  Modelling of stellar radiative intensities in various spectral pass-bands
plays an important role in stellar physics. At the same time the direct
calculations of the high-resolution spectrum and then integrating it over the
given spectral pass-band is computationally demanding due to the vast number of
atomic and molecular lines. This is particularly so when employing
three-dimensional (3D) models of stellar atmospheres. To accelerate the
calculations, one can employ approximate methods, e.g., the use of Opacity
Distribution Functions (ODFs). Generally, ODFs provide a good approximation of
traditional spectral synthesis i.e., computation of intensities through filters
with strictly rectangular transmission function. However, their performance
strongly deteriorates when the filter transmission noticeably changes within
its pass-band, which is the case for almost all filters routinely used in
stellar physics. In this context, the aims of this paper are a) to generalize
the ODFs method for calculating intensities through filters with arbitrary
transmission functions; b) to study the performance of the standard and
generalized ODFs methods for calculating intensities emergent from 3D models of
stellar atmosphere. For this purpose we use the newly-developed MPS-ATLAS
radiative transfer code to compute intensities emergent 3D cubes simulated with
the radiative magnetohydrodynamics code MURaM. The calculations are performed
in the 1.5D regime, i.e., along many parallel rays passing through the
simulated cube. We demonstrate that generalized ODFs method allows accurate and
fast syntheses of spectral intensities and their centre-to-limb variations.
