It looks to me like the HoG computation ignores half of the possible gradient values (see attached image). There are 15 lines visible in the attached HoG visualization, likewise there are 15 non-zero values in the feature vector returned by feature.hog(). Is this a bug? Am I doing something wrong? I'm using the hog example script from http://scikits-image.org/docs/0.5/auto_examples/plot_hog.html , modified to take a file from the command line. Best, David