SPRUIE9D May 2017 – May 2024 DRA74P , DRA75P , DRA76P , DRA77P
When configured CDS-on (ISP_CNF_EN[0] CNFA_EN) , chroma is downsampled before the chroma filter core, and upsampled after the chroma filter core.
For downsampling, a simple 2x2 non-overlapping averaging scheme is used. For upsampling, overlapping bilinear interpolation is used. Figure 9-110 shows the spatial relationship between the original and downsampled data points, and the downsampling/upsampling schems.
Chroma downsampling/upsampling is performed outside the chroma filter core (NSF3V reuse), so the core is not aware of the downsampling option. When CDS-on, the core should be programmed with half the width and height, as that’s the size of the image the core is receiving and producing.
As chroma downsampling utilizes non-overlapping 2x2 averaging, no special border treatment is needed.
Chroma upsampling, however, utilizes overlapping bilinear interpolation, and border treatment is needed. The left/right/top/bottom data points are replicated to produce the missing border data needed, as shown in the Figure 9-111
This is just the algorithm specification, and does not imply that the hardware implementation literally replicate border data points this way; hardware implementation seek to produce the equivalent outcome efficiently.