x	–	Data vector-array,
X	–	Matrix,
$\overrightarrow{\mathbf{x}}$	–	Point in 2D space (at X-ray plane detector),
$\overrightarrow{\mathbf{X}}$	–	Point in 3D space,
x	–	Constant,
x	–	Scalar variable,
X()	–	Scalar function, result is scalar variable (x),
$\overrightarrow{\mathrm{x}}()$	–	Vector function, result is point in 2D ($\overrightarrow{\mathbf{x}}$),
$\overrightarrow{\mathrm{X}}()$	–	Vector function, result is point in 3D space ($\overrightarrow{\mathbf{X}}$).