+ Real multiplier = feather_dir ? placements[LEFT] : 1.0;
+
+ Interval weights (1 - multiplier, multiplier);
+
+ if (feather_dir != LEFT)
+ weights.swap ();
+
+ // we need two translations: the normal one and
+ // the one of the lowest segment
+ int idx[] = {i, extreme};
+ Real translations[2];
+
+ for (int j = 0; j < 2; j++)
+ translations[j] = slope
+ * (segments[idx[j]].horizontal_[LEFT] - span.linear_combination (CENTER))
+ + pos.linear_combination (CENTER)
+ + beam_dy * segments[idx[j]].vertical_count_;
+
+ Real weighted_average = translations[0] * weights[LEFT] + translations[1] * weights[RIGHT];
+
+ /*
+ Tricky. The manipulation of the variable `weighted_average' below ensures
+ that beams with a RIGHT grow direction will start from the position of the
+ lowest segment at 0, and this error will decrease and decrease over the
+ course of the beam. Something with a LEFT grow direction, on the other
+ hand, will always start in the correct place but progressively accrue
+ error at broken places. This code shifts beams up given where they are
+ in the total span length (controlled by the variable `multiplier'). To
+ better understand what it does, try commenting it out: you'll see that
+ all of the RIGHT growing beams immediately start too low and get better
+ over line breaks, whereas all of the LEFT growing beams start just right
+ and get worse over line breaks.
+ */
+ Real factor = Interval (multiplier, 1 - multiplier).linear_combination (feather_dir);
+
+ if (segments[0].vertical_count_ < 0 && feather_dir)
+ weighted_average += beam_dy * (segments.size () - 1) * factor;
+
+ b.translate_axis (weighted_average, Y_AXIS);