  Ionization-parameter mapping (IPM) is a powerful technique for tracing the
optical depth of Lyman continuum radiation from massive stars. Using
narrow-band line-ratio maps, we examine trends in radiative feedback from
ordinary HII regions of the Magellanic Clouds and nearby starburst galaxies. We
find that the aggregate escape fraction for the Lyman continuum is sufficient
to ionize the diffuse, warm ionized medium in the Magellanic Clouds, and that
more luminous nebulae are more likely to be optically thin. We apply
ionization-parameter mapping to entire starburst galaxies, revealing ionization
cones in two nearby starbursts. Within the limits of our small sample, we
examine the conditions for the propagation of ionizing radiation beyond the
host galaxies.
