Fujifilm X-Trans color filter array layout has interesting symmetries

Fujifilm have revealed the X-Trans color filter array layout used on the X-Pro1 camera.

http://www.fujifilm.com/products/digital_cameras/x/fujifilm_x_pro1/features/

Two things come to mind:
1. The reds, greens and blues aren't quite balanced: there is a small excess of green. Not a big issue and perhaps a small advantage if it helps luminance contrast (acutance).

Of the 36 pixels in each 6x6 block 20 are green, 8 are blue and 8 are red. In a Bayer array 6x6 block there would be 18 green, 9 blue and 9 red. Green is where most of the luminance info (and contrast detail)

2. The array has an interesting set of symmetries: the 6x6 block is composed of four 3x3 blocks (of 2R5G2B) with the same layout which are related to each other by 90 degree clockwise rotations as you go around the four blocks in a clockwise order.
This symmetry drives the (small) imbalance in the green which appears in a center 2x2 block of green pels.

Aside from the central 2x2 block all the other 2x2 blocks in the 6x6 array have set of RGGB so they can sample color info with about half the resolution of the array (as you would expect).
When you tile the array you can see the "all green" 2x2 block repeats every 3 pels in both vertical and horizontal directions. The outside of this block is decorated with repeating GRB pels as you go around the "all green" block.

I suspect those 2x2 blocks of green pels improve the luminance sampling rate (reducing aliasing issues) or are possibly also useful (in pixel binning) for CD AF or scene brightness measurement?

All of this must come with a new demosaicing algorithm that makes the best use of the data provided by this "odd" CFA.

Of course Fuji gloss of the other aliasing issues (other than color moire) that come from removing the optical low pass filter but this layout may help more than just color moire. It won't be perfect (you can't remove alias artifact after the fact) but it may be "good enough".

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