up or downsample filter parameters. In the easiest case, there may be only a single filter for up and/or downsampling. However, in certain cases, it can be advantageous to allow more flexibility in filter design, and that may require to signaling of filter parameters. Such parameters may be selected through an index in a list of possible filter designs, the filter may be fully specified (for example through a list of filter coefficients, using suitable entropy coding techniques), the filter may be implicitly selected through up/downsample ratios according which in turn are signaled according to any of the mechanisms mentioned above, and so forth.
Henceforth, the description assumes the coding of a finite set of up/downsample factors (the same factor to be used in both X and Y dimension), indicated through a codeword. That codeword can advantageously be variable length coded, for example using the Ext-Golomb code common for certain syntax elements in video coding specifications such as H.264 and H.265. One suitable mapping of values to up/downsample factors can, for example, be according to the following table:
Codeword
Ext-Golomb Code
Original/Target resolution
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