In this example a kernel matrix is computed for a given real-valued data set.
The kernel used is the Chi2 kernel which operates on real-valued vectors. It
computes the chi-squared distance between sets of histograms. It is a very
useful distance in image recognition (used to detect objects). The preprocessor
LogPlusOne adds one to a dense real-valued vector and takes the logarithm of
each component of it. It is most useful in situations where the inputs are
counts: When one compares differences of small counts any difference may matter
a lot, while small differences in large counts don't. This is what this log
transformation controls for.
