Passive, active, and digital filters (3ed., CRC, 2009) - tiera.ru

28-2 Passive, Active, and Digital FiltersSensorFilterBeam-splitterCFASensorLensLensScene(a)FIGURE 28.1Scene(b)Illustration of optical paths for multichip and single-chip digital cameras.FIGURE 28.2 Several CFA designs are illustrated. From left to right: (a) This is the most commonly used CFApattern: the Bayer CFA pattern. It consists of red, green, and blue samples. It leads to very good color reproductionperformance. (b) This is the Bayer pattern with subtractive primaries: yellow, magenta, and cyan. The color filtershave high transmittance values; therefore, good performance in low-light conditions is expected. (c) This pattern usesred, green, blue, and emerald. It is recently used in some Sony cameras. (d) This is a pattern commonly used in videocameras; it consists of yellow, magenta, cyan, and green. (e) This pattern consists of yellow, cyan, and green filters,and unfiltered pixels. The unfiltered pixels improve light sensitivity. (f) This is a pattern that is introduced veryrecently by Kodak. It has red, green, blue, and unfiltered pixels.A variety of patterns exist for the color filter array. Some of these patterns are illustrated in Figure 28.2.Among these, the most common array is the Bayer color filter array. The Bayer array measures the greenimage on a quincunx grid and the red and blue images on rectangular grids. The green image is measuredat a higher sampling rate because the peak sensitivity of the human visual system lies in the mediumwavelengths, corresponding to the green portion of the spectrum (see Figure 28.3). Although this chapterdiscusses the demosaicking problem with reference to the Bayer CFA, the discussions and algorithms canin general be extended to other patterns.The simplest solution to the demosaicking problem is to apply a standard image interpolationtechnique to each channel separately. However, this neglects the correlation among color channels andresults in visible artifacts. For example, in Figure 28.4, Bayer sampling is applied on a full-color image andlater bicubic interpolation is applied on each channel. The resulting image suffers from color artifacts.This result motivates the need to find a specialized algorithm for the demosaicking problem. There havebeen many algorithms published on this topic; this chapter surveys the main approaches.28.2 Imaging ModelMost demosaicking algorithms model the imaging process as subsampling from a full-color image to amosaicked data. This is a sufficient model when the goal is only to estimate the missing color samples.(When the goal is to obtain a higher resolution image, then the modulation transfer function of thecamera should also be taken into account.)