Colour filter array decorrelation.

A Bayer mosaic interleaves four colour channels on a 2×2 grid, so horizontally adjacent samples measure different colours. Predictive image coders assume neighbouring samples are correlated, and that assumption is violated on every step — which is why compressing a mosaic as if it were a grayscale image leaves a large amount on the table.

Splitting the mosaic into four half-resolution planes, one per CFA position, restores the assumption: within a plane, every neighbour measures the same colour.

Measured on a Hasselblad X1D 3FR (8384×6304, 16-bit, ISO 400), lossless JXL:

12.8% smaller, and faster — the planes are quarter-size and encode in parallel. General-purpose compressors cannot do this because they do not know the data is a mosaic.

Why this operates on bytes

The transform is a pure reordering: samples are moved, never read. So it needs neither the CFA pattern (RGGB vs BGGR vs …) nor the byte order — which keeps it exactly reversible for any sensor, and avoids materialising the image as u16.