[GRASS-dev] Re: [GRASS-user] v.clean tool=prune - 'thresh' interpreted wrong?

[Markus Neteler]

Moritz Lennert wrote on 10/03/2006 06:26 PM:
> Maciej Sieczka wrote:
>> Moritz Lennert wrote:
>>
>>> http://freegis.org/cgi-bin/viewcvs.cgi/grass6/lib/vector/diglib/prune.c?rev=HEAD&content-type=text/vnd.viewcvs-markup
>>>
>>>
>>> There is an explanation of the algorithm at the beginning.
>>
>> Thanks, I should have looked into the source code first. My only (poor)
>> excuse is being a non-programmer and "mentally handicapped" due to my
>> disgraceful pre-Linux/FOSS times ;).
>>
>> Moritz suggested that maybe "the threshold only applies in cases of
>> change of direction (i.e. angles)". Please note there is no any
>> reference to angles or direction in the code or comments. It seems the
>> threshold refers to distance only.
>>
>> There is a following comment:
>>
>> * thresh - the distance that a string must wander from a straight
>> *          line before another point is selected.
>>
>> As I understand it, this means that if my vertcices are on a straight
>> line, at intervals of 10, only a thresh of 10 or more should prune
>> them. But, as you can see in my original example, any thresh>=0.03
>> prunes them. Do I missunderstand sthing (I'm not sure what the
>> "straight line" means here)?
>>
>> So we have 2 possibilities:
>>
>> 1. If in opposite to my interpretation of the source and manual,
>> angles/directions DO have something to do with the pruning thresh,
>> please somebody knowledgeable tell me what is that relation, so I can
>> fix the part of the manual, that reads "prune: remove vertices in
>> threshold from lines (...)".
>>
>> 2. If the pruning thresh doesn't depend on angles/directions, how do I
>> understand the fact that any thresh>=0.03 prunes points which are at
>> intervals of 10 on a straight line?
>>
>
> To plead in Markus' direction: there is a 3rd possibility:
>
> 3. Forget about the current pruning implementation and push the
> integration of another algorithm  (Douglas-Peucker). (I know you are
> working on a script, but could this wait until this is done ? Maybe if
> you push hard enough someone will "just do it" - Dylan mentioned it as
> a potentially "fun" project:
> http://grass.itc.it/pipermail/grassuser/2006-August/035529.html ;-)
> and actually links to an implementation in c++, which actually looks
> like more or less usable C to me).
>
> Markus you mentioned the source code on the GMT site. Could you be
> more precise on where to find it ?
Sure:
http://www.ngdc.noaa.gov/mgg/shorelines/data/gshhs/version1.3/
-> gshhs_1.5_src.tbz
   -> gshhs/gshhs_dp.c

Markus

PS: Let's reduce CC'ing to the DEV list to not spam hundreds of users
with this (now very detailed) thread.