3.2.8.1.4 Sub-Band Diffing
Sub-band diffing enables increased compression without any further quality loss by sending the differences of the quantized values between frames.
To compress each tile in a surface, the encoder stores the quantized DWT coefficients that the decoder most likely possesses. These coefficients differ slightly from the quantized coefficients of the previous frame due to the progressive entropy encoder and are known as the reference bits. See section 3.1.8.1.4 and the figure captioned "RemoteFX Progressive Codec encoding stages" in section 3.2.8.1.
The first phase of the Sub-Band Diffing Stage decides between sending the quantized DWT coefficients that have been calculated (section 3.2.8.1.3) or sending the differences with respect to the reference bits. This decision is made for each tile being encoded. If the quantized DWT coefficients of the tile are to be sent, then the tile is called an "original tile"; otherwise, it is referred to as a "difference tile".
A tile that is being encoded for the first time is always sent as an original tile.
The calculation to determine the difference is performed on all three color components. Each of the 1024 coefficients from the tile contained in the reference bits are subtracted from each of the 1024 coefficients from the most recently calculated tile. This data is used to construct the difference tile.

In the preceding formula, "QC" stands for "Quantized Coefficient", "DT" for "Difference Tile", "OT" for "Original Tile", and "RB" for "Reference Bits".
Zeros are counted in both the difference tile and the original tile in the Luma (Y) component and in all of the bands except for the LL3 band. The tile with the most number of zeros is selected to be sent to the RLGR Entropy Encoder. In the case of a tie, the original tile is preferred. If an original tile is selected over a difference tile, the reference bits MUST be cleared and filled with zeros.