self intersecting polygons

[Arvin-huang1]

Hello, I used this program to perform simple polygon Union and or Difference operations. The produced results have self intersecting polygons, and the result of self intersecting polygons were Union operations again, becoming simple polygons. Why is this?

[philstopford]

Would you be able to share some example code that highlights what you are asking about? Without that, it's difficult to understand or reply in context.

[Arvin-huang1]

thank for your reply,
this is example
`
void test() {
Paths64 subj = { MakePath("(0,0),(9,0),(9,12),(0,12)"),
MakePath("(0,2),(9,2),(9,10),(0,10)") };

Paths64 clip = { MakePath("(0,3),(9,3),(9,4),(0,4)"),
				 MakePath("(0,8),(9,8),(9,9),(0,9)"),
				 MakePath("(2,3),(3,3),(3,9),(2,9)"),
				 MakePath("(6,3),(7,3),(7,9),(6,9)") };

Paths64 sol = Difference(subj, clip, FillRule::NonZero);
std::cout << sol;

SvgWriter svgSrc,svgDst;
SvgAddSolution(svgSrc, subj, FillRule::NonZero, true);
SvgAddSolution(svgSrc, clip, FillRule::NonZero, true);
SvgSaveToFile(svgSrc, "svgSrc.svg", 800, 600, 10);

SvgAddSolution(svgDst, sol, FillRule::NonZero, true);
SvgSaveToFile(svgDst, "svgDst.svg", 800, 600, 10);

}
`

2 subject rectangle ABCD，EFGH
4 clip rectangle IJKL,MNOP,QRST,UVWX
the difference operation result sol is 3 path, ABCD, EFGHIJKLMNOP, QRST,
the result why not 5 sub rectangle ABJK, PECD, HIFG, QRST, LMNO?

thank you.

[philstopford]

This looks to be an edge case for the touching notes in the documentation here : http://www.angusj.com/clipper2/Docs/Overview.htm#touching

If .I extend the horizontal cutters by 1 unit each side, the solution seems to be clean. If I reverse the arrays for the paths, so that X <-> Y, the edges are also clean.

I did find that running a Union of the subj and clip before the clipping operation helped clean things up :

    subj = Clipper.Union(subj, FillRule.NonZero);
    clip = Clipper.Union(clip, FillRule.NonZero);