  Spatial frequency filtering is a fundamental enabler of information
processing methods in biological and technical imaging. Most filtering methods,
however, require either bulky and expensive optical equipment or some degree of
computational processing. Here we experimentally demonstrate on-chip,
all-optical spatial frequency filtering using a thin-film perfect absorber
structure. We give examples of edge enhancement in an amplitude image as well
as conversion of a phase gradient in a wave field into an intensity modulation.
