 example, the
    lower left pixel of a bitmap is delimited by the square (0,0)-(1,1), its
    center being at location (0.5,0.5).</p>

    <p>This introduces some differences when computing distances.  For
    example, the <em>length</em> in pixels of the line [0,0]-[10,0] is 11.
    However, the vectorial distance between (0,0)-(10,0) covers exactly
    10&nbsp;pixel centers, hence its length is&nbsp;10.</p>

    <center>
      <img src="grid_1.png"
           height=390 width=402
           alt="bitmap a