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Color Filter Array
Our eyes sense color by determining the amount of red, green, and blue contained within [1]. A computer monitor displays color based on the same principle - by mixing different amounts of red (R), green (G), and blue (B) light. Move the sliders in Figure 5 to control the amount of red, green, and blue that is mixed to form a custom color.
Most image sensors use a color filter array (CFA) to detect color [2]. A CFA is a grid of tiny color filters placed above the pixels. A red filter, like the one shown in Figure 6, allows just the red component of the incoming light to pass through. That pixel, then, can measure the red component of the incoming light, but no more. Similarly, green and blue filters allow pixels to measure the green and blue components of the incoming light. A popular design of the grid is the Bayer pattern [3] shown in Figure 7. It is a repeating 2x2 pattern that includes two green filters, one blue filter, and one red filter. Experiments have shown that our eyes are more sensitive to green than red and blue, which is why there are more green filters in the pattern [3].
Figure 8a shows a 3x3 grid of pixels. The number at each pixel represents the amount of red, green, or blue light it detected. The two colors that are missing at each pixel must be estimated to form a full color image. For example, the red component of the center pixel in Figure 8a is 175, but we do not know the pixel's green or blue components. However, we can estimate them from the green and blue measurements taken at the neighboring pixels. This process, seen in Figure 8b, is known as demosaicing or color interpolation ("interpolation" simply means estimating missing information). Figure 8b shows an easy way of estimating the green and blue components of the center pixel; the green and blue measurements at the neighboring pixels are averaged together. Click on the Play button in Figure 9 to see demosaicing in action.
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