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:

{{{#!c++
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.

{{{#!Spoiler
{{{#!c++
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:
[[Image(healpix-example.png)]].