Understanding camera raw formats

Modern digital video cameras are based on a single-chip, CMOS sensor design. The camera sensor is built with a Bayer-pattern (mosaic) configuration. This uses a color filter array that separates the light hitting the sensor into red, green, and blue wavelengths. This filtered light is received by photosites, which individually react to either red, green, or blue light, depending on its filter. These light values are translated into monochrome data.

The most common filter array design deploys twice as many green photosites as red or blue. This is considered a GRGB design. The exception to this configuration is the newest sensor from Blackmagic Design. It uses an equal normal of red, green, and blue photosites. These are combined with a number of white (clear) photosites in the array that equals the total of red, blue, and green elements.

When a video camera records a standard, full-color signal, the internal electronics of the camera processes the raw, monochrome sensor data into RGB pixels. The first step is called deBayering or demosaicing. The information from green pixels is combined with the data from the surrounding red and blue pixels to generate the RGB video pixels. This process varies with each camera and is controlled by the proprietary color science developed by that camera vendor. Different sensors use a different number and size of photosites. The number of photosites will not directly correspond to the number of pixels in the RGB video file. Along with deBayering, the camera’s internal electronics will also handle such tasks as noise management and detail enhancement.

A camera raw recording bypasses some or all of the deBayering step, as well as any in-camera signal processing. The monochrome sensor data is recorded internally or externally as a movie or image sequence file. It can be uncompressed or use different compression algorithms. Any steps that would have been handled through in-camera processing are moved to post-production.

Most camera raw formats are proprietary to a particular camera vendor and some manufacturers have created their own processing applications. In order for the raw file to be natively supported within another host application, the original camera manufacturer must create a custom plug-in or a software development kit (SDK) that outside developers may use to add support for that codec. There is no common standard for which raw controls are exposed, but ISO, exposure, temperature, and tint are available in most.