  In this paper we make a quantitative study of the hypothesis that the diffuse
H-alpha emitted from the discs of spiral galaxies owes its origin to the
ionizing photons escaping from HII regions. We use the H-alpha measurements of
the complete set of HII regions in the spiral NGC 157, for which an HI density
map was available, to derive a family of models which predict the ionizing
photon distribution in the disc of this galaxy. The predicted diffuse H-alpha
surface brightness distributions from our models were compared with the
observed distributions showing that, in general terms, the hypothesis of
density bounding for the HII regions allows us to predict well the spatial
distribution of the diffuse ionized gas. In the model yielding the best fit to
the data, the regions of lower luminosity lose a constant fraction of their
ionizing flux to their surroundings, while for HII region luminosities above a
specific transition value the ionizing escape fraction is a rising function of
the Ha luminosity.
