Channel (digital image)
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Color digital images are made of pixels, and pixels are made of combinations of primary colors. A channel in this context is the grayscale image of the same size as a color image, made of just one of these primary colors. For instance, an image from a standard digital camera will have a red, green and blue channel. A grayscale image has just one channel.
In the digital realm, there can be any number of conventional primary colors making up an image; a channel in this case is extended to be the grayscale image based on any such conventional primary color. By extension, a channel is any grayscale image the same size with the "proper" image, and associated with it.
It is important to understand that the channels are a conventional term used to refer to a certain component of an image. In reality, any image format can use any algorithm internally to store images. For instance, GIF images actually refer to the color in each pixel by an index number, which refers to a table where three color components are stored. However, regardless of how a specific format stores the images, discrete color channels can always be determined, as long as a final color image can be rendered.
The concept of channels is extended beyond the visible spectrum in multispectral and hyperspectral imaging. In that context, each channel corresponds to a range of wavelengths and contains spectroscopic information.
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[edit] Channel Types
Three main channel types (or color models) exist, and have respective strengths and weaknesses.
[edit] RGB
An RGB image has three channels: red, green, and blue. RGB channels roughly follow the color receptors in the human eye, and are used in computer displays and image scanners.
If the RGB image is 24-bit (the industry standard as of 2005), each channel has 8 bits, for red, green, and blue -- in other words, the image is composed of three grayscale images, where each grayscale image can store discrete pixels with conventional brightness intensities between 0 and 255. If the RGB image is 48-bit (very high resolution), each channel is made of 16-bit grayscale images.
[edit] RGB Color Sample
 A 24-bit RGB image |
 The RED channel of the original RGB image |
 The GREEN channel of the original RGB image |
 The BLUE channel of the original RGB image |
Notice how the grey trees have similar brightness in all channels, the red dress is much brighter in the red channel than in the other two, and how the green part of the picture is shown much brighter in the green channel.
[edit] CYMK
A CMYK image has four channels: cyan, magenta, yellow, and black. CYMK is the standard for print, where subtractive coloring is used.
A 32-bit CMYK image (the industry standard as of 2005) is made of four 8-bit channels, one for cyan, one for magenta, one for yellow, and one for black. 64-bit storage for CMYK images (16-bit per channel) is not common, given the fact that CMYK is usually device-dependent, whereas RGB is the generic standard for device-independent storage.
[edit] CMYK Color Sample
- As of 2005, the 32-bit CMYK image won't be displayed at all by major browsers--try saving the link below it to disk first. The RGB image from above is substituted in its place.
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A 32-bit CMYK image |
 The CYAN channel of the original CMYK image |
 The MAGENTA channel of the original CMYK image |
 The YELLOW channel of the original CMYK image |
 The BLACK (K) channel of the original CMYK image |
[edit] HSV
HSV, or Hue Saturation Value, stores color information in three channels, just like RGB, but one channel is devoted to brightness (Value), and the other two convey colour information. The value channel is exactly the same as the CYMK Black channel, or its negative.
HSV is especially useful in lossy video compression, as loss of color information less noticeable to the human eye. See Optimized channel sizes below.
[edit] Alpha Channel
The alpha channel stores transparency information--the higher the value, the more opaque that pixel is. No camera or scanner measures transparency, although physical objects certainly can possess transparency, but the alpha channel is extremely useful for compositing digital images together.
Bluescreen technology involves filming actors in front of a primary color background, then setting that color to transparent, and compositing it with a background.
GIF and PNG images are used extensively on the World Wide Web so that objects on web pages can appear to have an arbitrary shape even on a non-uniform background.
[edit] Bit Depth
In digitizing images, the color channels are converted to numbers. Since images contain thousands of pixels, each with multiple channels, channels are usually encoded in as few bits as possible. Typical values are 8 bits per channel or 16 bits per channel. Indexed color effectively gets rid of channels altogether to get, for instance, 3 channels into 8 bits (GIF) or 16 bits.
[edit] Optimized channel sizes
Images use a relatively large amount of digital "space". The huge majority of images/photographs being taken, generated, processed and/or reproduced are intended for human consumers. Given these two assumptions, scientists focused on saving digital space/bandwidth by creating image formats which yield good compromises between space usage and presentation. The most basic result of these studies is compressing the blue channel disproportionately more than the others, and compressing the green channel somewhere between red and blue. This type of "preferential" compression is the result of studies which show that the human retina actually uses the red channel to distinguish detail[citation needed], along with the green channel in a lesser measure, and uses the blue channel for background or environmental information.
Among other techniques, lossy video compression uses Chroma subsampling to reduce the bit depth in color channels (Hue and Saturation), while keeping all brightness information (Value in HSV).
16-bit HiColor stores red and blue in 5 bits, and green in 6 bits.
