Snapping leaves segments below tolerance

[strk]

Snapping a geometry to another should result in an output that does not move again on subsequent snapping to the same geometry. At least, I'd think such behaviour would be to be preferred, for stability.

In the following case it takes two iterations to reach that point:

WITH inp AS (                                                                   
 SELECT                                                                         
   0.001 as tol,                                                                
   'LINESTRING(599671.37338031 4889182.65265274,599671.369855744 4889982.62878372)'
    ::geometry as src,                                                          
   'LINESTRING(599671.37 4889664.32,599665.52 4889680.18,599671.37 4889683.4,599671.37 4889781.87)'
    ::geometry as tgt                                                           
)                                                                               
SELECT                                                                          
 ST_AsText(ST_Snap(src , tgt , tol)),                                           
 ST_AsText(ST_Snap(ST_Snap(src , tgt , tol), tgt, tol)),                        
 ST_AsText(ST_Snap(ST_Snap(ST_Snap(src , tgt , tol), tgt, tol), tgt, tol))
FROM inp;

[strk]

I shell note the problem is dependent on order of vertices in the target point set. Here's a simpler input:

src: LINESTRING(0 0,0 20)
tgt: LINESTRING(0.5 19,1 18,1.5 17)

Take the target in the given order and snapping is final after first iteration. Reverse the target and it takes 3 iterations to fully snap:

 SELECT
   0.8 as tol,
   
   'LINESTRING(0 0,0 20)'
    ::geometry as src,
   
   'LINESTRING(0.5 19,1 18,1.5 17)'
    ::geometry as tgt
)
SELECT
 ST_AsText(src) src,
 ST_AsText(ST_Snap(src , tgt , tol)) snap1,
 ST_AsText(ST_Snap(ST_Snap(src , tgt , tol), tgt, tol)) snap2,
 ST_AsText(ST_Snap(ST_Snap(ST_Snap(src , tgt , tol), tgt, tol), tgt, tol)) snap3,
 ST_AsText(ST_Snap(src , ST_Reverse(tgt) , tol)) snap1r,
 ST_AsText(ST_Snap(ST_Snap(src , ST_Reverse(tgt) , tol), ST_Reverse(tgt), tol)) snap2r,
 ST_AsText(ST_Snap(ST_Snap(ST_Snap(src , ST_Reverse(tgt) , tol), ST_Reverse(tgt), tol), ST_Reverse(tgt), tol)) snap3r,
 ST_Distance(src, ST_EndPoint(tgt)) dist_end,
 ST_Distance(src, ST_StartPoint(tgt)) dist_start,
 ST_Distance(src, ST_PointN(tgt,2)) dist_p2,
 ST_Distance(src, ST_PointN(tgt,3)) dist_p3
FROM inp;
-[ RECORD 1 ]---------------------------------------
src        | LINESTRING(0 0,0 20)
snap1      | LINESTRING(0 0,1.5 17,1 18,0.5 19,0 20)
snap2      | LINESTRING(0 0,1.5 17,1 18,0.5 19,0 20)
snap3      | LINESTRING(0 0,1.5 17,1 18,0.5 19,0 20)
snap1r     | LINESTRING(0 0,0.5 19,0 20)
snap2r     | LINESTRING(0 0,1 18,0.5 19,0 20)
snap3r     | LINESTRING(0 0,1.5 17,1 18,0.5 19,0 20)
dist_end   | 1.5
dist_start | 0.5
dist_p2    | 1
dist_p3    | 1.5

[strk]

Suggested fix: ​https://github.com/libgeos/libgeos/pull/59

[strk]

Ticket retargeted after milestone closed

[robe]

Retargeting in prep for GEOS 3.10.0 release