A covering can be extracted as a list of trixels, or lists of HtmID (start, end) pairs. In the latter case, the HtmIDs are always for level 20 trixels. The trixels making up the covering are either "inner" or "partial". Inner trixels are completely contained in the region. Partial trixels intersect the boundary of the region. Partial trixels follow the outline(s) of the region. The "outer" cover is the union of "inner" and "partial" covers. Each covering has an associated "cost," which is currently the number of (start, end) HtmID range pairs in the outer cover. It is used mostly in performance monitoring and statistics gathering. Internally, it is used in helping to compute a terminating condition.
The Cover object uses an internal "machine" that computes the covering. The machine runs in stages, each successive step generates a finer resolution covering. The number of these steps is exactly the depth of the partial trixels that are found in the covering.