American Cinematographer - June 2011 - 74

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Decoding Digital Imagers: Part 2
and vertically, if you look at them diagonally, you can see that there is a continuous stream of pixels. I marvel at the cleverness of the Bayer pattern, because what it’s effectively doing is diagonal sampling. Now, this is not intuitive, but what it adds up to is that it gives you the ‘beans’ in terms of the green-photosite sampling — you get the full benefit horizontally and vertically — but you lose resolution diagonally. That’s not intuitive. You would think that diagonal sampling would favor resolution diagonally.” Galt explains, “When you look around you in the real world, almost all the lines in our environment are either horizontal or vertical, so sampling in a diagonal direction is a good idea. You’re less likely to get aliases that way. In fact, it’s an old trick that was used in halftone photographic reproduction in newspapers. The early lithographers realized that if they rotated their [halftone] ‘screen’ by 45 degrees, the screen became less visible. That same concept applies to the Bayer pattern.” Alternative CFAs There are several alternative methods of color filtering a singleimager system to capture color information. Some of these alternatives are variations on the Bayer RGGB pattern, such as the RGBE [red, green, blue, emerald] filter arrangement, which splits half of the green-filtered photosites into alternating green and emerald [blue-green] filter overlays within the Bayer mosaic structure. The CYGM [cyan, yellow, green, magenta] CFA, however, introduces a wholly different color-pattern arrangement. Next to Bayer, perhaps the most popular CFA pattern in single-sensor digital motion-picture-imaging systems today is the striped-pattern CFA, which attacks the color-filtering concept in a different manner. With this CFA structure, the red, green and blue filters are arranged in vertical columns, with each featuring one continuous “stripe” of color. Red, green and blue are adjacent to one another, and this RGB pattern is repeated across the sensor. Panavision’s Genesis was the first camera to incorporate a Super 35-sized CCD imager with a striped CFA. With photosite dimensions measuring 5760 (H) x 2160 (V), the system uses “macro cell” groupings of two rows by three column stripes — one of each color — of its photosites to derive a full-bandwidth 1920 x 1080 RGB output. With this over-sampling structure, a single macro cell features two red, two green and two blue photosites to derive a single RGB digital-output pixel value. (See illustration below.) Global and Rolling Shutters Unlike film cameras, digital imagers do not require a physical shutter. However, all digital-imaging sensors perform some form of electronic shuttering — a prescribed interval of light gathering — by means of the sensor’s timing circuitry. There are two forms of electronic shuttering in today’s digital imagers: global and rolling. A global shutter exposes and captures the entire imager’s photosite array simultaneously. At the beginning of a set timing cycle, the entire frame of the sensor’s imaging area is exposed to the object-image being projected by the lens, and this image information is collected for a set period. When that predetermined period has elapsed, the sensor stops gathering light information and outputs its data before resetting itself. This is analogous to the way a film camera exposes a single frame of film. With a rolling shutter, the interval of information gathering does not occur simultaneously. A rolling shutter actually “exposes” different portions of the sensor’s frame at different points in time, rolling its scan progressively from top to bottom through the frame. Because a rolling shutter collects its exposure information row by row at different moments to create a single image frame, it can cause problems if you’re photographing certain types of motion or working in rapidly changing light levels. “With a rolling shutter, motion can be a problem both horizontally and vertically,” states Mahrer. “You can get sloping or tilting [distortion known as ‘skew’] when panning, or when you have fast-moving objects in frame. A worst-case scenario would be taking a CMOS camera with a slow rolling shutter into a badly vibrating helicopter. The whole image would ‘wobble’ and look like you’re using a lens made of Jell-O. “Global shuttering certainly is much more advantageous,” continues

June 2011

American Cinematographer

American Cinematographer - June 2011

Table of Contents for the Digital Edition of American Cinematographer - June 2011