[GRASSLIST:3518] [r.thin] doesn't seem to converge to zero

[kuszinger at giscom.hu]

Hello! 

I'm Robert and new to the list. I'm using grass (5pre3)
for two days. I used ARC/INFO, ERDAS, Mapinfo an ER Mapper before,
however I liked ARC/INFO the most. But now I found that GRASS
is somehow similar to the ARC/INFO unix versions I used a lot.
(The new ARC/NOF is annoying, slow, etc.) 

While I was experimenting I tried to generate countour line
vectors based on a paper map sheet (Hungarian National Survey,
Topography 1:100000). 

With color manipulations in GIMP I extracted the contour lines
on the scanned sheet.
I converted to B/W and imported into GRASS. 

(image size is 6200 x 4000 or so) 

When I ran <r.thin> the number of removed points was
something like this (approx.): 

1.st path
825000
2.
340000
3.
124000
4.
678000
5.
38700
6.
36200
7.
35788
8.
35788 (!)
9.
35787 

..... At this point I killed the process.
It took so much time and it did not seem to cenverge anymore. 

Is it a  bug? Or is the drawing 'too' complicated? 

What could be the problem?
Backgound is value is 0 and lines have value 1,
very dense lines usually 2-4 pixels wide. 

(Finally I'm planning to convert to vector,
assign height labels and generate a DTM
finally) 

Any help welcome 

thank you 

Robert