This conversion performs interpolation both vertically and horizontally. This is equivalent to a 2D separable filter implemented by cascading the 4:2:0 to 4:2:2 block and the 4:2:2 to 4:4:4 block. Quantized vertical filter results are filtered by the horizontal filter, which in turn quantizes results back to the [0 .. 2DW-1] range. (DW is the Data Width or number of bits per video component.)
Intermediate 4:2:2 chroma values are computed using This Equation. The resulting computation is shown in This Equation.
Next, the values are filtered according to This Equation. The resulting computation is shown in This Equation.
The predefined filter coefficients are the same as defined in Convert 4:2:0 to 4:2:2 and Convert 4:2:2 to 4:4:4. In the vertical direction, one input sample is replicated, and the other is interpolated with the filter [0.5 0.5]. The same then happens in the horizontal direction.