4:2:0 encoding contains horizontally and vertically sub-sampled chroma. Horizontal and vertical chroma positions are co-sited with alternate luma samples on alternate scanlines. The sampling positions are shown in the following figure.

- Implementation
- Between the three supported sub-sampling formats (4:4:4, 4:2:2, 4:2:0), there
are six conversions available. Conversion is achieved using a FIR filter
approach. Some require filtering in only the horizontal dimension or in only the
vertical dimension, and in some cases in both the horizontal and the vertical
dimensions. These are detailed in the following table along with default filter
information.
Table 1. Chroma Resampling Configuration Converter Filter Configuration 4:4:4 to 4:2:2 Horizontal anti-aliasing 4:4:4 to 4:2:0 Separable 2-D anti-aliasing 4:2:2 to 4:4:4 Horizontal Interpolation 4:2:2 to 4:2:0 Vertical anti-aliasing 4:2:0 to 4:4:4 Separable 2-D Interpolation 4:2:0 to 4:2:2 Vertical Interpolation Three implementation options are offered for each conversion operation:

- DSP48 based filter with programmable coefficients and programmable number of taps. 2D filters must be separable. Coefficients are in the range [-8, 8), represented in 16-bit signed, fixed-point format with four integer bits and 12 fractional bits.
- The predefined fixed coefficient, non-programmable filter with power of two coefficients (using only shifts and additions for filtering therefore no DSP48s are used). Default coefficients implement linear interpolation for the interpolation and anti-aliasing low pass filters.
- The simplest, lowest footprint solution is to simply drop (decimation) or replicate (interpolation) samples. For down sampling, some samples are passed directly to the output, but others are dropped entirely as appropriate. For up converters, replication of the previous input sample occurs.

- Convert 4:2:2 to 4:4:4
- This conversion is a 1:2 horizontal interpolation operation, implemented using a two-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.

- Convert 4:4:4 to 4:2:2
- This conversion is a horizontal 2:1 decimation operation, implemented using
a low-pass FIR filter to suppress chroma aliasing. In order to evaluate output
pixel o
_{x, y,}the FIR filter in the core convolves COEFk_HPHASE0, where k is the coefficient index, i_{x,y}are pixels from the input image, and [ ]^{M}_{m}represents rounding with clipping at M, and clamping at m. DW is the Data Width or number of bits per video component. N_{taps}is the number of filter taps.Figure 4. K

- Convert 4:2:0 to 4:2:2
- 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.

- Convert 4:2:2 to 4:2:0
- 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 o
_{x,y}, the FIR filter in the core convolves COEFk_VPHASE0, where k is the coefficient index, i_{x,y}are pixels from the input image, and [ ]M m represents rounding with clipping at M, and clamping at m. DW is the Data Width or number of bits per video component. N_{taps}is the number of filter taps.Figure 8. M

- Convert 4:2:0 to 4:4:4
- 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 Figure 8. The resulting computation is shown in the following equation.

Figure 10. NNext, the values are filtered according to Figure 1. The resulting computation is shown in the following equation.

Figure 11. O

- Convert 4:4:4 to 4:2:0
- This conversion performs decimation by 2 both vertically and horizontally.
This is equivalent to a 2D separable filter implemented by cascading the 4:4:4
to 4:2:2 block and the 4:2:2 to 4:2:0 block. Quantized horizontal filter results
are filtered by the vertical 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 the Figure 3. The resulting computation is shown in the following equation.

Figure 12. PNext, these values are filtered according to Figure 1. The resulting computation is shown in the following equation.

- Resampling Filters
- The upsampling and downsampling performed during the chroma format
conversion is implemented with low pass filters for the interpolation and
anti-aliasing.
The chroma resampling function offers a horizontal filter with a maximum of 10 taps and two phases, as well as a vertical filter with a maximum of 10 taps and two phases. For conversions requiring up/down sampling in both horizontal and vertical directions, 2D separable filters are offered.

The number of taps selected must be even (4, 6, 8, or 10). Depending on the conversion type and filter size selected, a subset of the coefficients can be used by setting the unnecessary coefficients to zero.

Each coefficient has 16 bits in 2's complement format: 4 integer bits (one sign bit) and 12 fractional bits. The sign bit is the MSB. For example, a coefficient with a value of 1 is represented with this bit vector`[0001000000000000].`

The coefficients should sum to exactly 1 to achieve unity gain. If they sum to less than 1, some loss of dynamic range is observed.

- Computation Bit Width Growth
- Full precision (DATA_WIDTH+16+log2(N
_{Taps}) bits) is maintained during the horizontal and/ or vertical FIR convolution operation.FIR filter outputs are rounded to DATA_WIDTH bits by adding half an output LSB in the full precision domain prior to truncation. Clipping and clamping of the output data prevents overflows and underflows. Data is clipped and clamped at 2

^{DATA_WIDTH}-1 and 0.

- Edge Padding
- The edge pixels of images are replicated prior to filtering to avoid image artifacts.