This conversion is a 1:2 vertical interpolation operation, implemented using a 2-phase polyphase FIR filter. One of the two output pixels is co-sited with one of the input sample. The ideal output is achieved simply by replicating this input sample.
To evaluate output pixel ox,y, the FIR filter in the core convolves COEFk_VPHASEpy, where k is the coefficient index, py is the interpolation phase, ix,y are pixels from the input image, and [ ]Mm represents rounding with clipping at M, and clamping at m. DW is the Data Width or number of bits per video component. Ntaps is the number of filter taps.
In phase 1, COEF00_VPHASE1 is the coefficient applied to the most recent input sample in the filter. This Figure illustrates coefficient use for a four tap filter example, with simplified nomenclature a= COEF00_VPHASE1, b= COEF01_VPHASE1, c= COEF02_VPHASE1, and d= COEF03_VPHASE1.
The predefined filters use the coefficients [0.5 0.5] to interpolate one of the output samples. The other output sample is a replication of the input sample.