Symmetry filters using optical correlation and convolution

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.