Demosaicking for a color image sensor with removal of blur due to an optical low-pass filter
Takashi Komatsu, Takahiro Saito · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 2004
As an optical low-pass filter, a doubly refractive crystal device is used and performs separation of its incident light into the normal light and the abnormal light shifted to the slightly different direction to the normal light. The actual optical low-pass filter is formed by combining two types of doubly refractive crystal device; the one separates its incident light into two traveling directions horizontally spaced each other by one pixel, and the other does vertically spaced by one pixel. The filter cannot sharply cut off high frequency components and it reduces the frequency components near the Nyquist frequency. Thus, images projected on the imaging surface are blurred. This paper presents a demosaicking method that can simultaneously remove image blurs caused by the optical low-pass filter. Most of the existing demosaicking methods do not try to remove the image blurs, whereas our sharpening approach to the demosaicking employs the Landweber-type iterative algorithm. For our sharpening-demosaicking approach, the Bayer’s pattern of the RGB primary color filter array is not necessarily proper, and hence we study another color-filter pattern, namely the WRB filter array that is preferable to the RGB filter array, where the W-filtering means that all the visible light passes through it. Our mathematical formulation of the sharpening-demosaicking has the form of the least square problem, but there exist multiple different least square solutions. To avoid its ambiguity, in the spatial frequency domain we introduce the pass-band limitation corresponding to the sub-sampling pattern of the mosaicking of color filters, into the iterative algorithm.