Geometry Boost List: [geometry] get_turns L/L, L/A and is_collinear

Subject: [geometry] get_turns L/L, L/A and is_collinear
From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2014-05-05 05:54:02

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 non-spike 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?

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