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<a name="Plotting-on-top-of-Images"></a>
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Next: <a href="Color-Conversion.html#Color-Conversion" accesskey="n" rel="next">Color Conversion</a>, Previous: <a href="Representing-Images.html#Representing-Images" accesskey="p" rel="prev">Representing Images</a>, Up: <a href="Image-Processing.html#Image-Processing" accesskey="u" rel="up">Image Processing</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Plotting-on-top-of-Images-1"></a>
<h3 class="section">32.4 Plotting on top of Images</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>(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>(1, 1)</em>. So, to plot
points given by row and column values on top of an image, one should
simply call <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>0.99</em>.
</p>
<div class="example">
<pre class="example">I = rand (100, 100);
[row, col] = find (I > 0.99);
hold ("on");
imshow (I);
plot (col, row, "ro");
hold ("off");
</pre></div>
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