See my Sigma mirrorless wish list at B&H Photo.
See Sigma SD Quattro Mirrorless Cameras for more info as well as my in-depth reviews of Sigma DP Merrill and Sigma dp Quattro cameras.
From the Sigma marketing page under “The special characteristics of the 1:1:4 structure”.
High resolution is a necessary condition of outstanding image quality. At the same time, adding megapixels to increase resolution also results in a bloated image file size. The risk of file bloat is especially pertinent in the case of image data in the .raw format containing RGB information for every pixel location.
The Foveon X3 Quattro direct image sensor retains the three-layer vertical color capture of previous Foveon sensors while featuring the new ability to capture luminance information in the top layer and apply that information to the middle and bottom layers. This new structure has been the key to preventing file bloat, which may eventually have proved a problem for the 1:1:1 structure of the previous generation of Foveon sensors.
That’s rich, given the new X3I format in which files are 320-390MB each, a massive file bloat that about doubles the Pentax K1 SuperRes pixel shift mode raw files, which are from a 36MP sensor (vs 19.6MP for Sigma dp Quattro).