1. A non-transitory computer-readable medium for storing data associated with a transform coefficient block, comprising: a data stream stored in the non-transitory computer-readable medium, the data stream comprising an encoded significance map indicating positions of significant transform coefficients within a transform coefficient block, and the significant transform coefficients within the transform coefficient block, wherein the significant transform coefficients are coded into the data stream by context-adaptive entropy encoding in a coefficient scan order such that each of a plurality of sub-blocks of the transform coefficient block is entropy coded using one or more contexts determined for that sub-block separately from the other of the plurality of sub-blocks, wherein, according to the coefficient scan order, the plurality of sub-blocks of the transform coefficient block are scanned in a sub-block scan order, wherein each of the plurality of sub-blocks includes more than one pixel, and the transform coefficients within each of the plurality of sub-blocks are scanned in a position sub-scan order separate from the sub-block scan order, wherein the position sub-scan order scans, within each of the plurality of sub-blocks, from a position within the respective sub-block corresponding to a highest frequency in both vertical and horizontal directions to a position of the respective sub-block corresponding to a lowest frequency in both the vertical and horizontal directions, wherein the significant transform coefficients are context-adapted entropy encoded using a selected set of contexts from a plurality of sets of contexts, wherein, for each of the plurality of sub-blocks, the selected set of contexts is determined based on a statistic of the transform coefficients within a previously traversed sub-block of the transform coefficient block, or a statistic of the transform coefficients of a co-located sub-block in a previously encoded transform coefficient block.