4:4:4 chroma sampling

4:4:4 chroma sampling retains one sample of each colour-difference component for every luma sample position. In a YCbCr image, the Y, Cb and Cr sample grids therefore have equal counts. The notation describes sampling density, not the number of bits in a sample.^[ref]

For an original $2\times 2$ eight-bit image, there are four Y samples, four Cb samples and four Cr samples: 12 active bytes. Declare the row-major component arrays to beEvery image location has its own colour-difference values. Averaging each pair of Cb values would discard the alternating pattern; 4:4:4 sampling itself has not performed that averaging.

A planar pixel format can store those three arrays separately. A packed pixel format can instead put each location's three samples together. Both can represent the same 4:4:4 data, while dimensions, framebuffer stride and component order determine how to address the stored bytes.

Full component sampling also does not promise a lossless pipeline. YCbCr conversion can include rounding, and an image compression method can discard information even when its input uses 4:4:4. Conversely, a decoder that expands 4:2:0 chroma subsampling to a full grid by chroma reconstruction has not necessarily restored the original missing colour detail. Equal output counts and preservation of the original samples are distinct properties.