Really I don't know what you mean by variable step length, but if you want to perform dimension reduction using feature (pattern) extraction you may use Fourier transform, wavelet transform or contourlet transform.

Hope this help you.

Dear Constantinos,

First we should define the pattern in case of an image extracting patterns it depends on several characteristics such as shape, size, position, orientation, texture. Normally texture is the most influential factor. In addition, if we take satellite images patterns are not consistent (Forests, crop fields...etc).

If you mean algorithms that uses variable length patterns for intrusion detection there is one named Teirisias pattern discovery algorithm