Polygon.Intersects() not working

[sreekanth-cb]

Not sure why the below polygon intersect unit test isn't working, it says the polygons intersect whereas the actual polygons aren't intersecting on the earth's surface.

Is there something the test does in the wrong way? Or
does anyone have any idea/clues as to what is going wrong here?

TEST(S2NonOverlapping, AllCases) {
  vector<S2Point> pts(7);
  S2LatLng latlng;
  
  pts[0] = latlng.FromDegrees(13.197164523281993, -282.9803466796875).ToPoint();
  pts[1] = latlng.FromDegrees(12.404388944669792, -282.919921875).ToPoint();
  pts[2] = latlng.FromDegrees(12.833226023521243, -282.1728515625).ToPoint();
  pts[3] = latlng.FromDegrees(12.95102921601837, -281.55761718749994).ToPoint();

  pts[4] = latlng.FromDegrees(13.443052132777558, -281.62353515625).ToPoint();
  pts[5] = latlng.FromDegrees(13.678013256725489, -282.19482421875).ToPoint();
  pts[6] = latlng.FromDegrees(13.720708401412068, -282.777099609375).ToPoint();
  
  vector<std::unique_ptr<S2Loop>> loops;
  loops.emplace_back(new S2Loop(pts));  
  S2Polygon polygon1 = S2Polygon(std::move(loops));

  pts.resize(19);
  pts[0] = latlng.FromDegrees(45.13745, -67.13734).ToPoint();
  pts[1] = latlng.FromDegrees(44.8097, -66.96466).ToPoint();
  pts[2] = latlng.FromDegrees(44.3252, -68.03252).ToPoint();
  pts[3] = latlng.FromDegrees(43.98, -69.06).ToPoint();

  pts[4] = latlng.FromDegrees(43.68405, -70.11617).ToPoint();
  pts[5] = latlng.FromDegrees(43.09008, -70.64573).ToPoint();
  pts[6] = latlng.FromDegrees(43.08003, -70.75102).ToPoint();
  pts[7] = latlng.FromDegrees(43.21973, -70.79761).ToPoint();

  pts[8] = latlng.FromDegrees(43.36789, -70.98176).ToPoint();
  pts[9] = latlng.FromDegrees(43.46633, -70.94416).ToPoint();
  pts[10] = latlng.FromDegrees(45.30524, -71.08482).ToPoint();
  pts[11] = latlng.FromDegrees(45.46022, -70.66002).ToPoint();

  pts[12] = latlng.FromDegrees(45.91479, -70.30495).ToPoint();
  pts[13] = latlng.FromDegrees(46.69317, -70.00014).ToPoint();
  pts[14] = latlng.FromDegrees(47.44777, -69.23708).ToPoint();
  pts[15] = latlng.FromDegrees(47.18479, -68.90478).ToPoint();

  pts[16] = latlng.FromDegrees(47.35462, -68.2343).ToPoint();
  pts[17] = latlng.FromDegrees(47.06624, -67.79035).ToPoint();
  pts[18] = latlng.FromDegrees(45.70258, -67.79035).ToPoint();
 
  loops.clear();
  loops.emplace_back(new S2Loop(pts));  

  auto polygon2 = S2Polygon(std::move(loops));

  bool intersection = polygon1.Intersects(&polygon2);
  EXPECT_EQ(false,intersection); 
 
}

[jmr]

Check the area of both polygons. If one is near 4π then it's backwards and encloses the whole sphere except what you probably intended.

s2geometry/src/s2/s2loop.h

Line 61 in ad1489e

// last. All loops are defined to have a CCW orientation, i.e. the interior of

[sreekanth-cb]

Really appreciate your quick response!! Thanks a lot!
@jmr , Aside from ensuring that the incoming edges of the polygon are in CCW order,
Is there anything the higher levels(calling app) can do to identify and avoid this before calling Intersect() method?

What is the recommended action to avoid this scenario?

EDIT - the area of the latter polygon is 4π. (12.564235290078345).
what would be the course of action for figuring out whether the polygons intersect or not here.

[jmr]

The edges are always CCW by definition, so you can't check that. You can check for suspiciously large loops/polygons, though.

You the conditions documented in the comments also need to be satisfied.

s2geometry/src/s2/s2polygon.h

Line 101 in ad1489e

// Polygons have the following restrictions:

s2geometry/src/s2/s2loop.h

Line 61 in ad1489e

// last. All loops are defined to have a CCW orientation, i.e. the interior of

You can use the S2Debug::DISABLE constructor then IsValid to check whether you have something valid.

s2geometry/src/s2/s2polygon.h

Line 130 in ad1489e

// With override == S2Debug::DISABLE, the automatic validity checking

Of course, in your example everything was valid AFAIK.

EDIT - the area of the latter polygon is 4π. (12.564235290078345).
what would be the course of action for figuring out whether the polygons intersect or not here.

They intersect. What are you asking?

[sreekanth-cb]

Due to the whole sphere enclosure internal techniques as you hinted above,
polygon1.Intersects(&polygon2) is returning that the polygons intersect in the earlier test.

But the original polygons on the map don't intersect. (one is from US and the other is from india).
My query is - how can I detect and prevent/workaround the cases of backward sphere enclosure and successfully invoke intersect() or check whether the polygons intersect or not.

[jmr]

pts[0] = latlng.FromDegrees(13.197164523281993, -282.9803466796875).ToPoint();

I missed this before, but -282 is not a valid longitude.

But the original polygons on the map don't intersect. (one is from US and the other is from india).
My query is - how can I detect and prevent/workaround the cases of backward sphere enclosure and successfully invoke intersect() or check whether the polygons intersect or not.

Apparently you entered the complement of the second polygon. Reverse the vertices.

  pts[0] = latlng.FromDegrees(45.70258, -67.79035).ToPoint();
  pts[1] = latlng.FromDegrees(47.06624, -67.79035).ToPoint();
  ...

[sreekanth-cb]

@jmr, my bad. It's all working fine!
thank you very much.