y edge detector</a>.</li>
  <li><strong>max_height</strong>: the maximum height for a letter.</li>
  <li><strong>min_height</strong>: the minimum height for a letter.</li>
  <li><strong>aspect_ratio</strong>: the maximum aspect ratio for a letter.</li>
  <li><strong>variance_ratio</strong>: the inner-class variance when grouping letters.</li>
  <li><strong>thickness_ratio</strong>: the allowable thickness variance when grouping letters.</li>
  <li><strong>height_ratio</strong>: the allowable height variance