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<h3 class="section" id="Plotting-on-top-of-Images-1"><span>32.4 Plotting on top of Images<a class="copiable-link" href="#Plotting-on-top-of-Images-1"> ¶</a></span></h3>
<p>If gnuplot is being used to display images it is possible to plot on
top of images. Since an image is a matrix it is indexed by row and
column values. The plotting system is, however, based on the
traditional <em class="math">(x, y)</em> system. To minimize the difference between
the two systems Octave places the origin of the coordinate system in
the point corresponding to the pixel at <em class="math">(1, 1)</em>. So, to plot
points given by row and column values on top of an image, one should
simply call <code class="code">plot</code> with the column values as the first argument
and the row values as the second. As an example the following code
generates an image with random intensities between 0 and 1, and shows
the image with red circles over pixels with an intensity above
<em class="math">0.99</em>.
</p>
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<div class="group"><pre class="example-preformatted">I = rand (100, 100);
[row, col] = find (I > 0.99);
hold ("on");
imshow (I);
plot (col, row, "ro");
hold ("off");
</pre></div></div>
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