31. The video coding device of claim 29 , wherein the video coding device comprises a video encoding device, further comprising: means for transforming residual pixel values into transform coefficients within the first transform unit having the first size; means for generating the retained coefficient block of the second size within the first transform unit by zeroing out a subset of transform coefficients of the first transform unit having the first size; means for selecting contexts for the transform coefficients within the retained coefficient block according to the joint context model; and means for entropy encoding the transform coefficients of the retained coefficient block within the first transform unit having the first size based on the selected contexts.
31. The video coding device of claim 29 , wherein the video coding device comprises a video encoding device, further comprising: means for transforming residual pixel values into transform coefficients within the first transform unit having the first size; means for generating the retained coefficient block of the second size within the first transform unit by zeroing out a subset of transform coefficients of the first transform unit having the first size; means for selecting contexts for the transform coefficients within the retained coefficient block according to the joint context model; and means for entropy encoding the transform coefficients of the retained coefficient block within the first transform unit having the first size based on the selected contexts. 33. The video coding device of claim 31 , further comprising means for scanning the transform coefficients of the retained coefficient block within the first transform unit having the first size using a scan order for the first transform unit of the first size modified to skip transform coefficients of the first transform unit not included in the retained coefficient block.