RAW
维基百科，自由的百科全书
| -{zh-cn:扩展名;zh-tw:副檔名名}-: | .raf (富士) |
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| 类型: | 图像文件格式 |
Raw格式的文件包含着从数码相机或扫描仪的图像传感器上直接得到的仅经过最少处理的原始数据。Raw之所以这样定义，是因为这些数据实际上未经任何处理，可以任由你使用位图编辑器进行编辑，打印，或者在浏览器上直接显示。这些图像文件在实际应用前必须经过处理后并输出成RGB格式，诸如TIFF或JPEG。
目录 |
[编辑] Contents
RAW格式并不是单一标准的，不同的相机设备制造商使用各自特有的标准，这些标准一般都被认为是RAW格式。RAW格式的文件包含图像传感器上每个像素点的信息，一般被记录为12位或14位的数据。 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 known as Demosaicing, but this process is referred to by many manufacturers as Digital Development.
The contents of RAW files are often considered to be of 'higher quality' than the RGB converted results. Each pixel in RAW is represented by a higher range number. Transformations which affect brightness or color curve lose less information when performed on the raw data. This does not mean that an 8Mb RAW file is of higher quality than an 8Mb RGB file. It simply means that less information is lost in these particular transformations.
Other sensors, such as the Foveon X3 sensor capture information directly in RGB form.
[编辑] 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.
[编辑] 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).
The time taken in the image workflow is an important factor for choosing RAW instead of ready to be used formats.
[编辑] 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) and Image Broadway. 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). 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 directly to the system, as part of the ImageIO framework, which adds RAW support automatically 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.
Picasa, a free image editing and cataloguing program from Google, supports many RAW formats.
UFRaw is free software based on dcraw. It can be used as a Gimp plugin and is availabe for most operating systems.
[编辑] 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.
[编辑] See also
- Image file formats
- dcraw
[编辑] External links
- 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.
- jrawio Project jrawio: An Open Source Java program used to read RAW data from various digital camera models.
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
- Ken Rockwell: JPG vs RAW vs TIFF: Get it Right the First Time; arguments against raw
- 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?
