About RAW Images

The term raw, sometimes capitalized RAW, can be used in at least four fundamentally different ways, in addition to any number of minor distinctions. The four major usages are:

RAW 1 -- The vague concept of an image that hasn't been processed yet.
RAW 2 -- A form of low-level circuit-board modification that can be performed on some webcams.
RAW 3 -- A file format in which absolutely no meta-information is provided which would be necessary in order to properly interpret the file contents as an image.
RAW 4 -- A file format which stores the direct output of a digital CCD sensor without performing any alterations on the data.

Here follows a brief description of each of the four types...

RAW 1

Raw, usually written lowercase, is used throughout the documentation and throughout the astrophotography community to refer to a single unstacked unprocessed image that has been brought into the workspace. Such images have not, for example, undergone alignment, darkframe subtraction, flat field division, denoising, etc. Since this usage of raw refers to a general notion of an image and not to a specific file format, it makes no prescription on the incoming file format. Such images can originate from digital camera RAWs (type 4 below), TIFFs, JPEGs, or any other conceivable file format.

RAW 2

RAW refers to a low-level circuit board (soldering) modification on webcams. It was developed by Steve Chambers and permits the capture of images comparable to the RAW 4 format described below, but without all the proprietary corporate malarkey. This usage of the term RAW is briefly referenced in the documentation in the section on DeBayering.

RAW 3

RAW refers to a file format in which pixel values are laid out in an single contiguous array in the file with no meta-information (as would often be stored in a file header for example) and for which no agreed-upon convention or standard indicates the file's structure. For example, there is no explicit way to know the dimensions of the image, the bit depth used to record color values, the byte order (big vs. little endian) or whether the RGB channels are interlaced or separated. Such information must be provided at the time a RAW file is opened so that the file can be interpreted. Many image programs support this file format including Graphic Converter, Adobe Photoshop, and Keith's Image Stacker.

RAW 4

RAW refers to the proprietary file formats used to store CCD data on digital cameras without any processing, such as gamma correction, bit depth projection, or JPEG compression. Every camera manfacturer (and perhaps every camera model) uses a different RAW file format and those formats are private corporate entitites, and thus are relatively inaccessible to most programmers, short of reverse engineering. Professional software packages, such as Photoshop, are able to support RAW files because companies like Adobe pay licensing fees to the camera companies to use their file formats. As such, these kinds of RAW files are inaccessible by Keith's Image Stacker.