Note
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The first stage of decoding aims to reconstruct the DWT data of a tile.
The decoder MUST maintain a copy of the unquantized DWT data ("Current frame" in the figure captioned "RemoteFX Progressive Codec decoding stages" in section 3.3.8.2) as well as a tri-state value for each element in a tile that has not yet been fully upgraded ("Persistent progressive state" in the same figure). The tri-state value records whether the data that has been received for an element sums up to a positive value, a negative value, or zero.
A coefficient either is encoded with the SRL encoder, or its absolute value is written as raw bits (section 3.2.8.1.5). The decoder MUST determine which of these two methods was used and what sign to apply to the decoded element. The sign can be determined by using the tri-state value associated with each element.
If the input data is for the first progressive chunk of a tile (or it contains all of the data for a tile), then the Persistent progressive state MUST be cleared. Furthermore, if the tile is an original tile (not a difference tile), then the tile MUST be zeroed out in the current frame. The result of the entropy decode operation MUST be added to the current frame.