't see any special mention of
negative angles, but here is what XDrawArc(3X11) says:

"...and negative angles indicate clockwise motion.  If the magnitude of
angle2 is greater than 360 degrees, XDrawArc or XDrawArcs truncates it
to 360 degrees."

I don't know if that applies only to the delta or also to the start
angle.

> > Perhaps a better solution (thinking about it) is to make sure the angle
> > never over-flows in the first place. Can PCB's internal data-structures
> > accommodate any necessary range limitation?

While I didn't grok all of PCB's code, it does look like
RotateArcLowLevel() does limit the range of Arc->StartAngle:

  /* add Number*90 degrees to the startangle and check for overflow */
  Arc->StartAngle = (Arc->StartAngle + Number * 90) % 360;

> > You'd then perform the limit wrapping at each load / modification to the
> > data-structure rather than at draw time. This prevents any internal
> > overflows too, whilst keeping drawing faster.
> 
> This seems like a better way of doing it.  Fixing at draw time is a bit of a 
> hack.

The HID still has to fix whatever it breaks as a result of coordinate
transformation; PCB's angular origin is at 9 o'clock whilst GDK and X
have theirs at 9 o'clock.

- -- 
Jim Summers was right - you've completely burned out.  It's sad.
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