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Raw image format

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RAW image file
Filename extension
.raf (Fuji)
.crw .cr2 (Canon)
.mrw (Minolta)
.nef (Nikon)
.orf (Olympus)
.dng (Adobe)
(Open raw) etc.
.ptx (Pentax)
Type of formatImage file formats

A raw image file contains minimally processed data from the image sensor of a digital camera or image scanner. Raw files are so named because they are not yet processed and ready to use by a bitmap graphics editor, printed, or displayed by a typical web browser. The image must be processed and converted to an RGB format such as TIFF or JPEG before it can be manipulated.

Contents

Raw files contain pixel data from the image sensor usually at 12 or 14 bit per an individual sensor bucket. These pixels are a mosaic of either red, blue or green values. The sensor is filtered with dye to direct the correct color of light into each bucket, this is called a Bayer filter. To retrieve an image from a RAW file this mosaic must be converted into an RGB image. This is know as Demosaicing, but this process is refered to by many manufacturers as Digital Development.

The contents of RAW files are often times considered to be of higher quality than the RGB converted results. Each pixel in RAW is represented by a higher range number. Tranformations which affect brightness or color curve lose less information when performed on the raw data. This does not mean that a 8Mbit RAW file is higher quality than an 8Mbit RGB file. It simply means that less information is lost in these particular transformations.

In another new type of sensor no demosaicing is necessary. The Foveon X3 sensor captures information directly in RGB form.

Benefits

Nearly all digital cameras can process the image from the sensor into a JPEG file using settings for white balance, color saturation, contrast, and sharpness that are either selected automatically or entered by the photographer before taking the picture. Cameras that support raw files save these settings in the file, but defer the processing. This results in an extra step for the photographer, so raw is normally only used when additional computer processing is intended. However, RAW permits much greater control than JPEG for several reasons:

  • Finer control is easier for the settings when a mouse and keyboard are available to set them. For example, the white point can be set to any value, not just discrete values like "daylight" or "incandescent".
  • The settings can be previewed and tweaked to obtain the best quality image or desired effect. (With in-camera processing, the values must be set before the exposure). This is especially pertinent to the white balance setting since color casts can be difficult to correct after the conversion to RGB is done.
  • Raw files can store up to 14 independent bits of information per pixel, whereas JPEG stores only 8-bit values which have been lossily compressed, leading to the loss of fine detail and a lack of contrast, especially in darker areas of the image.
  • Different demosaicing algorithms can be used, not just the one coded into the camera.

Drawbacks

Camera raw files are 2-6 times larger than JPEG files. Some raw formats do not use compression, some implement lossless data compression to reduce the size of the files without affecting image quality and others use lossy data compression where quantization and filtering is performed on the image data. This avoids or reduces the compression artifacts inherent in JPEG, but means that fewer images can fit on a given memory card. It also takes longer for the camera to write raw images to the card, so fewer pictures can be taken in quick succession (affecting the ability to take, for example, a sports sequence).

The time taken in the image workflow is an important factor for choosing RAW instead of ready to be used formats.

Software support

Cameras that support raw files typically come with proprietary software for conversion of their raw format to TIFF or JPEG. Other conversion programs and plugins are available from vendors that have either licensed the technology from the camera manufacturer or reverse-engineered the particular raw format. A portable open source program, dcraw, supports most raw formats and can be made to run on operating systems such as Unix not supported by most commercial software.

Raw file formats are proprietary, and differ greatly from one manufacturer to another, and sometimes between cameras made by one manufacturer. In 2004 Adobe Systems published the Digital Negative Specification (DNG), which is intended to be a unified raw format. Adobe Photoshop CS2 contains extensive support of RAW as does Adobe Lightroom (beta version at time of writing 2006). As of 2005, a few camera manufacturers have announced support for DNG, including Leica (native camera support) and Hasselblad (export). Other manufacturers, however, appear to have little interest making their raw files easier to read: cameras from Canon, Nikon, Sony and others include elements of encryption designed to make it harder for others to decode the format. The Leica Digital Modul-R (DMR) was the first camera to use DNG as its native format.

Microsoft too is now including support for RAW, releasing thumbnail viewers, and it appears that they plan on embedding support into future versions of Windows.

In 2005, Apple Computer introduced several products which offered RAW file support. In January, Apple released iPhoto 5, which offered basic support for viewing and editing RAW files. In April, Apple introduced a new version of its operating system, Mac OS X 10.4 Tiger, which added RAW support to Preview, its PDF and image viewing application. Finally, in October, Apple released Aperture, a photo post-production software package for professionals whose chief feature is full support for RAW files.

Picasa, a free image editing and cataloguing program from Google, supports many RAW formats.

Processing

There is no single standard algorithm for converting data from a Bayer filter or Foveon sensor into RGB format. Instead, a number of different algorithms have been proposed, and some have been patented in the USA. Thus, different programs are likely to give slightly different results, of better or worse subjective quality, for any particular image.

RAW files are sometimes referred to as CCD-RAW (even for CMOS sensors).

Although the term "raw" describes files in the classical sense of "raw data" vs. "cooked data", raw files typically are slightly processed in the camera. In general, this processing is limited to algorithms that require direct access to the camera's hardware. This includes "long exposure noise reduction" (aka "dark frame subtraction") and the mapping out of "hot" (too bright) or "dead" (too dim) pixels. It also often includes rudimentary noise reduction. Noise reduces the effectiveness of compression algorithms, so files are compressed more efficiently when noise reduction is applied before compression.

See also

Photographers views

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