#12 Union of polygons with colinear edges (2)

[Anonymous]

Hi Timo,

Thank you for your response at the ticket 11. I tried to continue there the discussion without succeding so I'm opening a new ticket.

I tried the new release you sent me and I noticed that you solved the problem I reported you. However, a new problem that did not occoured before your fix, it occours now.

Consider this code:

test = [[{"X":393750,"Y":1134375},{"X":403125,"Y":1134375},{"X":403125,"Y":1143750},{"X":393750,"Y":1143750}],[{"X":393750,"Y":1143750},{"X":412500,"Y":1143750},{"X":412500,"Y":1162500},{"X":393750,"Y":1162500}],[{"X":375000,"Y":1143750},{"X":393750,"Y":1143750},{"X":393750,"Y":1162500},{"X":375000,"Y":1162500}],[{"X":375000,"Y":1162500},{"X":412500,"Y":1162500},{"X":412500,"Y":1200000},{"X":375000,"Y":1200000}],[{"X":412500,"Y":1162500},{"X":450000,"Y":1162500},{"X":450000,"Y":1200000},{"X":412500,"Y":1200000}],[{"X":412500,"Y":1125000},{"X":450000,"Y":1125000},{"X":450000,"Y":1162500},{"X":412500,"Y":1162500}],[{"X":412500,"Y":1087500},{"X":450000,"Y":1087500},{"X":450000,"Y":1125000},{"X":412500,"Y":1125000}],[{"X":375000,"Y":1087500},{"X":412500,"Y":1087500},{"X":412500,"Y":1125000},{"X":375000,"Y":1125000}],[{"X":375000,"Y":1050000},{"X":412500,"Y":1050000},{"X":412500,"Y":1087500},{"X":375000,"Y":1087500}],[{"X":412500,"Y":1050000},{"X":431250,"Y":1050000},{"X":431250,"Y":1068750},{"X":412500,"Y":1068750}]]

cpr = new ClipperLib.Clipper()
paths = new ClipperLib.Paths()
clipper_union = new ClipperLib.Paths()

cpr.AddPaths(test, ClipperLib.PolyType.ptSubject, true)
succeeded = cpr.Execute(ClipperLib.ClipType.ctUnion, clipper_union)

console.log clipper_union

You can find it here.

Thank you for your help!
Fabio

[Timo Kähkönen]

This has to be some bug in original C# Clipper. I tested in the last stable C# Clipper version 6.2.1 and the solution was the same as in JS Clipper 6.2.1.0:

C# Console Demo, solution.txt:
------------------------------
2
4
403125, 1143750
393750, 1143750
393750, 1134375
403125, 1134375
12
450000, 1087500
450000, 1200000
375000, 1200000
375000, 1143750
412500, 1143750
412500, 1125000
375000, 1125000
375000, 1050000
431250, 1050000
431250, 1068750
412500, 1068750
412500, 1087500

The mysubj.txt was:
-------------------
10
4
393750,1134375,
403125,1134375,
403125,1143750,
393750,1143750
4
393750,1143750,
412500,1143750,
412500,1162500,
393750,1162500
4
375000,1143750,
393750,1143750,
393750,1162500,
375000,1162500
4
375000,1162500,
412500,1162500,
412500,1200000,
375000,1200000
4
412500,1162500,
450000,1162500,
450000,1200000,
412500,1200000
4
412500,1125000,
450000,1125000,
450000,1162500,
412500,1162500
4
412500,1087500,
450000,1087500,
450000,1125000,
412500,1125000
4
375000,1087500,
412500,1087500,
412500,1125000,
375000,1125000
4
375000,1050000,
412500,1050000,
412500,1087500,
375000,1087500
4
412500,1050000,
431250,1050000,
431250,1068750,
412500,1068750

The command was:
ConsoleDemo.exe UNION mysubj.txt mysubj.txt 0 0

[Timo Kähkönen]

Because current stable C# Clipper is not perfect in common line joining, you can use an offset trick. The idea is to "bolden" every polygon a bit before UNION operation. Polygons can be boldened using Clipper's Offsetting feature. The offset delta (offset amount) is so small, that it does not affect to the final coordinates.

This like case is trivial, but think about a case where you rotate the polygonal shape (using double coordinates) and make then an UNION operation. Without boldening trick it is sure that all common lines are not joined (while some will be). The boldening ensures that you get an expected result in all cases (all cases = I presume so).

The downside is performance decrease, but if you must be sure, then there are not much choices.

Please check this little demo about the offsetting trick:

http://output.jsbin.com/sowiju

[Timo Kähkönen]

This is fixed in upcoming 6.4.2.1.