avoid gaps at the end

lib/elements/satin_column.py

@@ -613,7 +613,7 @@ class SatinColumn(EmbroideryElement):
         index1 = 0
         last_index1 = len(paths[1]) - 1
 
-        while index0 < last_index0 and index1 < last_index1:
+        while index0 < last_index0 or index1 < last_index1:
             add_pair(pos0, pos1)
 
             old_pos0 = pos0
@@ -625,47 +625,50 @@ class SatinColumn(EmbroideryElement):
             pos0, index0 = self.walk(paths[0], pos0, index0, spacing0)
             pos1, index1 = self.walk(paths[1], pos1, index1, spacing1)
 
-            if pos0 == old_pos0 or pos1 == old_pos1:
+            try:
+                # Adjust for rails that contract or expand from each other.
+                # Without any compensation, rail sections that spread out or come
+                # together are longer than parallel rails, and we'll plot stitches
+                # too densely as a result.  We can compensate by using some trig,
+                # as described here:
+                #
+                # https://github.com/inkstitch/inkstitch/issues/379#issuecomment-467262685
+                stitch_direction = (pos1 - pos0).unit()
+                peak_to_peak0 = pos0 - old_pos0
+                peak_to_peak1 = pos1 - old_pos1
+
+                # The dot product of two unit vectors is the cosine of the angle
+                # between them.  We want the cosine of the angle minus 90 degrees,
+                # so we rotate left by 90 degrees first.
+                #
+                # We take the absolute value to correct for the different direction
+                # of the angles on the opposing rails.
+                cos1 = abs(peak_to_peak0.unit() * stitch_direction.rotate_left())
+                cos2 = abs(peak_to_peak1.unit() * stitch_direction.rotate_left())
+
+                # Use the smaller of the two angles to avoid spacing out
+                # too far on the other rail.  Note that the cosine of 0
+                # is 1, so we use min here to mean a bigger angle.
+                cos = min(cos1, cos2)
+
+                # Beyond 0.55 (about 56 degrees), we end up distorting the
+                # stitching and it looks bad.
+                cos = max(cos, 0.55)
+
+                pos0, index0 = self.walk(paths[0], pos0, index0, spacing0 / cos - spacing0)
+                pos1, index1 = self.walk(paths[1], pos1, index1, spacing1 / cos - spacing1)
+            except ZeroDivisionError:
+                # These can occur in unit() if the vector has a length of zero,
+                # which can happen in certain cases.  We'll just skip the
+                # compensation.
                 continue
 
-            # Adjust for rails that contract or expand from each other.
-            # Without any compensation, rail sections that spread out or come
-            # together are longer than parallel rails, and we'll plot stitches
-            # too densely as a result.  We can compensate by using some trig,
-            # as described here:
-            #
-            # https://github.com/inkstitch/inkstitch/issues/379#issuecomment-467262685
-            stitch_direction = (pos1 - pos0).unit()
-            peak_to_peak0 = pos0 - old_pos0
-            peak_to_peak1 = pos1 - old_pos1
-
-            # The dot product of two unit vectors is the cosine of the angle
-            # between them.  We want the cosine of the angle minus 90 degrees,
-            # so we rotate left by 90 degrees first.
-            #
-            # We take the absolute value to correct for the different direction
-            # of the angles on the opposing rails.
-            cos1 = abs(peak_to_peak0.unit() * stitch_direction.rotate_left())
-            cos2 = abs(peak_to_peak1.unit() * stitch_direction.rotate_left())
-
-            # Use the smaller of the two angles to avoid spacing out
-            # too far on the other rail.  Note that the cosine of 0
-            # is 1, so we use min here to mean a bigger angle.
-            cos = min(cos1, cos2)
-
-            # Beyond 0.55 (about 56 degrees), we end up distorting the
-            # stitching and it looks bad.
-            cos = max(cos, 0.55)
-
-            pos0, index0 = self.walk(paths[0], pos0, index0, spacing0 / cos - spacing0)
-            pos1, index1 = self.walk(paths[1], pos1, index1, spacing1 / cos - spacing1)
-
         # We're off by one in the algorithm above, so we need one more
         # pair of points.  We'd like to add points at the very end to
         # make sure we match the vectors on screen as best as possible,
         # but we avoid doing so if the stitches will stack up too closely.
 
-        if (pos0 - old_pos0).length() > 0.1 * spacing or \
+        if (pos0 - old_pos0).length() > 0.1 * spacing and \
            (pos1 - old_pos1).length() > 0.1 * spacing:
             add_pair(pos0, pos1)