<!-- The input is the "SourceGraphic image within the filter region"
         so the output should be a 200x200 two-shaded rectangle.
    -->
    <filter id="f1" filterUnits="userSpaceOnUse"
            x="50" y="50" width="200" height="200">
      <feConvolveMatrix kernelMatrix="1 1 1  1 1 1  1 1 1"
                        x="50" y="50" width="200" height="200"
                        preserveAlpha="false" bias=".5" />
    </filter>
  </defs>

  <rect x="100" y="100" width="100" height="100" fill="blue"
  