Demosaicking: Color Filter Array Interpolation in Single-Chip Digital Cameras

Digital cameras have become popular and many people are choosing to take their pictures with digital cameras instead of film cameras. When a digital image is recorded, the camera needs to perform a significant amount of processing to provide the user a viewable image. This processing includes white balance adjustment, gamma correction, compression and more (reference “Color Image Processing Pipeline in Digital Still Cameras” in this issue). A very important part of this image processing chain is color filter array interpolation or demosaicking. A color image requires at least three color samples at each pixel location. Computer images often use red, green, and blue. A camera would need three separate sensors to completely measure the image. Using multiple sensors to detect different parts of the visible spectrum requires splitting the light entering the camera so that the scene is imaged onto each sensor. Precise registration is then required to align the three images. These additional requirements add a large expense to the system. Thus, many cameras use a single sensor array with a color filter array. The color filter array allows only one part of the spectrum to pass to the sensor so that only one color is measured at each pixel. This means that the camera must estimate the missing two color values at each pixel. This process is known as demosaicking. Several patterns exist for the filter array. The most common array is the Bayer color filter array, shown in Figure 1. The Bayer array measures the green image on a quincunx grid and the red and blue images on rectangular grids. The green image is measured at a higher sampling rate because the peak sensitivity of the human visual system lies in the medium wavelengths, corresponding to the green portion of the spectrum. Other patterns exist the Nikon Coolpix 990 uses a CMYG grid, where each of the four images (cyan, magenta, yellow, green) are sampled using rectangular grids. A CMY-based system has the advantage of