American Cinematographer - June 2011 - 72

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Decoding Digital Imagers: Part 2
the middle and live with a certain amount of green aliasing and a higher amount of red and blue aliasing,” he concludes. [See figure #3.] “You will find this aliasing with a test chart very easily, but fortunately, the real world is pretty forgiving!” Bayer-pattern single-sensor systems are being used more and more in digital-imaging platforms today, including Arri’s D-20, D-21 and Alexa; Red Digital Cinema’s One and Epic; Vision Research’s Phantom cameras; and Weisscam’s HS-2. Sony, long a purveyor of CCD technology, has also begun using Bayer-pattern CMOS sensors with such cameras as the PMW-F3, as well as the new F65, which uses an 8K diagonally rotated Bayer-pattern super 35 CMOS sensor. Most DSLR cameras capable of recording HD video also utilize Bayerpattern sensors. Demosaic Processing After the full-spectrum incident light projected by the lens onto the sensor has passed through the colormosaic filter structure and is collected by the photosites below, the camera’s digital image-processing functions interpolate that information and assemble usable color information for each derived digital-output pixel. With a Bayer-pattern imager, this process is called demosaicing. The process of interpolating, or “deBayering,” the color-matrix information from a Bayer CFA sensor can first be looked at from a single, fourpixel cluster with two diagonally oriented green photosites bracketed by a single value for red and a single value for blue. Using this information to derive full RGB values for each of the photosites, however, cannot yield 4:4:4 sampling. (For more details on color sampling, see “The Color-Space Conundrum,” AC Jan. ’05 and April ’05, or visit www.theasc.com/magazine/ jan05/conundrum/index.html.) In this worst-case scenario, these four pixels would represent a color sampling of 4:2:0. One red pixel would

A breakdown of the Bayer CFA into its separate RGB color paths.

combine and interpolate a full-spectrum composite of red, green and blue values for each point in the digital-output pixels. Today, many systems allow this demosaic processing to be done incamera (generating a video-output signal that can be monitored on set and recorded to tape or tapeless formats), while others save the raw, unprocessed Bayer data for later processing, demosaicing and transcoding into various delivery specs. “You have a fundamental dilemma with the Bayer CFA that you do not have with a three-chip camera,” notes Thorpe. “With the Bayer pattern, you will see lots of green photosites in an odd quincunx structure, each surrounded by two reds and two blues. If you break them apart to look at them separately, you will find the resulting patterns for R, G and B [pictured above]. “We see that with the Bayer pattern, half the samples, both horizontally and vertically, are green, while the remaining half are allotted to red and blue. The red and blue patterns are arranged in a classic cardinal structure
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— they don’t have that quincunx look — but there are big gaps between them. The fill factor is very big in the red and the blue. The dilemma with the Bayer pattern is that depending on the total number of photosites, you may have a good sampling resolution with the green, but only half that resolution with the red and blue. “With any sampling,” he continues, “you have to contend with aliasing, but with a single-sensor system you can only use one Optical Low-Pass Filter. So which sampling resolution do you design it for? Do you favor the green or the red and blue? If you opt to filter for the green, you’ll get good green resolution performance with a degree of green-aliasing control, but there will be a whole lot of aliasing on the red and the blue. [See figure #1.] “The other option is to use an Optical LPF that properly handles the red and blue aliasing, leaving you a respectable red and blue response but really clobbering the green resolution. [See figure #2.] “I recommend a compromise: use an Optical LPF that falls somewhere in
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American Cinematographer - June 2011

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