The use of four three-dimensional filters, one for each side of the box where the prediction is being made, can eliminate the bad predictions at the boundaries. Only four filters are needed, since reversing the order of the coefficients along all the filter axes produces a similar prediction. Thus, a separate filter is needed for each side of the box, but the front and back faces use the same set of filters.
In a three-dimensional volume, the use of four filters might also be justified through classifying dipping events by the direction of the dip. An examination of the comparative amplitudes of the filter coefficients for the four filters calculated from a single dipping plane within a volume generally shows that only one filter makes a significant prediction, with the other filters being comparatively weak. Since each filter predicts one direction well, the filters are referred to here as directional filters. Thus four filters are used to completely predict the input volume.