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In OpenCV, coordinates can be 2-dimensional, 3-dimensional, or 4-dimensional.

The number "2", "3", and "4" refers to the number of elements (components) in the coordinate vector.

The value for each element can be stored as:

  • signed 32-bit integer, denoted with letter "i",
  • 32-bit floating point (single precision), denoted with letter "f", and
  • 64-bit floating point (double precision), denoted with letter "d".

Therefore, a 2D vector of integer values is named Point2i. Likewise, a 3D vector of 32-bit single-precision floating point values is named Point3f.

OpenCV defines a C++ type alias, named Point, is a shorthand for Point2i. This is merely for convenience and for reduce code clutter.