38. The non-transitory computer-readable medium of claim 36 , wherein the instructions comprises instructions for encoding video data, further comprising instructions that cause the processor 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.
38. The non-transitory computer-readable medium of claim 36 , wherein the instructions comprises instructions for encoding video data, further comprising instructions that cause the processor 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. 40. The non-transitory computer-readable medium of claim 38 , further comprising instructions that cause the processor 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 first transform unit of the first size modified to skip transform coefficients of the first transform unit not included in the retained coefficient block.