private static CoordinateSequence fill(double buffer) {
    double min = -256d * buffer;
    double max = 256d - min;
    return new PackedCoordinateSequence.Double(new double[]{
      min, min,
      max, min,
      max, max,
      min, max,
      min, min
    }, 2, 0);
  }
Adds a new point to {@code out} where the line segment from (ax,ay) to (bx,by) crosses a horizontal line at y=y.
planetiler-core/src/main/java/com/onthegomap/planetiler/render/TiledGeometry.java