18. The video coding device of claim 15 , wherein the video coding device comprises a video encoding device, wherein the processor is configured to: transform residual pixel values into transform coefficients within the first transform unit having the first size; generate 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; select contexts for the transform coefficients within the retained coefficient block according to the joint context model; and entropy encode the transform coefficients of the retained coefficient block within the first transform unit having the first size based on the selected contexts.
18. The video coding device of claim 15 , wherein the video coding device comprises a video encoding device, wherein the processor is configured to: transform residual pixel values into transform coefficients within the first transform unit having the first size; generate 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; select contexts for the transform coefficients within the retained coefficient block according to the joint context model; and entropy encode the transform coefficients of the retained coefficient block within the first transform unit having the first size based on the selected contexts. 20. The video coding device of claim 18 , wherein the processor is configured to scan the transform coefficients of the retained coefficient block within the first transform unit having the first size using a scan order for the second transform unit of the second size.