question: RegionCoverer interface about s2

[missinglink]

I'm not sure if S2Polygon::GetCellUnionBound actually removes holes from the CellUnion or not 🤷‍♂️

I was a bit disappointed to see that the Go Port (p *Polygon) CellUnionBound() function returns the CellUnion of the Cap! So it's just returning a coverage of outer rings and not considering interior rings 😢

[missinglink]

I might have had the wrong assumptions about RegionCoverer, I assumed it would cover the surface of the shape rather than the exterior boundary, looking at the C++ code that doesn't seem to be the case.

#include "s2/s2point.h"
#include "s2/s2cell_id.h"

#include "s2/s2builder.h"
#include "s2/s2builderutil_s2polygon_layer.h"
#include "s2/s2polygon.h"
#include "s2/s2loop.h"

#include "s2/s2text_format.h"

#include "s2/s2region.h"
#include "s2/s2region_coverer.h"

using s2builderutil::S2PolygonLayer;
using s2textformat::MakeLoopOrDie;
using s2textformat::MakePolygonOrDie;
using s2textformat::ToString;

using absl::make_unique;
using std::cout;
using std::endl;

string CellUnionTokens(const S2CellUnion &cell_union)
{
  string out;
  for (S2CellId cell_id : cell_union)
  {
    if (!out.empty())
      out += ",";
    out += cell_id.ToToken();
  }
  return out;
}

int main()
{
    S2Builder::Options options;
    S2Builder builder(options);

    S2Polygon output;
    builder.StartLayer(make_unique<S2PolygonLayer>(&output));

    auto outer = MakeLoopOrDie("-32.364:153.207, -35.364:153.207, -35.364:158.207");
    builder.AddLoop(*outer);
    cout << "-- outer --" << endl;
    cout << ToString(*outer) << endl;

    auto inner1 = MakeLoopOrDie("-34.364:155.207, -34.364:154.207, -33.364:154.207");
    builder.AddLoop(*inner1);
    cout << "-- inner1 --" << endl;
    cout << ToString(*inner1) << endl;

    // auto inner2 = MakeLoopOrDie("-34.364:157.207, -34.364:156.207, -33.364:156.207");
    // builder.AddLoop(*inner2);
    // cout << "-- inner2 --" << endl;
    // cout << ToString(*inner2) << endl;

    cout << "-- build --" << endl;
    S2Error error;
    builder.Build(&error);
    cout << error << endl;

    cout << "-- output --" << endl;
    cout << ToString(output) << endl;

    S2RegionCoverer coverer;
    S2CellUnion ret = coverer.GetCovering(output);

    cout << "-- cell union --" << endl;
    cout << CellUnionTokens(ret) << endl;

    return 0;
}

-- outer --
-32.364:153.207, -35.364:153.207, -35.364:158.207
-- inner1 --
-34.364:155.207, -34.364:154.207, -33.364:154.207
-- build --

-- output --
-32.364:153.207, -35.364:153.207, -35.364:158.207;
-33.364:154.207, -34.364:154.207, -34.364:155.207
-- cell union --
6b63,6b65,6b6f,6b71,6b77,6b7c,6c82c,6c9d

[missinglink]

I found this function GetInteriorCovering() which solves my issue 🎉

  auto outer = MakeLoopOrDie("-35.364:153.207, -35.364:158.207, -32.364:158.207, -32.364:153.207");
  auto inner1 = MakeLoopOrDie("-34.364:154.207, -34.364:155.207, -33.364:155.207, -33.364:154.207");
  auto inner2 = MakeLoopOrDie("-34.364:156.207, -34.364:157.207, -33.364:157.207, -33.364:156.207");

  S2RegionCoverer coverer;
  coverer.mutable_options()->set_min_level(0);
  coverer.mutable_options()->set_max_level(16);
  coverer.mutable_options()->set_max_cells(100);

  S2CellUnion outerCells = coverer.GetCovering(*outer);
  S2CellUnion inner1Cells = coverer.GetInteriorCovering(*inner1);
  S2CellUnion inner2Cells = coverer.GetInteriorCovering(*inner2);

  auto output = outerCells.Difference(inner1Cells).Difference(inner2Cells);
  cout << CellUnionTokens(output) << endl;

result: https://s2.sidewalklabs.com/regioncoverer/?center=-33.958564%2C155.768571&zoom=8&cells=6b6295%2C6b62b%2C6b62d%2C6b62e4%2C6b62ec%2C6b634%2C6b6454%2C6b645c%2C6b64604%2C6b64c%2C6b654%2C6b659%2C6b65a4%2C6b65ac%2C6b65bd5%2C6b6e3%2C6b6e5%2C6b6ed%2C6b6ef%2C6b6f1%2C6b6f24%2C6b6f3c%2C6b6f4004%2C6b6f7%2C6b6fc%2C6b7001%2C6b7003%2C6b70041%2C6b70045%2C6b70047%2C6b7004c%2C6b70054%2C6b7005c%2C6b7006c%2C6b70073%2C6b70075%2C6b70077%2C6b700c3%2C6b700c5%2C6b700cc%2C6b700d4%2C6b700dc%2C6b700e4%2C6b700e9%2C6b700ef%2C6b700f4%2C6b700fc%2C6b7014%2C6b701c%2C6b703%2C6b705%2C6b7064%2C6b706c%2C6b70704%2C6b7070c%2C6b70711%2C6b70717%2C6b7071c%2C6b70724%2C6b7072c%2C6b70731%2C6b70733%2C6b7073b%2C6b7073d%2C6b707b%2C6b707d%2C6b7085%2C6b70864%2C6b70869%2C6b7086f%2C6b70871%2C6b70877%2C6b7087c%2C6b708c%2C6b7094%2C6b7099%2C6b709a4%2C6b709ac%2C6b709b4%2C6b709bb%2C6b709bd%2C6b70a24%2C6b70a2c%2C6b70a34%2C6b70a39%2C6b70a3b%2C6b70a3f%2C6b70a5%2C6b70a7%2C6b70ac%2C6b70b4%2C6b70bc%2C6b70d%2C6b70f%2C6b711%2C6b713%2C6b71b%2C6b71d%2C6b76d%2C6b76f%2C6b774%2C6b7784%2C6b779c%2C6b77a01%2C6b782c%2C6b7834%2C6b785%2C6b78b4%2C6b78b9%2C6b78d%2C6b78f%2C6b794%2C6b79c%2C6b7a04%2C6b7a084%2C6b7a08c%2C6b7a091%2C6b7a093%2C6b7a09b%2C6b7a09d%2C6b7a0f4%2C6b7a0fc%2C6b7a104%2C6b7a109%2C6b7a10b%2C6b7a1c%2C6b7a21%2C6b7a23%2C6b7a244%2C6b7a25c%2C6b7a27%2C6b7a29%2C6b7a2b%2C6b7a2ec%2C6b7a2f3%2C6b7a2f5%2C6b7aa8c%2C6b7aa94%2C6b7aa9c%2C6b7aab%2C6b7aad%2C6b7aae4%2C6b7aaec%2C6b7aaf4%2C6b7aafb%2C6b7aafd%2C6b7aaff%2C6b7ab4%2C6b7ab84%2C6b7ab9c%2C6b7aba4%2C6b7ac43%2C6b7ac45%2C6b7ac4c%2C6b7ac54%2C6b7ac5c%2C6b7ac64%2C6b7ac6c%2C6b7ac7c%2C6b7acc%2C6b7ad4%2C6b7ad9%2C6b7adb%2C6b7adc1%2C6b7adc7%2C6b7adec%2C6b7adf4%2C6b7b05%2C6b7b07%2C6b7b0c%2C6b7b14%2C6b7b19%2C6b7b1b%2C6b7b20c%2C6b7b214%2C6b7b219%2C6b7b21b%2C6b7b23%2C6b7b25%2C6b7b27%2C6b7b2c%2C6b7b34%2C6b7b3c%2C6b7b5%2C6b7b7%2C6b7b9%2C6b7ba4%2C6b7ba9%2C6b7baf%2C6b7bb1%2C6b7bb7%2C6b7bbc%2C6b7bc4%2C6b7bc9%2C6b7bcf%2C6b7bd1%2C6b7bd43%2C6b7bd45%2C6b7bd4d%2C6b7bd4f%2C6b7bd54%2C6b7bd5c%2C6b7bd7%2C6b7bdc%2C6b7bf%2C6b7c04%2C6b7c0c%2C6b7c11%2C6b7c13%2C6b7c144%2C6b7c14c%2C6b7c151%2C6b7c153%2C6b7c15c%2C6b7c164%2C6b7c16c%2C6b7c174%2C6b7c179%2C6b7c17b%2C6b7c183%2C6b7c185%2C6b7c18c%2C6b7c194%2C6b7c19c%2C6b7c1b%2C6b7c1d%2C6b7c1e4%2C6b7c1ec%2C6b7c1f4%2C6b7c1fb%2C6b7c1fd%2C6b7c603%2C6b7c605%2C6b7c60d%2C6b7c60f%2C6b7c614%2C6b7c61c%2C6b7c63%2C6b7c65%2C6b7c664%2C6b7c66c%2C6b7c674%2C6b7c67b%2C6b7c67d%2C6b7c69%2C6b7c6a3%2C6b7c6a5%2C6b7c6ac%2C6b7c6b4%2C6b7c6bc%2C6b7c6d%2C6b7c6f%2C6b7c74%2C6b7c7c%2C6b7c84%2C6b7c89%2C6b7c8a4%2C6b7c8ac%2C6b7c8b1%2C6b7c8b7%2C6b7c8bc%2C6b7c8c4%2C6b7c8c9%2C6b7c8cf%2C6b7c8d1%2C6b7c8d5%2C6b7c8d7%2C6b7c8dc%2C6b7c8f%2C6b7c94%2C6b7c9c%2C6b7cb%2C6b7cd%2C6b7ce4%2C6b7cec%2C6b7cf04%2C6b7cf09%2C6b7cf0f%2C6b7cf14%2C6b7cf1c%2C6b7cf24%2C6b7cf2c%2C6b7cfa3%2C6b7cfa5%2C6b7cfa7%2C6b7cfac%2C6b7cfb4%2C6b7cfbc%2C6b7cfd%2C6b7cfe1%2C6b7cfe7%2C6b7d014%2C6b7d01c%2C6b7d03%2C6b7d05%2C6b7d064%2C6b7d06c%2C6b7d07b%2C6b7d07d%2C6b7d084%2C6b7d08c%2C6b7d094%2C6b7d099%2C6b7d09b%2C6b7d09f%2C6b7d0b%2C6b7d0d%2C6b7d0f%2C6b7d14%2C6b7d1c%2C6b7d3%2C6b7d5%2C6b7d7%2C6b7d84%2C6b7d8c%2C6b7d9d4%2C6b7d9dc%2C6b7d9e1%2C6b7d9e3%2C6b7d9fd%2C6b7d9ff%2C6b7da04%2C6b7df4%2C6b7dfc%2C6b7e04%2C6b7e154%2C6b7e15c%2C6b7e1c%2C6b7e3%2C6b7e5%2C6b7e61%2C6b7e624%2C6b7e663%2C6b7e665%2C6b7e66c%2C6b7e674%2C6b7e67c%2C6b7e684%2C6b7e689%2C6b7e68f%2C6b7e69c%2C6b7e6a4%2C6b7e6a9%2C6b7e6ab%2C6b7e6af%2C6b7e893%2C6b7e895%2C6b7e897%2C6b7e89c%2C6b7e8b%2C6b7e8d%2C6b7e8e4%2C6b7e8ec%2C6b7e903%2C6b7e905%2C6b7e90d%2C6b7e90f%2C6b7e914%2C6b7e91c%2C6b7e93%2C6b7e95%2C6b7e97%2C6b7ea05%2C6b7ea07%2C6b7ea0c%2C6b7ea14%2C6b7ea1c%2C6b7ea3%2C6b7ea5%2C6b7ea7%2C6b7eac%2C6b7eb4%2C6b7ebc%2C6b7ed%2C6b7ef%2C6b7f1%2C6b7f3%2C6b7f44%2C6b7f4c%2C6b7f7%2C6b7fc%2C6b8001%2C6b80021%2C6c7e2c%2C6c7e33%2C6c7fcd5%2C6c7fd4%2C6c7fdc%2C6c7fe4%2C6c7ffc%2C6c804%2C6c8084%2C6c808c%2C6c80f4%2C6c80fc%2C6c8104%2C6c810c%2C6c8174%2C6c817c%2C6c81c%2C6c824%2C6c82c%2C6c8304%2C6c831c%2C6c8324%2C6c832c%2C6c83ac%2C6c83b4%2C6c83cc%2C6c83d4%2C6c9cd4%2C6c9d3%2C6c9d5