The HealPix concept (and the original libraries) are described at ​https://healpix.jpl.nasa.gov/index.shtml.

A HealPix implementation for ROOT is available at ​https://github.com/akira-okumura/RootHealPix. You can compile that into your Mars libarary.

After checking out Mars from the repository (see InstallingMars), simple extract the attached zip-file into your Mars source directory or get RootHealPix from git:

[0] cd Mars
[1] git clone https://github.com/akira-okumura/RootHealPix.git

If RootHealPix is located in your Mars source directory, cmake should find it automatically and compile the healpix classes into the Mars library, otherwise call cmake with the corresponding path:

[2] cmake -DHEALPIX_PREFIX=<path-to-RootHealPix>

To test the installation, start root in your Mars build directory and try something like:

THealPixD hp("name", "", 2); 
for (int i=0; i<10000; i++) 
   hp.Fill(gRandom->Uniform()*90, gRandom->Uniform()*180); 
hp.Draw("COLZ HAMMER");

The following example on reading and writing to and from FITS files is adapted from RootHealPix/tutorials/ReadAndWrite.C.

Reveal Spoiler >>

void ReadAndWrite()
{
  THealPixD hpd1("hpd1", "data1", 5);
  THealPixD hpd2("hpd2", "data2", 4);
  hpd1.SetUnit("MeV"); // Set the unit of entries
  hpd2.SetUnit("GeV");
  hpd1.SetDegree(); // change the unit of Fill(theta, phi)
  hpd2.SetDegree();

  for(Int_t i = 0; i<1800; i++)
  {
     for(Int_t j = 0; j<3600; j++)
     {
        hpd1.Fill(i/10., j/10., 1);
        hpd2.Fill(i/10., j/10., 5);
     } 
  } 

  vector<THealPix*> vec;
  vec.push_back(&hpd1);
  vec.push_back(&hpd2);

  THealUtil::SaveToFits("!test.fits", vec); // hpd1/2 are saved in one file

  // Read only "data2" from FITS file
  THealPixD* hpd = THealPixD::ReadFits("test.fits", "data2");

  cout << "Order: " << hpd->GetOrder() << endl;
  cout << "Nside: " << hpd->GetNside() << endl;
  cout << "Npix : " << hpd->GetNpix()  << endl;
  cout << "Unit : " << hpd->GetUnit()  << endl;
  cout << "Title: " << hpd->GetTitle() << endl;
}

The documentation is not very complete, but you can find it here: ​https://www.fact-project.org/RootHealPix/.

The available Draw/Paint options are described at ​https://www.fact-project.org/RootHealPix/THealPainter.html#THealPainter:Paint

The example above should produce something like this: .