[GRASS-user] Running canny edge detection on thinned binary image

[Venka]

Hi Micha,

Thanks for your response.

Is there any other way to generate edge_map and angle_map
from single pixel width areas?

Venka

On 7/24/2023 6:23 PM, Micha Silver wrote:
> 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.
> 
> It's a bit counter-intuitive, but the algorithm will not work on single pixel
> width areas.
> 
> 
> On 24/07/2023 6:28, Venka wrote:
>> Hi All,
>>
>> I have a question about Canny Edge Detection using i.edge
>>
>> 1) I produce a thinned binary image as below
>>
>> r.thin --overwrite input=line_element output=thinned_line_element
>>
>> a) the "line_element" represents valleys that are, at places, few pixel
>> wide
>>
>> b) the output "thinned_line_element" represents valley lines that are
>> a single pixel wide
>>
>> 2) I run the Canny edge detector on "thinned_line_element" as below
>> i.edge --overwrite input=thin_line_element output=edge_map angles_map=angle_map
>>
>>
>> The resultant "edge_map" produces a monotone (no edges) edge map and the 
>> "angle_map" outputs the angles
>> correctly
>>
>> 3) Running the Canny edge detector on the un-thinned "line_element"
>>
>> i.edge --overwrite input=line_element at PERMANENT output=edge_map1 
>> angles_map=angle_map1
>>
>> produces both "edge_map1" and "angles_map1" correctly.
>>
>> Any idea why the Canny edge detector does not produce edge map on thinned image?
>>
>> Kind regards,
>>
>> Venka
>> _______________________________________________
>> grass-user mailing list
>> grass-user at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/grass-user
> 
> -- 
> Micha Silver
> Ben Gurion Univ.
> Sde Boker, Remote Sensing Lab
> cell: +972-523-665918
>