Clipping open subject creates a lot of single-point polylines about clipper2

[AngusJohnson]

Question: I understand that it is garbage input, but is there any reason to generate single-point output polys like this? Is this intended?

I suspect this is because your clipping path isn't as straight as (I'm guessing) you think it is.

[olologin]

@AngusJohnson
Sorry, I don't understand what you mean by "Straight". Do you mean SUBJECTS_OPEN is not exact subpoly of CLIPS? I have no idea how it is possible.
In OpenClip.txt you can search for whole sequence of points from SUBJECTS_OPEN, it will be found 2 times, once in SUBJECTS_OPEN and once as subsequence of points in CLIPS.

[AngusJohnson]

Sorry, I don't understand what you mean by "Straight".

What I mean is the edges of the clip path and subject path where they're almost touching aren't straight (see the image below). So while the subject path outlines a seemingly single very long single subject 'line', this subject polygon is actually broken into numerous polygons (ie the outlines of numerous line segments). So even if you just Unioned this subject, you'll still get multiple paths in the clipping solution.

[olologin]

Yes, I am fully aware that my clip and subject polylines are not single straight lines and contain a lot of noise.

I spent some time on minimizing inputs, still I need ~600 points clipping poly to reproduce this behavior, but at least now I can fit it in test like this, cannot reduce it further:

TEST(Clipper2Tests, TestOpenClipProducesSinglePoints1)
{
	Clipper2Lib::ClipType ct = Clipper2Lib::ClipType::Intersection;
	Clipper2Lib::FillRule fr = Clipper2Lib::FillRule::EvenOdd;
	int64_t stored_area = 0, stored_count = 0;

	Clipper2Lib::Paths64 solution, solution_open;
	Clipper2Lib::Clipper64 c;
	Clipper2Lib::Paths64 subject;
	Clipper2Lib::Paths64 subject_open = { {{561987188, 277759647}, {561989245, 277759632}, {561991269, 277760004}, {561993309, 277760183}, {561997389, 277760540}, {562005550, 277761254}, {562021872, 277762682}, {562054515, 277765538}, {562119802, 277771250}, {562250375, 277782673}, {562511521, 277805521}, {563033814, 277851215}, {563556107, 277896910}, {563817254, 277919757}, {563947827, 277931181}, {564013114, 277936893}, {564045757, 277939749}, {564062078, 277941177}, {564070239, 277941891}, {564074320, 277942248}, {564076360, 277942426}, {564078417, 277942411}} };
	Clipper2Lib::Paths64 clip = { {{639288598, 284522449}, {689987154, 0}, {0, 228592139}, {2057, 228592124}, {2089172, 228774918}, {2091229, 228774903}, {4178343, 228957697}, {4180401, 228957682}, {6267515, 229140476}, {6269572, 229140461}, {8356687, 229323255}, {8358744, 229323239}, {10445858, 229506034}, {10447916, 229506018}, {12535030, 229688812}, {12537087, 229688797}, {14624202, 229871591}, {14626259, 229871576}, {16713374, 230054370}, {16715431, 230054355}, {18802545, 230237149}, {18804602, 230237134}, {20891717, 230419928}, {20893774, 230419912}, {22980889, 230602707}, {22982946, 230602691}, {25070060, 230785485}, {25072118, 230785470}, {27159232, 230968264}, {27161289, 230968249}, {29248404, 231151043}, {29250461, 231151028}, {31337576, 231333822}, {31339633, 231333807}, {33426747, 231516601}, {33428804, 231516586}, {35515919, 231699380}, {35517976, 231699364}, {37605091, 231882158}, {37607148, 231882143}, {39694262, 232064937}, {39696320, 232064922}, {41783434, 232247716}, {41785491, 232247701}, {43872606, 232430495}, {43874663, 232430480}, {45961777, 232613274}, {45963835, 232613259}, {48050949, 232796053}, {48053006, 232796037}, {50140121, 232978832}, {50142178, 232978816}, {52229293, 233161610}, {52231350, 233161595}, {54318464, 233344389}, {54320521, 233344374}, {56407636, 233527168}, {56409693, 233527153}, {58496808, 233709947}, {58498865, 233709932}, {60585979, 233892726}, {60588037, 233892710}, {62675151, 234075505}, {62677208, 234075489}, {64764323, 234258283}, {64766380, 234258268}, {66853494, 234441062}, {66855552, 234441047}, {68942666, 234623841}, {68944723, 234623826}, {71031838, 234806620}, {71033895, 234806605}, {73121010, 234989399}, {73123067, 234989383}, {75210181, 235172178}, {75212238, 235172162}, {77299353, 235354956}, {77301410, 235354941}, {79388525, 235537735}, {79390582, 235537720}, {81477696, 235720514}, {81479754, 235720499}, {83566868, 235903293}, {83568925, 235903278}, {85656040, 236086072}, {85658097, 236086057}, {87745212, 236268851}, {87747269, 236268835}, {89834383, 236451629}, {89836440, 236451614}, {91923555, 236634408}, {91925612, 236634393}, {94012727, 236817187}, {94014784, 236817172}, {96101898, 236999966}, {96103956, 236999951}, {98191070, 237182745}, {98193127, 237182730}, {100280242, 237365524}, {100282299, 237365508}, {102369413, 237548303}, {102371471, 237548287}, {104458585, 237731081}, {104460642, 237731066}, {106547757, 237913860}, {106549814, 237913845}, {108636929, 238096639}, {108638986, 238096624}, {110726100, 238279418}, {110728157, 238279403}, {112815272, 238462197}, {112817329, 238462181}, {114904444, 238644976}, {114906501, 238644960}, {116993615, 238827754}, {116995673, 238827739}, {119082787, 239010533}, {119084844, 239010518}, {121171959, 239193312}, {121174016, 239193297}, {123261130, 239376091}, {123263188, 239376076}, {125350302, 239558870}, {125352359, 239558854}, {127439474, 239741649}, {127441531, 239741633}, {129528646, 239924427}, {129530703, 239924412}, {131617817, 240107206}, {131619874, 240107191}, {133706989, 240289985}, {133709046, 240289970}, {135796161, 240472764}, {135798218, 240472749}, {137885332, 240655543}, {137887390, 240655528}, {139974504, 240838322}, {139976561, 240838306}, {142063676, 241021100}, {142065733, 241021085}, {144152848, 241203879}, {144154905, 241203864}, {146242019, 241386658}, {146244076, 241386643}, {148331191, 241569437}, {148333248, 241569422}, {150420363, 241752216}, {150422420, 241752201}, {152509534, 241934995}, {152511591, 241934979}, {154598706, 242117774}, {154600763, 242117758}, {156687878, 242300552}, {156689935, 242300537}, {158777049, 242483331}, {158779107, 242483316}, {160866221, 242666110}, {160868278, 242666095}, {162955393, 242848889}, {162957450, 242848874}, {165044565, 243031668}, {165046622, 243031652}, {167133736, 243214447}, {167135793, 243214431}, {169222908, 243397225}, {169224965, 243397210}, {171312080, 243580004}, {171314137, 243579989}, {173401251, 243762783}, {173403309, 243762768}, {175490423, 243945562}, {175492480, 243945547}, {177579595, 244128341}, {177581652, 244128325}, {179668766, 244311120}, {179670824, 244311104}, {181757938, 244493898}, {181759995, 244493883}, {183847110, 244676677}, {183849167, 244676662}, {185936282, 244859456}, {185938339, 244859441}, {188025453, 245042235}, {188027510, 245042220}, {190114625, 245225014}, {190116682, 245224999}, {192203797, 245407793}, {192205854, 245407777}, {194292968, 245590571}, {194295026, 245590556}, {196382140, 245773350}, {196384197, 245773335}, {198471312, 245956129}, {198473369, 245956114}, {200560484, 246138908}, {200562541, 246138893}, {202649655, 246321687}, {202651712, 246321672}, {204738827, 246504466}, {204740884, 246504450}, {206827999, 246687245}, {206830056, 246687229}, {208917170, 246870023}, {208919227, 246870008}, {211006342, 247052802}, {211008399, 247052787}, {213095514, 247235581}, {213097571, 247235566}, {215184685, 247418360}, {215186743, 247418345}, {217273857, 247601139}, {217275914, 247601123}, {219363029, 247783918}, {219365086, 247783902}, {221452201, 247966696}, {221454258, 247966681}, {223541372, 248149475}, {223543429, 248149460}, {225630544, 248332254}, {225632601, 248332239}, {227719716, 248515033}, {227721773, 248515018}, {229808887, 248697812}, {229810945, 248697796}, {231898059, 248880591}, {231900116, 248880575}, {233987231, 249063369}, {233989288, 249063354}, {236076402, 249246148}, {236078460, 249246133}, {238165574, 249428927}, {238167631, 249428912}, {240254746, 249611706}, {240256803, 249611691}, {242343918, 249794485}, {242345975, 249794470}, {244433089, 249977264}, {244435146, 249977248}, {246522261, 250160042}, {246524318, 250160027}, {248611433, 250342821}, {248613490, 250342806}, {250700604, 250525600}, {250702662, 250525585}, {252789776, 250708379}, {252791833, 250708364}, {254878948, 250891158}, {254881005, 250891143}, {256968120, 251073937}, {256970177, 251073921}, {259057291, 251256716}, {259059348, 251256700}, {261146463, 251439494}, {261148520, 251439479}, {263235635, 251622273}, {263237692, 251622258}, {265324806, 251805052}, {265326863, 251805037}, {267413978, 251987831}, {267416035, 251987816}, {269503150, 252170610}, {269505207, 252170594}, {271592321, 252353389}, {271594379, 252353373}, {273681493, 252536167}, {273683550, 252536152}, {275770665, 252718946}, {275772722, 252718931}, {277859837, 252901725}, {277861894, 252901710}, {279949008, 253084504}, {279951065, 253084489}, {282038180, 253267283}, {282040237, 253267267}, {284127352, 253450062}, {284129409, 253450046}, {286216523, 253632840}, {286218581, 253632825}, {288305695, 253815619}, {288307752, 253815604}, {290394867, 253998398}, {290396924, 253998383}, {292484038, 254181177}, {292486096, 254181162}, {294573210, 254363956}, {294575267, 254363941}, {296662382, 254546735}, {296664439, 254546719}, {298751554, 254729513}, {298753611, 254729498}, {300840725, 254912292}, {300842782, 254912277}, {302929897, 255095071}, {302931954, 255095056}, {305019069, 255277850}, {305021126, 255277835}, {307108240, 255460629}, {307110298, 255460614}, {309197412, 255643408}, {309199469, 255643392}, {311286584, 255826187}, {311288641, 255826171}, {313375756, 256008965}, {313377813, 256008950}, {315464927, 256191744}, {315466984, 256191729}, {317554099, 256374523}, {317556156, 256374508}, {319643271, 256557302}, {319645328, 256557287}, {321732442, 256740081}, {321734499, 256740065}, {323821614, 256922860}, {323823671, 256922844}, {325910786, 257105638}, {325912843, 257105623}, {327999957, 257288417}, {328002015, 257288402}, {330089129, 257471196}, {330091186, 257471181}, {332178301, 257653975}, {332180358, 257653960}, {334267473, 257836754}, {334269530, 257836738}, {336356644, 258019533}, {336358701, 258019517}, {338445816, 258202311}, {338447873, 258202296}, {340534988, 258385090}, {340537045, 258385075}, {342624159, 258567869}, {342626217, 258567854}, {344713331, 258750648}, {344715388, 258750633}, {346802503, 258933427}, {346804560, 258933412}, {348891674, 259116206}, {348893732, 259116190}, {350980846, 259298984}, {350982903, 259298969}, {353070018, 259481763}, {353072075, 259481748}, {355159190, 259664542}, {355161247, 259664527}, {357248361, 259847321}, {357250418, 259847306}, {359337533, 260030100}, {359339590, 260030085}, {361426705, 260212879}, {361428762, 260212863}, {363515876, 260395658}, {363517934, 260395642}, {365605048, 260578436}, {365607105, 260578421}, {367694220, 260761215}, {367696277, 260761200}, {369783391, 260943994}, {369785449, 260943979}, {371872563, 261126773}, {371874620, 261126758}, {373961735, 261309552}, {373963792, 261309536}, {376050907, 261492331}, {376052964, 261492315}, {378140078, 261675109}, {378142135, 261675094}, {380229250, 261857888}, {380231307, 261857873}, {382318422, 262040667}, {382320479, 262040652}, {384407593, 262223446}, {384409651, 262223431}, {386496765, 262406225}, {386498822, 262406210}, {388585937, 262589004}, {388587994, 262588988}, {390675109, 262771782}, {390677166, 262771767}, {392764280, 262954561}, {392766337, 262954546}, {394853452, 263137340}, {394855509, 263137325}, {396942624, 263320119}, {396944681, 263320104}, {399031795, 263502898}, {399033853, 263502883}, {401120967, 263685677}, {401123024, 263685661}, {403210139, 263868456}, {403212196, 263868440}, {405299310, 264051234}, {405301368, 264051219}, {407388482, 264234013}, {407390539, 264233998}, {409477654, 264416792}, {409479711, 264416777}, {411566826, 264599571}, {411568883, 264599556}, {413655997, 264782350}, {413658054, 264782334}, {415745169, 264965129}, {415747226, 264965113}, {417834341, 265147907}, {417836398, 265147892}, {419923512, 265330686}, {419925570, 265330671}, {422012684, 265513465}, {422014741, 265513450}, {424101856, 265696244}, {424103913, 265696229}, {426191027, 265879023}, {426193085, 265879007}, {428280199, 266061802}, {428282256, 266061786}, {430369371, 266244580}, {430371428, 266244565}, {432458543, 266427359}, {432460600, 266427344}, {434547714, 266610138}, {434549771, 266610123}, {436636886, 266792917}, {436638943, 266792902}, {438726058, 266975696}, {438728115, 266975681}, {440815229, 267158475}, {440817287, 267158459}, {442904401, 267341253}, {442906458, 267341238}, {444993573, 267524032}, {444995630, 267524017}, {447082745, 267706811}, {447084802, 267706796}, {449171916, 267889590}, {449173973, 267889575}, {451261088, 268072369}, {451263145, 268072354}, {453350260, 268255148}, {453352317, 268255132}, {455439431, 268437927}, {455441489, 268437911}, {457528603, 268620705}, {457530660, 268620690}, {459617775, 268803484}, {459619832, 268803469}, {461706946, 268986263}, {461709004, 268986248}, {463796118, 269169042}, {463798175, 269169027}, {465885290, 269351821}, {465887347, 269351805}, {467974462, 269534600}, {467976519, 269534584}, {470063633, 269717378}, {470065690, 269717363}, {472152805, 269900157}, {472154862, 269900142}, {474241977, 270082936}, {474244034, 270082921}, {476331148, 270265715}, {476333206, 270265700}, {478420320, 270448494}, {478422377, 270448478}, {480509492, 270631273}, {480511549, 270631257}, {482598663, 270814051}, {482600721, 270814036}, {484687835, 270996830}, {484689892, 270996815}, {486777007, 271179609}, {486779064, 271179594}, {488866179, 271362388}, {488868236, 271362373}, {490955350, 271545167}, {490957407, 271545152}, {493044522, 271727946}, {493046579, 271727930}, {495133694, 271910724}, {495135751, 271910709}, {497222865, 272093503}, {497224923, 272093488}, {499312037, 272276282}, {499314094, 272276267}, {501401209, 272459061}, {501403266, 272459046}, {503490381, 272641840}, {503492438, 272641825}, {505579552, 272824619}, {505581609, 272824603}, {507668724, 273007398}, {507670781, 273007382}, {509757896, 273190176}, {509759953, 273190161}, {511847067, 273372955}, {511849124, 273372940}, {513936239, 273555734}, {513938296, 273555719}, {516025411, 273738513}, {516027468, 273738498}, {518114582, 273921292}, {518116640, 273921276}, {520203754, 274104071}, {520205811, 274104055}, {522292926, 274286849}, {522294983, 274286834}, {524382098, 274469628}, {524384155, 274469613}, {526471269, 274652407}, {526473326, 274652392}, {528560441, 274835186}, {528562498, 274835171}, {530649613, 275017965}, {530651670, 275017949}, {532738784, 275200744}, {532740842, 275200728}, {534827956, 275383522}, {534830013, 275383507}, {536917128, 275566301}, {536919185, 275566286}, {539006299, 275749080}, {539008357, 275749065}, {541095471, 275931859}, {541097528, 275931844}, {543184643, 276114638}, {543186700, 276114623}, {545273815, 276297417}, {545275872, 276297401}, {547362986, 276480195}, {547365043, 276480180}, {549452158, 276662974}, {549454215, 276662959}, {551541330, 276845753}, {551543387, 276845738}, {553630501, 277028532}, {553632559, 277028517}, {555719673, 277211311}, {555721730, 277211296}, {557808845, 277394090}, {557810902, 277394074}, {559898017, 277576869}, {559900074, 277576853}, {561987188, 277759647}, {561989245, 277759632}, {561991269, 277760004}, {561993309, 277760183}, {561997389, 277760540}, {562005550, 277761254}, {562021872, 277762682}, {562054515, 277765538}, {562119802, 277771250}, {562250375, 277782673}, {562511521, 277805521}, {563033814, 277851215}, {563556107, 277896910}, {563817254, 277919757}, {563947827, 277931181}, {564013114, 277936893}, {564045757, 277939749}, {564062078, 277941177}, {564070239, 277941891}, {564074320, 277942248}, {564076360, 277942426}, {564078417, 277942411}, {578700562, 279221878}, {578702619, 279221863}, {580789734, 279404657}, {580791791, 279404642}, {582878905, 279587436}, {582880962, 279587420}, {584968077, 279770215}, {584970134, 279770199}, {587057249, 279952993}, {587059306, 279952978}, {589146420, 280135772}, {589148478, 280135757}, {591235592, 280318551}, {591237649, 280318536}, {593324764, 280501330}, {593326821, 280501315}, {595413935, 280684109}, {595415993, 280684094}, {597503107, 280866888}, {597505164, 280866872}, {599592279, 281049666}, {599594336, 281049651}, {601681451, 281232445}, {601683508, 281232430}, {603770622, 281415224}, {603772679, 281415209}, {605859794, 281598003}, {605861851, 281597988}, {607948966, 281780782}, {607951023, 281780767}, {610038137, 281963561}, {610040195, 281963545}, {612127309, 282146340}, {612129366, 282146324}, {614216481, 282329118}, {614218538, 282329103}, {616305653, 282511897}, {616307710, 282511882}, {618394824, 282694676}, {618396881, 282694661}, {620483996, 282877455}, {620486053, 282877440}, {622573168, 283060234}, {622575225, 283060218}, {624662339, 283243013}, {624664396, 283242997}, {626751511, 283425791}, {626753568, 283425776}, {628840683, 283608570}, {628842740, 283608555}, {630929854, 283791349}, {630931912, 283791334}, {633019026, 283974128}, {633021083, 283974113}, {635108198, 284156907}, {635110255, 284156891}, {637197370, 284339686}, {637199427, 284339670}} };

	c.AddOpenSubject(subject_open);
        // Uncomment line below to make clipper produce other output
        // clip[0].erase(clip[0].begin() + 14);
	c.AddClip(clip);
	if (c.Execute(ct, fr, solution, solution_open))
	{
		for (size_t i = 0; i < solution_open.size(); ++i)
		{
			if (solution_open[i].size() <= 1)
			{
				ASSERT_TRUE(false);
			}
		}
	}
}

As with previous input, subject_open is subsequence of clip.
Whole scene now looks like this (Every point is shown as arrow, a bit messy here):

If we zoom in to our subject poly (yellow here) it looks like this:

Zoom in even closer to the second point of subject (which is produced as single-point result):

With this input clipper produces single-point poly (which is second point of subject {561989245, 277759632}).
But if you remove any point from clip, for example 14th point - it is starting to produce 21-points poly, even though any component of clipping poly that should affect clipping is not changed, 14th point is located far away from subject poly.

Is such behavior expected? I just want to make sure it is not some unknown bug. In my opinion such input should result in empty output, or subject-poly output (because subject is on clipping poly, and we can define this as "outside" or "inside" respectively).

[AngusJohnson]

Hi again, and thank you for your patience and persistence. Despite you very clearly indicating that your issue was when clipping open paths, I failed to register that until now. Sorry. (My ADHD strikes again!) Nevertheless, I didn't close this Issue because I suspected I hadn't fully understood your problem.

I think it's fair to state that (currently) where open paths overlap clipping boundaries, whether these overlapping segments are, or are not included in clipping solutions is undefined.

[olologin]

Ok, thanks for explaining it.
I think if you are aware of such behavior and it is not a bug - we can close it.
Thanks again for making second version of this great library :)