at moves along a column, as
<code class="docutils literal"><span class="pre">memory_based_step_iterator&lt;XIterator&gt;</span></code> with a step equal to the number of
memory units (bytes or bits) per row. Again, one is free to provide their own
y-iterator.</p>
<p>Then we can instantiate
<code class="docutils literal"><span class="pre">memory_based_2d_locator&lt;memory_based_step_iterator&lt;XIterator&gt;</span> <span class="pre">&gt;</span></code> to obtain
a 2D pixel locator, as the diagram indicates:</