<html style="direction: ltr;">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<style id="bidiui-paragraph-margins" type="text/css">body p { margin-bottom: 0cm; margin-top: 0pt; } </style>
</head>
<body bidimailui-charset-is-forced="true" style="direction: ltr;">
<p><font size="4"><font face="Helvetica, Arial, sans-serif">AFAIK,
the Canny algorithm requires large, wide areas of black
pixels, adjacent to large wide areas of white pixels in order
to find the edge. It compares the change in value over a wide
"strip" to determine the edge.</font></font></p>
<p><font size="4"><font face="Helvetica, Arial, sans-serif">It's a
bit counter-intuitive, but the algorithm will not work on
single pixel width areas.</font></font></p>
<p><br>
</p>
<div class="moz-cite-prefix">On 24/07/2023 6:28, Venka wrote:<br>
</div>
<blockquote type="cite"
cite="mid:ed72e5bb-45d7-afde-e1d7-42d5be866f28@gmail.com">Hi All,
<br>
<br>
I have a question about Canny Edge Detection using i.edge
<br>
<br>
1) I produce a thinned binary image as below
<br>
<br>
r.thin --overwrite input=line_element output=thinned_line_element
<br>
<br>
a) the "line_element" represents valleys that are, at places, few
pixel
<br>
wide
<br>
<br>
b) the output "thinned_line_element" represents valley lines that
are
<br>
a single pixel wide
<br>
<br>
2) I run the Canny edge detector on "thinned_line_element" as
below
<br>
i.edge --overwrite input=thin_line_element output=edge_map
angles_map=angle_map
<br>
<br>
<br>
The resultant "edge_map" produces a monotone (no edges) edge map
and the "angle_map" outputs the angles
<br>
correctly
<br>
<br>
3) Running the Canny edge detector on the un-thinned
"line_element"
<br>
<br>
i.edge --overwrite input=line_element@PERMANENT output=edge_map1
angles_map=angle_map1
<br>
<br>
produces both "edge_map1" and "angles_map1" correctly.
<br>
<br>
Any idea why the Canny edge detector does not produce edge map on
thinned image?
<br>
<br>
Kind regards,
<br>
<br>
Venka
<br>
_______________________________________________
<br>
grass-user mailing list
<br>
<a class="moz-txt-link-abbreviated" href="mailto:grass-user@lists.osgeo.org">grass-user@lists.osgeo.org</a>
<br>
<a class="moz-txt-link-freetext" href="https://lists.osgeo.org/mailman/listinfo/grass-user">https://lists.osgeo.org/mailman/listinfo/grass-user</a>
<br>
</blockquote>
<pre class="moz-signature" cols="72">--
Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918</pre>
</body>
</html>