can be computed using
 linear or non-linear transformations of I, various choices implemented
 here are described below. The only constraint is that channels must be
 translationally invariant (i.e. translating the input image or the
 resulting channels gives the same result). This allows for fast object
 detection, as the channels can be computed once on the entire image
 rather than separately for each overlapping detection window.

 Currently, three channel types are available by default (to date, these
