WIP

5 files changed, 210 insertions, 119 deletions

diff --git a/gerbonara/cad/kicad/symbols.py b/gerbonara/cad/kicad/symbols.py
index 45f8ec0..972dd55 100644
--- a/gerbonara/cad/kicad/symbols.py
+++ b/gerbonara/cad/kicad/symbols.py
@@ -285,7 +285,7 @@ class Arc:
         x2, y2 = self.mid.x-x1, self.mid.y-x2
         x3, y3 = (self.end.x - x1)/2, (self.end.y - y1)/2
         clockwise = math.atan2(x2*y3-x3*y2, x2*x3+y2*y3) > 0
-        return arc_bounds(x1, y1, self.end.x, self.end.y, cx-x1, cy-y1, clockwise)
+        return arc_bounds(x1, y1, self.end.x, self.end.y, cx, cy, clockwise)
 
 
     def to_svg(self, colorscheme=Colorscheme.KiCad):
diff --git a/gerbonara/graphic_objects.py b/gerbonara/graphic_objects.py
index 2b9dc3e..14cfc66 100644
--- a/gerbonara/graphic_objects.py
+++ b/gerbonara/graphic_objects.py
@@ -19,9 +19,9 @@
 import math
 import copy
 from dataclasses import dataclass, astuple, field, fields
-from itertools import zip_longest
+from itertools import zip_longest, pairwise, islice, cycle
 
-from .utils import MM, InterpMode, to_unit, rotate_point, sum_bounds
+from .utils import MM, InterpMode, to_unit, rotate_point, sum_bounds, approximate_arc, sweep_angle
 from . import graphic_primitives as gp
 from .aperture_macros import primitive as amp
 
@@ -278,9 +278,15 @@ class Region(GraphicObject):
      * A region is always exactly one connected component.
      * A region must not overlap itself anywhere.
      * A region cannot have holes.
+     * The last outline point of the region must be equal to the first.
 
     There is one exception from the last two rules: To emulate a region with a hole in it, *cut-ins* are allowed. At a
     cut-in, the region is allowed to touch (but never overlap!) itself.
+
+    When ``arc_centers`` is empty, this region has only straight outline segments. When ``arc_centers`` is not empty,
+    the i-th entry defines the i-th outline segment, with a ``None`` entry designating a straight line segment.
+    An arc is defined by a ``(clockwise, (cx, cy))`` tuple, where ``clockwise`` can be ``True`` for a clockwise arc, or
+    ``False`` for a counter-clockwise arc. ``cx`` and ``cy`` are the absolute coordinates of the arc's center. 
     """
 
     def __init__(self, outline=None, arc_centers=None, *, unit=MM, polarity_dark=True):
@@ -304,8 +310,8 @@ class Region(GraphicObject):
     def _rotate(self, angle, cx=0, cy=0):
         self.outline = [ gp.rotate_point(x, y, angle, cx, cy) for x, y in self.outline ]
         self.arc_centers = [
-                (arc[0], gp.rotate_point(*arc[1], angle, cx-p[0], cy-p[1])) if arc else None
-                for p, arc in zip_longest(self.outline, self.arc_centers) ]
+                (arc[0], gp.rotate_point(*arc[1], angle, cx, cy)) if arc else None
+                for arc in self.arc_centers ]
 
     def _scale(self, factor):
         self.outline = [ (x*factor, y*factor) for x, y in self.outline ]
@@ -322,6 +328,10 @@ class Region(GraphicObject):
             (x, y+h),
             ], unit=unit)
 
+    @classmethod
+    def from_arc_poly(kls, arc_poly, polarity_dark=True, unit=MM):
+        return kls(arc_poly.outline, arc_poly.arc_centers, polarity_dark=polarity_dark, unit=unit)
+
     def append(self, obj):
         if obj.unit != self.unit:
             obj = obj.converted(self.unit)
@@ -331,48 +341,49 @@ class Region(GraphicObject):
         self.outline.append(obj.p2)
 
         if isinstance(obj, Arc):
-            self.arc_centers.append((obj.clockwise, obj.center_relative))
+            self.arc_centers.append((obj.clockwise, obj.center))
         else:
             self.arc_centers.append(None)
 
-    def close(self):
-        if not self.outline:
-            return
+    def iter_segments(self, tolerance=1e-6):
+        for points, arc in zip_longest(pairwise(self.outline), self.arc_centers):
+            if arc:
+                if points:
+                    yield *points, arc
+                else:
+                    yield self.outline[-1], self.outline[0], arc
+                    return
+            else:
+                if not points:
+                    break
+                yield *points, (None, (None, None))
 
-        if self.outline[-1] != self.outline[0]:
-            self.outline.append(self.outline[0])
+        # Close outline if necessary.
+        if math.dist(self.outline[0], self.outline[-1]) > tolerance:
+            yield self.outline[-1], self.outline[0], (None, (None, None))
 
     def outline_objects(self, aperture=None):
-        for p1, p2, arc in zip_longest(self.outline, self.outline[1:] + self.outline[:1], self.arc_centers):
-            if arc:
-                clockwise, pc  = arc
-                yield Arc(*p1, *p2, *pc, clockwise, aperture=aperture, unit=self.unit, polarity_dark=self.polarity_dark)
+        for p1, p2, (clockwise, center) in self.iter_segments():
+            if center:
+                yield Arc(*p1, *p2, *center, clockwise, aperture=aperture, unit=self.unit, polarity_dark=self.polarity_dark)
             else:
                 yield Line(*p1, *p2, aperture=aperture, unit=self.unit, polarity_dark=self.polarity_dark)
 
-    def _aperture_macro_primitives(self, max_error=1e-2, unit=MM):
+    def _aperture_macro_primitives(self, max_error=1e-2, clip_max_error=True, unit=MM):
         # unit is only for max_error, the resulting primitives will always be in MM
         
         if len(self.outline) < 2:
             return
 
-        points = [self.outline[0]]
-        for p1, p2, arc in zip_longest(self.outline[:-1], self.outline[1:], self.arc_centers):
-            if arc:
-                clockwise, pc  = arc
-                #r = math.hypot(*pc) # arc center is relative to p1.
-                #d = math.dist(p1, p2)
-                #err = r - math.sqrt(r**2 - (d/(2*n))**2)
-                #n = math.ceil(1/(2*math.sqrt(r**2 - (r - max_err)**2)/d))
-                arc = Arc(*p1, *p2, *pc, clockwise, unit=self.unit, polarity_dark=self.polarity_dark, aperture=None)
-                for line in arc.approximate(max_error=max_error, unit=unit):
-                    points.append(line.p2)
-
+        points = []
+        for p1, p2, (clockwise, center) in self.iter_segments():
+            if center:
+                for p in approximate_arc(*center, *p1, *p2, clockwise,
+                                             max_error=max_error, clip_max_error=clip_max_error):
+                    points.append(p)
+                    points.pop()
             else:
-                points.append(p2)
-
-        if points[-1] != points[0]:
-            points.append(points[0])
+                points.append(p1)
 
         yield amp.Outline(self.unit, int(self.polarity_dark), len(points)-1, tuple(coord for p in points for coord in p))
 
@@ -389,6 +400,9 @@ class Region(GraphicObject):
             yield gp.ArcPoly(conv_outline, conv_arc, polarity_dark=self.polarity_dark)
 
     def to_statements(self, gs):
+        if len(self.outline) < 3:
+            return
+
         yield from gs.set_polarity(self.polarity_dark)
         yield 'G36*'
         # Repeat interpolation mode at start of region statement to work around gerbv bug. Without this, gerbv will
@@ -398,32 +412,24 @@ class Region(GraphicObject):
 
         yield from gs.set_current_point(self.outline[0], unit=self.unit)
 
-        for point, arc_center in zip_longest(self.outline[1:], self.arc_centers):
-            if point is None and arc_center is None:
+        for previous_point, point, (clockwise, center) in self.iter_segments():
+            if point is None and center is None:
                 break
 
-            if arc_center is None:
-                yield from gs.set_interpolation_mode(InterpMode.LINEAR)
+            x = gs.file_settings.write_gerber_value(point[0], self.unit)
+            y = gs.file_settings.write_gerber_value(point[1], self.unit)
 
-                x = gs.file_settings.write_gerber_value(point[0], self.unit)
-                y = gs.file_settings.write_gerber_value(point[1], self.unit)
+            if clockwise is None:
+                yield from gs.set_interpolation_mode(InterpMode.LINEAR)
                 yield f'X{x}Y{y}D01*'
 
-                gs.update_point(*point, unit=self.unit)
-
             else:
-                clockwise, (cx, cy) = arc_center
-                x2, y2 = point
                 yield from gs.set_interpolation_mode(InterpMode.CIRCULAR_CW if clockwise else InterpMode.CIRCULAR_CCW)
-
-                x = gs.file_settings.write_gerber_value(x2, self.unit)
-                y = gs.file_settings.write_gerber_value(y2, self.unit)
-                # TODO are these coordinates absolute or relative now?!
-                i = gs.file_settings.write_gerber_value(cx, self.unit)
-                j = gs.file_settings.write_gerber_value(cy, self.unit)
+                i = gs.file_settings.write_gerber_value(center[0]-previous_point[0], self.unit)
+                j = gs.file_settings.write_gerber_value(center[1]-previous_point[1], self.unit)
                 yield f'X{x}Y{y}I{i}J{j}D01*'
 
-                gs.update_point(x2, y2, unit=self.unit)
+            gs.update_point(*point, unit=self.unit)
 
         yield 'G37*'
 
@@ -605,22 +611,8 @@ class Arc(GraphicObject):
         :returns: Angle in clockwise radian between ``0`` and ``2*math.pi``
         :rtype: float
         """
-        cx, cy = self.cx + self.x1, self.cy + self.y1
-        x1, y1 = self.x1 - cx, self.y1 - cy
-        x2, y2 = self.x2 - cx, self.y2 - cy
-
-        a1, a2 = math.atan2(y1, x1), math.atan2(y2, x2)
-        f = abs(a2 - a1)
-        if not self.clockwise:
-            if a2 > a1:
-                return a2 - a1
-            else:
-                return 2*math.pi - abs(a2 - a1)
-        else:
-            if a1 > a2:
-                return a1 - a2
-            else:
-                return 2*math.pi - abs(a1 - a2)
+
+        return sweep_angle(self.cx+self.x1, self.cy+self.y1, self.x1, self.y1, self.x2, self.y2, self.clockwise)
 
     @property
     def p1(self):
@@ -677,34 +669,16 @@ class Arc(GraphicObject):
         :returns: list of :py:class:`~.graphic_objects.Line` instances.
         :rtype: list
         """
-        # TODO the max_angle calculation below is a bit off -- we over-estimate the error, and thus produce finer
-        # results than necessary. Fix this.
-            
-        r = math.hypot(self.cx, self.cy)
 
         max_error = self.unit(max_error, unit)
-        if clip_max_error:
-            # 1 - math.sqrt(1 - 0.5*math.sqrt(2))
-            max_error = min(max_error, r*0.4588038998538031)
-
-        elif max_error >= r:
-            return [Line(*self.p1, *self.p2, aperture=self.aperture, polarity_dark=self.polarity_dark, unit=self.unit)]
-
-        # see https://www.mathopenref.com/sagitta.html
-        l = math.sqrt(r**2 - (r - max_error)**2)
-
-        angle_max = math.asin(l/r)
-        sweep_angle = self.sweep_angle()
-        num_segments = math.ceil(sweep_angle / angle_max)
-        angle = sweep_angle / num_segments
-
-        if not self.clockwise:
-            angle = -angle
-
-        cx, cy = self.center
-        points = [ rotate_point(self.x1, self.y1, i*angle, cx, cy) for i in range(num_segments + 1) ]
-        return [ Line(*p1, *p2, aperture=self.aperture, polarity_dark=self.polarity_dark, unit=self.unit)
-                for p1, p2 in zip(points[0::], points[1::]) ]
+        return [Line(*p1, *p2, aperture=self.aperture, polarity_dark=self.polarity_dark, unit=self.unit)
+                for p1, p2 in pairwise(approximate_arc(
+                    self.cx+self.x1, self.cy+self.y1,
+                    self.x1, self.y1,
+                    self.x2, self.y2,
+                    self.clockwise,
+                    max_error=max_error,
+                    clip_max_error=clip_max_error))]
 
     def _rotate(self, rotation, cx=0, cy=0):
         # rotate center first since we need old x1, y1 here
@@ -726,7 +700,7 @@ class Arc(GraphicObject):
         w = self.aperture.equivalent_width(unit) if self.aperture else 0
         return gp.Arc(x1=conv.x1, y1=conv.y1,
                 x2=conv.x2, y2=conv.y2,
-                cx=conv.cx, cy=conv.cy,
+                cx=conv.cx+conv.x1, cy=conv.cy+conv.y1,
                 clockwise=self.clockwise,
                 width=w,
                 polarity_dark=self.polarity_dark)
diff --git a/gerbonara/graphic_primitives.py b/gerbonara/graphic_primitives.py
index ea8fd9f..e136efb 100644
--- a/gerbonara/graphic_primitives.py
+++ b/gerbonara/graphic_primitives.py
@@ -19,7 +19,7 @@
 import math
 import itertools
 
-from dataclasses import dataclass, replace
+from dataclasses import dataclass, replace, field
 
 from .utils import *
 
@@ -79,6 +79,10 @@ class Circle(GraphicPrimitive):
         color = fg if self.polarity_dark else bg
         return tag('circle', cx=prec(self.x), cy=prec(self.y), r=prec(self.r), fill=color)
 
+    def to_arc_poly(self):
+        return ArcPoly([(self.x-self.r, self.y), (self.x+self.r, self.y)],
+                       [(True, (self.x, self.y)), (True, (self.x, self.y))])
+
 
 @dataclass(frozen=True)
 class ArcPoly(GraphicPrimitive):
@@ -88,28 +92,51 @@ class ArcPoly(GraphicPrimitive):
     #: connected.
     outline : list
     #: Must be either None (all segments are straight lines) or same length as outline.
-    #: Straight line segments have None entry.
-    arc_centers : list = None
+    #: Straight line segments have None entry. Arc segments have (clockwise, (cx, cy)) tuple with cx, cy being absolute
+    #: coords.
+    arc_centers : list = field(default_factory=list)
 
     @property
     def segments(self):
         """ Return an iterator through all *segments* of this polygon. For each outline segment (line or arc), this
-        iterator will yield a ``(p1, p2, center)`` tuple. If the segment is a straight line, ``center`` will be
-        ``None``.
+        iterator will yield a ``(p1, p2, (clockwise, center))`` tuple. If the segment is a straight line, ``clockwise``
+        will be ``None``.
         """
-        ol = self.outline
-        return itertools.zip_longest(ol, ol[1:] + [ol[0]], self.arc_centers or [])
+        for points, arc in itertools.zip_longest(itertools.pairwise(self.outline), self.arc_centers):
+            if arc:
+                if points:
+                    yield *points, arc
+                else:
+                    yield self.outline[-1], self.outline[0], arc
+                    return
+            else:
+                if not points:
+                    break
+                yield *points, (None, (None, None))
+
+        # Close outline if necessary.
+        if math.dist(self.outline[0], self.outline[-1]) > 1e-6:
+            yield self.outline[-1], self.outline[0], (None, (None, None))
+
+    def approximate_arcs(self, max_error=1e-2, clip_max_error=True):
+        outline = []
+        for p1, p2, (clockwise, center) in self.segments():
+            if clockwise is None:
+                outline.append(p1)
+            else:
+                outline.extend(approximate_arc(cx, cy, x1, y1, x2, y2, clockwise,
+                                               max_error=max_error, clip_max_error=clip_max_error))
+                outline.pop() # remove arc end point
+        return type(self)(outline)
 
     def bounding_box(self):
         bbox = (None, None), (None, None)
-        for (x1, y1), (x2, y2), arc in self.segments:
-            if arc:
-                clockwise, (cx, cy) = arc
-                bbox = add_bounds(bbox, arc_bounds(x1, y1, x2, y2, cx, cy, clockwise))
-
-            else:
+        for (x1, y1), (x2, y2), (clockwise, (cx, cy)) in self.segments:
+            if clockwise is None:
                 line_bounds = (min(x1, x2), min(y1, y2)), (max(x1, x2), max(y1, y2))
                 bbox = add_bounds(bbox, line_bounds)
+            else:
+                bbox = add_bounds(bbox, arc_bounds(x1, y1, x2, y2, cx, cy, clockwise))
         return bbox
 
     @classmethod
@@ -149,6 +176,9 @@ class ArcPoly(GraphicPrimitive):
         color = fg if self.polarity_dark else bg
         return tag('path', d=' '.join(self.path_d()), fill=color)
 
+    def to_arc_poly(self):
+        return self
+
 
 @dataclass(frozen=True)
 class Line(GraphicPrimitive):
@@ -191,6 +221,24 @@ class Line(GraphicPrimitive):
         return tag('path', d=f'M {float(self.x1):.6} {float(self.y1):.6} L {float(self.x2):.6} {float(self.y2):.6}',
                 fill='none', stroke=color, stroke_width=str(width))
 
+    def to_arc_poly(self):
+        l = math.dist((self.x1, self.y1), (self.x2, self.y2))
+        dx, dy = self.x2-self.x1, self.y2-self.y1
+        nx, ny = -dy/l, dx/l
+        rx, ry = nx*self.width/2, ny*self.width/2
+        return ArcPoly([
+                    (self.x1+rx, self.y1+ry),
+                    (self.x1-rx, self.y1-ry),
+                    (self.x2-rx, self.y2-ry),
+                    (self.x2+rx, self.y2+ry),
+                ], [
+                    (True, (self.x1, self.y1)),
+                    None,
+                    (True, (self.x2, self.y2)),
+                    None,
+                ])
+
+
 @dataclass(frozen=True)
 class Arc(GraphicPrimitive):
     """ Circular arc with line width ``width`` going from ``(x1, y1)`` to ``(x2, y2)`` around center at ``(cx, cy)``. """
@@ -202,9 +250,9 @@ class Arc(GraphicPrimitive):
     x2 : float
     #: End Y coodinate
     y2 : float
-    #: Center X coordinate relative to ``x1``
+    #: Center X coordinate (absolute)
     cx : float
-    #: Center Y coordinate relative to ``y1``
+    #: Center Y coordinate (absolute)
     cy : float
     #: ``True`` if this arc is clockwise from start to end. Selects between the large arc and the small arc given this
     #: start, end and center
@@ -214,11 +262,10 @@ class Arc(GraphicPrimitive):
 
     @property
     def is_circle(self):
-        return math.isclose(self.x1, self.x2) and math.isclose(self.y1, self.y2)
+        return math.isclose(self.x1, self.x2, abs_tol=1e-6) and math.isclose(self.y1, self.y2, abs_tol=1e-6)
 
     def flip(self):
-        return replace(self, x1=self.x2, y1=self.y2, x2=self.x1, y2=self.y1,
-            cx=(self.x1 + self.cx) - self.x2, cy=(self.y1 + self.cy) - self.y2, clockwise=not self.clockwise)
+        return replace(self, x1=self.x2, y1=self.y2, x2=self.x1, y2=self.y1, clockwise=not self.clockwise)
 
     def bounding_box(self):
         r = self.width/2
@@ -232,6 +279,25 @@ class Arc(GraphicPrimitive):
         return tag('path', d=f'M {float(self.x1):.6} {float(self.y1):.6} {arc}',
                 fill='none', stroke=color, stroke_width=width)
 
+    def to_arc_poly(self):
+        r = math.dist((self.x1, self.y1), (self.cx, self.cy))
+        dx1, dy1 = self.x1-self.cx, self.y1-self.cy
+        nx1, ny1 = dx1/r * self.width/2, dy1/r * self.width/2
+        dx2, dy2 = self.x2-self.cx, self.y2-self.cy
+        nx2, ny2 = dx2/r * self.width/2, dy2/r * self.width/2
+        return ArcPoly([
+                    (self.x1+nx1, self.y1+nx1),
+                    (self.x1-nx1, self.y1-nx1),
+                    (self.x2-nx2, self.y2-nx2),
+                    (self.x2+nx2, self.y2+nx2),
+                ], [
+                    (self.clockwise, (self.x1, self.y1)),
+                    (self.clockwise, (self.cx, self.cy)),
+                    (self.clockwise, (self.x2, self.y2)),
+                    (self.clockwise, (self.cx, self.cy)),
+                ])
+
+
 @dataclass(frozen=True)
 class Rectangle(GraphicPrimitive):
     #: **Center** X coordinate
diff --git a/gerbonara/tests/test_kicad_footprints.py b/gerbonara/tests/test_kicad_footprints.py
index 07d64a7..663037b 100644
--- a/gerbonara/tests/test_kicad_footprints.py
+++ b/gerbonara/tests/test_kicad_footprints.py
@@ -142,7 +142,7 @@ def _parse_path_d(path):
                     cx = mx - nx*nl
                     cy = my - ny*nl
 
-            (min_x, min_y), (max_x, max_y) = arc_bounds(last_x, last_y, ax, ay, cx-last_x, cy-last_y, clockwise=(not sweep))
+            (min_x, min_y), (max_x, max_y) = arc_bounds(last_x, last_y, ax, ay, cx, cy, clockwise=(not sweep))
             min_x -= sr
             min_y -= sr
             max_x += sr
diff --git a/gerbonara/utils.py b/gerbonara/utils.py
index c7336e6..892b217 100644
--- a/gerbonara/utils.py
+++ b/gerbonara/utils.py
@@ -244,6 +244,59 @@ def rotate_point(x, y, angle, cx=0, cy=0):
             cy + (x - cx) * math.sin(-angle) + (y - cy) * math.cos(-angle))
 
 
+def sweep_angle(cx, cy, x1, y1, x2, y2, clockwise):
+    """ Calculate absolute sweep angle of arc. This is always a positive number.
+
+    :returns: Angle in clockwise radian between ``0`` and ``2*math.pi``
+    :rtype: float
+    """
+    x1, y1 = x1-cx, y1-cy
+    x2, y2 = x2-cx, y2-cy
+
+    a1, a2 = math.atan2(y1, x1), math.atan2(y2, x2)
+    f = abs(a2 - a1)
+    if not clockwise:
+        if a2 > a1:
+            return a2 - a1
+        else:
+            return 2*math.pi - abs(a2 - a1)
+    else:
+        if a1 > a2:
+            return a1 - a2
+        else:
+            return 2*math.pi - abs(a1 - a2)
+
+
+def approximate_arc(cx, cy, x1, y1, x2, y2, clockwise, max_error=1e-2, clip_max_error=True):
+    # TODO the max_angle calculation below is a bit off -- we over-estimate the error, and thus produce finer
+    # results than necessary. Fix this.
+        
+    r = math.dist((x1, y1), (cx, cy))
+
+    if clip_max_error:
+        # 1 - math.sqrt(1 - 0.5*math.sqrt(2))
+        max_error = min(max_error, r*0.4588038998538031)
+
+    elif max_error >= r:
+        yield (x1, y1)
+        yield (x2, y2)
+        return
+
+    # see https://www.mathopenref.com/sagitta.html
+    l = math.sqrt(r**2 - (r - max_error)**2)
+
+    angle_max = math.asin(l/r)
+    sweep_angle = sweep_angle(cx, cy, x1, y1, x2, y2, clockwise)
+    num_segments = math.ceil(sweep_angle / angle_max)
+    angle = sweep_angle / num_segments
+
+    if not clockwise:
+        angle = -angle
+
+    for i in range(num_segments + 1):
+        yield rotate_point(x1, y1, i*angle, cx, cy)
+
+
 def min_none(a, b):
     """ Like the ``min(..)`` builtin, but if either value is ``None``, returns the other. """
     if a is None:
@@ -340,11 +393,9 @@ def arc_bounds(x1, y1, x2, y2, cx, cy, clockwise):
     # This solution manages to handle circular arcs given in gerber format (with explicit center and endpoints, plus
     # sweep direction instead of a format with e.g. angles and radius) without any trigonometric functions (e.g. atan2).
     #
-    # cx, cy are relative to p1.
+    # cx, cy are in absolute coordinates.
 
     # Center arc on cx, cy
-    cx += x1
-    cy += y1
     x1 -= cx
     x2 -= cx
     y1 -= cy
@@ -461,25 +512,25 @@ def point_line_distance(l1, l2, p):
 
 
 def svg_arc(old, new, center, clockwise):
-    """ Format an SVG circular arc "A" path data entry given an arc in Gerber notation (i.e. with center relative to
-    first point).
+    """ Format an SVG circular arc "A" path data entry given an arc in Gerber notation (but with center in absolute
+    coordinates).
 
     :rtype: str
     """
-    r = float(math.hypot(*center))
+    r = float(math.dist(old, center))
     # invert sweep flag since the svg y axis is mirrored
     sweep_flag = int(not clockwise)
     # In the degenerate case where old == new, we always take the long way around. To represent this "full-circle arc"
     # in SVG, we have to split it into two.
     if math.isclose(math.dist(old, new), 0):
-        intermediate = old[0] + 2*center[0], old[1] + 2*center[1]
+        intermediate = old[0] + 2*(center[0]-old[0]), old[1] + 2*(center[1]-old[1])
         # Note that we have to preserve the sweep flag to avoid causing self-intersections by flipping the direction of
         # a circular cutin
         return f'A {r:.6} {r:.6} 0 1 {sweep_flag} {float(intermediate[0]):.6} {float(intermediate[1]):.6} ' +\
                f'A {r:.6} {r:.6} 0 1 {sweep_flag} {float(new[0]):.6} {float(new[1]):.6}'
 
     else: # normal case
-        d = point_line_distance(old, new, (old[0]+center[0], old[1]+center[1]))
+        d = point_line_distance(old, new, center[0], center[1])
         large_arc = int((d < 0) == clockwise)
         return f'A {r:.6} {r:.6} 0 {large_arc} {sweep_flag} {float(new[0]):.6} {float(new[1]):.6}'