We describe a novel method based on optical cross-correlation
and autoconvolution for the selective enhancement of image components
with specific symmetry. Such a symmetry filter can operate in real time
by using nonlinear optical four-wave mixing. Instead of correlating with
a set of preselected references, the input picture is correlated with its own
rotated version, which is generated by simple optics. Any patterns that
are invariant under the rotation will exhibit strong correlation signals.