[GRASS-user] r.thin quirk

[Dwight Needels]

On Jun 11, 2009, at 12:01 AM, Dwight Needels wrote:

> On Jun 6, 2009, at 11:36 PM, Dwight Needels wrote:
>
>> Hi all. I have run into a quirk using r.thin, but I am not sure  
>> whether or not it is a bug. I have a raster that was generated from  
>> multiple GPS tracks using v.rast followed by r.buffer. Using r.thin  
>> followed by r.to.vect creates a vector that must then be cleaned  
>> using v.clean to remove dangles (so far, so good).
>>
>> The quirk is that r.thin occasionally creates a triangle at the  
>> intersection of two lines instead of two intersecting lines. All of  
>> the examples I have seen involve relatively acute angles (less than  
>> 25 degrees??). I have attached a screenshot that shows the raster  
>> in magenta, the vector with dangles in white, and the cleaned  
>> vector as (a thinner) green. There are numerous acute angles that  
>> generate the vectors I would expect, with one exception in the  
>> center.
>>
>> Is this expected behavior? If so, is there an easy way to search  
>> for all such resulting triangles in a vector or to remove all such  
>> extra lines in a vector? If not, is there a way to fix it?
>>
>> Thanks, -Dwight
>>
>> <r.thin.png>
>> _______________________________________________
>> grass-user mailing list
>> grass-user at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/grass-user
>
> Although I figured out how to file a bug report on this, http://trac.osgeo.org/grass/ticket/636 
> , I am not sure how to respond directly to comments on the ticket.
>
> Hamish, I used the screenshot showing the vectors because I had  
> already created it to look for artifacts. The extra line that makes  
> the triangle is also present in the thinned raster (screenshot  
> attached).
>
> Thanks, -Dwight
>
> <r.thin_raster.png>

Follow-up... the two examples of this happening in this particular  
file each have a single white pixel surrounded on 7 out of 8  
directions by red pixels, but still touching a white pixel on a corner  
in the 8th direction (see two screenshots). I suspect that r.thin is  
treating this as a hole even though it is touching an edge. Another  
way of saying this is that the two red pixels touching in 1 out of the  
8 directions are treated as a line.

Perhaps there could be a flag to control this behavior.

Thanks, -Dwight

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