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((Refimprove|date=December 2018))
[[Color]] digital images are made of [[pixel]]s, and pixels are made of combinations of [[primary color]]s represented by a series of code. A '''channel''' in this context is the [[grayscale]] image of the same size as a color image,((Citation needed|date=January 2021)) 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 [[geographic information system]]s, channels are often referred to as '''raster bands'''.<ref>((cite web
| url=http://help.arcgis.com/en/geodatabase/10.0/sdk/arcsde/concepts/rasters/basicprinciples/bands.htm
| title=Raster Bands
| work=ArcSDE SDK C and Java APIs 10.0
| publisher=[[ArcGIS]]
| access-date=2020-07-28))</ref> Another closely related concept is '''feature maps''', which are used in [[convolutional neural network]]s.

==Overview==
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 of the same dimension as and associated with the original image((Citation needed|date=January 2021)).

''Channel'' is 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, [[Graphics Interchange Format|GIF]] images actually refer to the color in each pixel by an [[CLUT|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 [[multi-spectral image|multispectral]] and [[hyperspectral]] imaging. In that context, each channel corresponds to a range of wavelengths and contains [[spectroscopic]] information. The channels can have multiple widths and ranges.

Three main channel types (or [[color model]]s) exist, and have respective strengths and weaknesses.

===RGB images===
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 display]]s and [[image scanner]]s.

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 images (one for each channel), where each image can store discrete pixels with conventional brightness intensities between 0 and 255. If the RGB image is 48-bit (very high color-depth), each channel has 16-bit per pixel color, that is 16-bit red, green, and blue for each per pixel.

====RGB color sample====
<gallery>
Image:Channel digital image RGB color.jpg|A 24-bit RGB image
Image:Channel digital image red.jpg|The red channel, displayed as grayscale
Image:Channel digital image green.jpg|The green channel, displayed as grayscale
Image:Channel digital image blue.jpg|The blue channel, displayed as grayscale
</gallery>
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.

===YUV===
[[YUV]] images are an [[affine transformation]] of the RGB colorspace, originated in broadcasting. The Y channel correlates approximately with perceived intensity, whilst the U and V channels provide colour information.

===CMYK===
A [[CMYK color model|CMYK]] image has four channels: cyan, magenta, yellow, and key (black). CMYK is the standard for print, where [[subtractive color]]ing 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 key color (typically is black). 64-bit storage for CMYK images (16-bit per channel) is not common, since CMYK is usually device-dependent, whereas RGB is the generic standard for device-independent storage.

====CMYK color sample====
((hatnote|((As of|2011)), the 32-bit CMYK image will not be displayed by some major browsers. The RGB image from above is substituted in its place with the link below it. Try saving the link to disk and opening it in another program if it will not display in your browser.))
<gallery>
Image:Channel digital image RGB color.jpg|[[:Image:Channel digital image CMYK color.jpg|Example 32-bit CMYK image]]
Image:Channel digital image cyan.jpg|Cyan channel
Image:Channel digital image magenta.jpg|Magenta channel
Image:Channel digital image yellow.jpg|Yellow channel
Image:Channel digital image black.jpg|[[Key plate|Key (black)]] channel
</gallery>

===HSV===
[[HSV color space|HSV]], or [[hue]] [[saturation (color theory)|saturation]] [[color value|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 similar to (but not exactly the same as) the CMYK black channel, or its [[negative (photography)|negative]].

HSV is especially useful in [[lossy data compression|lossy]] [[video compression]], where [[channel (digital image)#Optimized channel sizes|loss of color information is less noticeable to the human eye]].

==CA Tak==
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.

[[Chroma key|Bluescreen]] technology involves filming actors in front of a primary color background, then setting that color to transparent, and compositing it with a background.

The [[GIF]] and [[Portable Network Graphics|PNG]] image formats use alpha channels on the [[World Wide Web]] to merge images on [[web page]]s so that they appear to have an arbitrary shape even on a non-uniform background.

==Other channels==
In [[3D computer graphics]], multiple channels are used for additional control over material rendering; e.g., controlling [[specularity]] and so on.

==Bit depth==
((Main|Color 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.

==Optimized channel sizes==
Since the brain does not necessarily perceive distinctions in each channel to the same degree as in other channels, it is possible that differing the number of bits allocated to each channel will result in more optimal storage; in particular, for RGB images, compressing the blue channel the most and the red channel the least may be better than giving equal space to each.((Citation needed|date=April 2011)) 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|date=February 2007)) 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 (color theory)|saturation]]), while keeping all [[brightness]] information (value in [[HSV color space|HSV]]).

16-bit [[HiColor]] stores red and blue in 5 bits, and green in 6 bits.

==References==
((Reflist))

((DEFAULTSORT:Channel (Digital Image)))
[[Category:Computer graphics]]
[[Category:Digital photography]]

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