RAW image format
From Wikipedia, the free encyclopedia
| RAW image file | |
| File extension: | .raf (Fuji) |
|---|---|
| Type of format: | Image 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 be used with a bitmap graphics editor or printed. Normally, the image will be processed by a raw converter in a wide-gamut internal colorspace where precise adjustments can be made before conversion to an RGB file format such as TIFF or JPEG for storage, printing, or further manipulation.
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[edit] Contents
There is no single RAW format: different manufacturers use their own proprietary formats, which are collectively known as RAW format. RAW files contain pixel data from the image sensor, usually at 12 or 14 bits per sensor bucket. The sensor is almost invariably overlaid with a so-called Bayer filter, consisting of a mosaic of red, blue and green filters in alternating rows of RG and GB. Given that three colors fit uncomfortably in a rectangular grid, green was chosen to be doubly present, since the human eye is more sensitive to it. Green also often serves as the luminance channel, and as the dominant channel for in-camera B&W conversions. To retrieve an image from a RAW file, this mosaic of data must be converted into a full RGB image. This is known as Demosaicing, but is referred to by many manufacturers as Digital Development.
The contents of RAW files are often considered to be of 'higher quality' than the final converted results, usually JPG. Each pixel in a RAW file has a greater bit-depth, and can thus store more subtle variations in color, and has more detail specifically in the shadow areas. Hence, large transformations of the data, such as increasing the exposure of a dramatically under-exposed photo, result in less visible artifacts. This leaves more scope for both corrections and artistic manipulations, without resulting in images with visible flaws such as posterization.
One variation on the Bayer scheme is the RGBE sensor of the Sony DSC-F828, which experimented with exchanging the Green in the RG rows with Emerald (cyan). This was claimed to increase the gamut of the response, but was gone again in the follow-up model. Other sensors, such as the Foveon X3 sensor capture information directly in RGB form, having three photosites in each pixel, one for each colour component.
[edit] 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.
- Camera raw files have 12 or 14 bits of brightness information. But you cannot compare this number alone to other methods. JPEG stores a brightness gradient in an 8-bit number every 4 or 8 pixels and stores color values even more infrequently depending on the parameters used. Because of this JPEG loses fine details and is ill-suited for major color or brightness changes. By comparison the mosaicing used by the Bayer filter in raw files changes colors every 3-4 pixels and brightness every 1-2 pixels producing much finer resolution detail in a same size image. And because it is 12-bit each of these values are far more precise.
- The working color space can be set to whatever is desired.
- Different demosaicing algorithms can be used, not just the one coded into the camera.
[edit] Drawbacks
Camera raw files are typically 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).
There is still no widely-accepted standard RAW format; Adobe's DNG format has been put forward as a standard, but is not adopted by major camera companies. Numerous different RAW formats are currently in use; new RAW formats keep being developed, and others orphaned.
More specific software may be required to open RAW files on some systems, as opposed to standard formats like JPEG or TIFF.
The time taken in the image workflow is an important factor for choosing between RAW and ready-to-use image formats.
[edit] 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 Photoshop Lightroom. As of 2006, a few camera manufacturers have started to announce support for DNG in newer camera models, including Leica & Pentax (native camera support) and Hasselblad (export). The Leica Digital-Modul-R (DMR) was first to use DNG as its native format.
Microsoft's Digital Image 2006 is able to recognize and organize RAW image formats such as .crw, .cr2, and .nef, which are file formats produced by Canon and Nikon.[1] For the Windows platform, there is a free download available for Windows XP that integrates viewing and printing into other included photo tools.[2]
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 v10.4, which added RAW support directly to the system, as part of the ImageIO framework, which adds RAW support automatically to the majority of Mac OS X applications (such as Preview, Mac OS X's 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.
There are many other "RAW workflow applications" designed to provide efficient post-processing of Raw images, including Helicon Filter, Phase One's Capture One and Bibble Labs' Bibble Pro. Like Apple Aperture and Adobe Photoshop Lightroom, these programs provide sophisticated controls for processing the information stored in the Raw file and converting RAW files to JPEG or TIFF. Picasa, a free image editing and cataloging program from Google, can read and display many RAW formats, but like iPhoto, Picasa provides only limited tools for processing the data in a RAW file.
UFRaw is free software based on dcraw. It can be used as a GIMP plugin and is available for most operating systems.
RawShooter Essentials 2005/6 was free software developed by Pixmantec. In 2006 Adobe Systems Inc acquired the assets of Pixmantec ApS. RawShooter Essentials is no longer be updated (The last update added support for the Canon 5D and the Nikon D200). It could still be downloaded as a free product until Adobe's Photoshop Lightroom 1.0 was released in March 2007. The software was fully featured, including wide support for various RAW formats, file priority sorting and batch processing.
[edit] Processing
There is no single standard algorithm for converting data from a Bayer filter or Foveon sensor into RGB format; a number of different algorithms have been proposed, and some have been patented in the USA. Different programs may give slightly different results, better or worse subjectively, 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. Image noise reduces the effectiveness of compression algorithms, so rudimentary noise reduction is often applied to the raw image before compression, to reduce file sizes.
[edit] References
[edit] See also
[edit] External links
- shootcameraraw A website advocating the raw file format.
- Open RAW A working group of photographers and other people interested in advocating the open documentation of digital camera raw files
- Adobe: Understanding Raw Files(PDF); Background on how camera sensors treat raw files
- DCraw Raw Digital Photo Decoding in Linux Dave Coffin: software to process RAW data from various models of camera. Also shows all current cameras that support RAW. (supported models)
- libopenraw, an ongoing effort to provide a free software implementation for camera RAW files decoding.
- jrawio: An Open Source Java library for reading camera raw files from various digital camera models.
- RAW Data 48 bit RAW data processing (High Dynamic Range Imaging)
- Sample Olympus RAW file (15MB)
- Pixmantec ApS Free download of RawShooter Essentials, a RAW batch converter, until Adobe's Lightroom is released (Early 2007).
Photographers' views
- Bob Atkins: Raw, JPEG, and TIFF; common file formats compared.
- Sean T. McHugh: RAW Files with a Digital Camera; Why should a photographer use raw? Advantages and disadvantages of raw formats, as compared to JPEG
- John Roling: A RAW Deal: Using the RAW image format; explanation and arguments for raw formats.
- Jim M. Goldstein: RAW vs JPEG: Is Shooting RAW Format For Me?
