To register color images, digital camera sensors are equipped with a filter matrix. The vast majority of cameras use the so-called Bayer matrix, which is composed of 50% green filters (green being the color to which the human eye is most sensitive), 25% red filters, and 25% blue filters. Placed in front of the photosites, this matrix creates rows of alternating blue/green and red/green pixels.
The X-Trans sensor in most of Fuji cameras does not use a Bayer matrix, but rather a very different matrix whose pixels are arranged in rows of three colors — a technology inspired by the naturally irregular distribution of silver halide crystals on film.
The demosaicing algorithms of DxO PhotoLab 4 that allow it to process digital camera RAW photo files were designed to process the output of sensors using the Bayer matrix. Further, some of the algorithms that are used for denoising, exposure, or white balance corrections are also based on the Bayer matrix.
As DxO PhotoLab 4 is not currently able to process RAW files from Fuji cameras with X-Trans sensor, DxO Labs has chosen instead to focus — as it always has — on the best image quality, and thus for the moment has decided not to support cameras for which it cannot provide results that perfectly fulfill this criterion.