Correctly find the center for single quadrant arcs

gerber/rs274x.py

@@ -21,6 +21,7 @@
 import copy
 import json
 import re
+import sys
 
 try:
     from cStringIO import StringIO
@@ -30,6 +31,7 @@ except(ImportError):
 from .gerber_statements import *
 from .primitives import *
 from .cam import CamFile, FileSettings
+from .utils import sq_distance
 
 
 def read(filename):
@@ -548,7 +550,7 @@ class GerberParser(object):
             else:
                 i = 0 if stmt.i is None else stmt.i
                 j = 0 if stmt.j is None else stmt.j
-                center = (start[0] + i, start[1] + j)
+                center = self._find_center(start, end, (i, j))
                 if self.region_mode == 'off':
                     self.primitives.append(Arc(start, end, center, self.direction, self.apertures[self.aperture], quadrant_mode=self.quadrant_mode, level_polarity=self.level_polarity, units=self.settings.units))
                 else:
@@ -579,6 +581,35 @@ class GerberParser(object):
                     self.primitives.append(primitive)
 
         self.x, self.y = x, y
+        
+    def _find_center(self, start, end, offsets):
+        """
+        In single quadrant mode, the offsets are always positive, which means there are 4 possible centers.
+        The correct center is the only one that results in an arc with sweep angle of less than or equal to 90 degrees
+        """
+        
+        if self.quadrant_mode == 'single-quadrant':
+            
+            # The Gerber spec says single quadrant only has one possible center, and you can detect 
+            # based on the angle. But for real files, this seems to work better - there is usually
+            # only one option that makes sense for the center (since the distance should be the same
+            # from start and end). Find the center that makes the most sense
+            sqdist_diff_min = sys.maxint
+            center = None
+            for factors in [(1, 1), (1, -1), (-1, 1), (-1, -1)]:
+                
+                test_center = (start[0] + offsets[0] * factors[0], start[1] + offsets[1] * factors[1])
+                
+                sqdist_start = sq_distance(start, test_center)
+                sqdist_end = sq_distance(end, test_center)
+                
+                if abs(sqdist_start - sqdist_end) < sqdist_diff_min:
+                    center = test_center
+                    sqdist_diff_min = abs(sqdist_start - sqdist_end)
+                
+            return center
+        else:
+            return (start[0] + offsets[0], start[1] + offsets[1])
 
     def _evaluate_aperture(self, stmt):
         self.aperture = stmt.d

gerber/utils.py

@@ -297,3 +297,9 @@ def nearly_equal(point1, point2, ndigits = 6):
     '''Are the points nearly equal'''
     
     return round(point1[0] - point2[0], ndigits) == 0 and round(point1[1] - point2[1], ndigits) == 0
+
+def sq_distance(point1, point2):
+    
+    diff1 = point1[0] - point2[0]
+    diff2 = point1[1] - point2[1]
+    return diff1 * diff1 + diff2 * diff2