HEALPix is an acronym for graded equal-area iso latitude pixels of a sphere. This pixelization produces a subdivision of a sphere, in which each pixel covers the same surface area as other pixels. HEALPix projection with H=4 and K=3 is the most commonly used projection, which is widely used in cosmology to store and manipulate cosmic microwave background images. It was approved by the FITS Working Group of the International Astronomical Union as part of the official FITS world coordinate system on April 26th, 2006.

The main requirement of HEALPix development is to create a mathematical structure to support the proper discretization of functions on the sphere with high enough resolution, and to promote the rapid and accurate statistics and astrophysical analysis of a large number of all-day data sets.

HEALPIx meets these requirements because it has the following three basic attributes:

The 12 "basic resolution pixels" of HEALPIx projection with H=4 and K=3 can be regarded as the faces of rhombic dodecahedron. A pixel is a square on a plane (a quadrilateral with non-geodesic edges that can be backprojected on a three-dimensional sphere), and each vertex connects four pixels, except for eight vertices, each vertex only connects three pixels.

The rhombic dodecahedron has many interesting properties, such as filling space, just as a hexagon can fill a plane.

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