docs: public double relativeFill() {
      double acc = 0.0;
      final int cols = widths.size();
      final int rows = heights.size();
      {
        for(int y = 0; y < rows; y++) {
          for(int x = 0; x < cols; x++) {
            if(usage.get(y).get(x) != null) {
              acc += widths.get(x) * heights.get(y);
            }
          }
        }
      }
      return acc / (twidth * theight);
    }