30.3.5. 422 and 420 Chroma Sampled Data Scaling
However, support for scaling 444, 422 and 420 chroma sampling on a field-variable basis adds significant ALM and memory cost to the resource footprint of the scaler. The scaler includes three parameters (444 chroma sampling, 422 chroma sampling and 420 chroma sampling) to allow you to select which chroma sampling formats the IP should support. Intel recommends that you only turn on chroma samplings formats that your scaler expects to receive in your system.
When the scaler processes 444 sampled data, each color plane is scaled independently, and each color plane has the same input and output dimensions. When the scaler processes 422 sampled data, each color plane is still processed independently (using the same filters as the 444 case), but the width of the chroma planes at the input and output is half that of the luma plane. Likewise, for 420 sampled data, each color plane is scaled independently, and the width and height of the chroma planes is half that of the luma plane. In all cases, the scaling ratio applied to each color plane must be the same. The scaler requires that the input and output field width are even for 422 sampled data, and the input and output width and height are even for 420 sampled data.
For 420 sampled data, each video line contains data from either the blue chroma (Cb) or red chroma (Cr) color plane. To supply both N lines of Cb and Cr data to an N tap vertical scaling filter, 2N lines of video data must be stored in line buffer, which is twice the storage required to process 444 or 422 sampled data. To reduce the on-chip memory footprint of the scaler with 420 chroma sampling enabled, you can turn on Mirror 420 chroma data. When on, the size of the line buffer is reduced back the N lines (for an N tap filter) and the IP mirrors N/2 lines of Cb and Cr data available in the buffer to fill the available taps. This parameter might give a small reduction in the quality of the scaled results for 420 chroma sampled data. You must assess any reduction in memory usage against the potential reduction in quality.
For 420 sampled data, the IP packs two luma samples into the three color planes that make up a standard pixel for either 444 or 422 chroma sampling. The scaler applies the horizontal scaling algorithm to the luma plane independently of the chroma planes. To allow for the double packing of the luma samples the scaler must, by default, implement the luma scaling with processing for twice the number of pixels in parallel you specify. However, you might not require the full processing bandwidth of the interface using 420 sampling. For example, if you have an HDMI 2.0 interface that supports up to 4k60 processing in both 444 and 420 sampling modes, the 420 mode only uses half the bandwidth allowed by the interface. In this example, the scaler need not use double the pixels in parallel to process the luma data and you should turn on the Half rate 420 parameter. This parameter specifies that the IP does not require internal doubling of pixels in parallel for processing the luma plane.