
Geometry : 
Subject: Re: [geometry] get_turns L/L, L/A and is_collinear
From: Menelaos Karavelas (menelaos.karavelas_at_[hidden])
Date: 20140505 07:54:38
Hi Adam.
On 05/05/2014 12:54 Î¼Î¼, Adam Wulkiewicz wrote:
> Hi,
>
> I added tests for is_collinear and fixed some errors yesterday. I also
> unified the value of is_collinear for the first endpoint, it's FALSE
> since there is no preceeding segment for the first endpoint.
>
> For spikes collinear to some segment 2 turns are generated for the
> spike point, one operation_block and one operation_intersection, both
> has is_colinear set to TRUE (in the nonspike IP, if a turn has
> operation_intersection then is_collinear wil be FALSE because the
> Linestring is going in from the outside of the second Linestring).
>
> An interesting edge case is the intersection of 2 equal spikes. e.g. for:
>
> "MULTILINESTRING((0 0,1 1,0 0))",
> "MULTILINESTRING((0 0,1 1,0 0))"
>
> those turns will be created:
>
> ("tii++")("tix+=")("txi=+")("txx==")("ecc==")
>
> but for the opposite case (2 spikes of the same length and in the same
> place but opposite)
>
> "MULTILINESTRING((0 0,1 1,0 0))",
> "MULTILINESTRING((1 1,0 0,1 1))"
>
> those will be generated:
>
> ("tix+=")("tii+=")("txx==")("txi==")
> ("txi=+")("tii=+")("txx==")("tix=="))
>
> because for each spike (duplicated) there are 2 intersecting segments
> and we have 2 ends of which gives 2*2*2=8 turns.
>
> Are you ok with this behavior?
>
AFAIC, this behavior is fine with me (from the point of view of set
operations for linear geometries).
FYI, I just added these two examples in the unit tests for difference
and intersection of linear geometries.
 m.
> You may test additional cases adding them to the
> test/algorithms/overlay/get_turns_linear_linear.cpp
>
> FYI, each string defines the turn like this:
> <method><operation for G1><operation for G2><is_collinear for
> G1><is_collinear for G2>
>
> t  method_touch
> e  method_equal
> i  operation_intersection
> x  operation_blocked
> =  is_collinear == TRUE
> +  is_collinear == FALSE
>
> Regards,
> Adam
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