  Aims. To provide a schematic mathematical description of the imaging concept
of the Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter.
The derived model is the fundamental starting point for both the interpretation
of STIX data and the description of the data calibration process. Methods. We
describe the STIX indirect imaging technique which is based on spatial
modulation of the X-ray photon flux by means of tungsten grids. We show that
each of 30 STIX imaging sub-collimators measures a complex Fourier component of
the flaring X-ray source corresponding to a specific angular frequency. We also
provide details about the count distribution model, which describes the
relationship between the photon flux and the measured pixel counts. Results. We
define the image reconstruction problem for STIX from both visibilities and
photon counts. We provide an overview of the algorithms implemented for the
solution of the imaging problem, and a comparison of the results obtained with
these different methods in the case of the SOL2022-03-31T18 flaring event.
