This conversion is a vertical 2:1 decimation operation, implemented using a low-pass FIR filter to suppress chroma aliasing. In order to evaluate output pixel ox,y, the FIR filter in the core convolves COEFk_VPHASE0, where k is the coefficient index, 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 0, COEF00_VPHASE0 is the coefficient applied to the most recent input sample in the filter. This Figure illustrates coefficient use for a five tap filter example, with simplified nomenclature a= COEF00_VPHASE0, b= COEF01_VPHASE0, c= COEF02_VPHASE0, d= COEF03_VPHASE0, and e= COEF04_VPHASE0.
The predefined filter coefficients are [0.25 0.5 0.25].