In the D21 Arri uses a CMOS chip with a Bayer mask, which is one of the many forms of Color Filter Arrays (CFA) that have been devised to allow three colors to be recorded with a single chip. On a Bayer mask chip each pixel has one color filter, one next to the other. The first row of pixels has green and red color filters (green, red, green, red, green, red, green, etc. ), the next row has blue and green filters (blue, green, blue, green, blue green, etc. ) and the next one again green and red, and so on.
row 1: G R G R G R G R G R ...
row 2: B G B G B G B G B G ...
row 3: G R G R G R G R G R ...
row 4: B G B G B G B G B G ...
...
The raw "Bayer data" is essentially a monochrome image where each pixel corresponds to only one specific color value. In order to get a color image, the colors have to be "reconstructed" based on the Bayer data.
f you look at the pixels you will notice that each red pixel, for instance, is surrounded by four green and four blue pixels. Also, because there is an overlap in the color spectra of red, green and blue, the available red value is at least in part the result of light in another color. Based on the knowledge of what the colors and values of those neighbor-pixels are, and based on the knowledge of the overlap in the color spectra, it is now possible to work out (reconstruct) what the green and blue values for that red pixel should be.
This process is more accurate than the interpolation used to increase the size (i.e. pixel count) of an image. In interpolation, completely new pixels are "made up" based on what the neighboring pixels look like. In Bayer data reconstruction we already have pixels, we just don't know two of the three color values. Since we do know the colors and values of neighbor pixels and since there is a color spectrum overlap, we can reconstruct the missing information very accurately.
Please note that the actual color reconstruction is more complicated than the method described here. For instance, to determine a given color value for a given pixel we use more than just the eight neighboring pixels. Furthermore, it is also possible to improve the result by incorporating certain assumptions about real world images in the algorithms (e.g. colors coincide at edges, etc.). We have simplified the process in this description to aid in understanding.
But what about resolution? First of all, we have to clearly define resolution. Resolution tells us how small the smallest structures (e.g. alternating black and white stripes) are that an optical or opto-electronic system is capable of reproducing. In digital photography, there is a tendency to describe resolution in terms of the number of pixels on the chip. Depending on the technology used however, the actual pixel count of a chip does not directly correspond to the resolution the system is capable of reproducing. The D21, for instance, is designed to accurately reproduce images at HD resolution (1920 horizontal pixels). In order to achieve this goal, a Bayer mask CMOS chip of a higher pixel count is necessary.
On the Bayer mask chip itself the full number of pixels is not available for each color. For a 2880 x 2160 chip, the red channel for instance does not have a resolution corresponding to 2880 x 2160 pixels. One could assume that since every second pixel is red in every second row, we have half the resolution for red (1440 x 1080). But that is not accurate either, since for most natural images the missing color pixel values can be reconstructed very accurately, so the resolution of the red channel is somewhere between 2880 x 2160 and 1440 x 1080.
Our goal with the D21 design is to output a very high quality HD image with a resolution corresponding to 1920 horizontal pixels. In order to achieve such an image output from a Bayer mask chip we need substantially more than 1920 horizontal pixels, which is the reason the chip's pixel count (2880 x 2160) is much higher than the desired image output resolution. The raw Bayer data at 2880 x 2160 goes through the color reconstruction process to fill in the missing color information and is downscaled to a pixel count that corresponds more closely to its actual resolution. This allows the D21 to create a high definition image that looks as good as if not better than the images produced by CCD high definition cameras.
Marc Shipman-Mueller
Courtesy of Arri
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