Channel (digital image)
Encyclopedia
Color
digital images are made of pixel
s, and pixels are made of combinations of primary color
s. 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.
"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, 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. The channels can have multiple widths and ranges.
Three main channel types (or color models) exist, and have respective strengths and weaknesses.
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 resolution), each channel is made of 16-bit images.
image has four channels: cyan, magenta, yellow, and 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 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.
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 CMYK Black channel, or its negative
.
HSV is especially useful in lossy
video compression, where loss of color information is less noticeable to the human eye
. See Optimized channel sizes below.
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.
The GIF and 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.
actually uses the red channel to distinguish detail, 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.
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
digital images are made of pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
s, and pixels are made of combinations of primary color
Primary color
Primary colors are sets of colors that can be combined to make a useful range of colors. For human applications, three primary colors are usually used, since human color vision is trichromatic....
s. 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
Digital camera
A digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor. It is the main device used in the field of digital photography...
will have a red, green and blue channel. A grayscale
Grayscale
In photography and computing, a grayscale or greyscale digital image is an image in which the value of each pixel is a single sample, that is, it carries only intensity information...
image has just one channel.
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 the same size with the "proper" image, and associated with it."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, GIF images actually refer to the color in each pixel by an index number
CLUT
A colour look-up table is a mechanism used to transform a range of input colours into another range of colours. It can be a hardware device built into an imaging system or a software function built into an image processing application...
, 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
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
in multispectral
Multi-spectral image
A multispectral image is one that captures image data at specific frequencies across the electromagnetic spectrum. The wavelengths may be separated by filters or by the use of instruments that are sensitive to particular wavelengths, including light from frequencies beyond the visible light range,...
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 models) exist, and have respective strengths and weaknesses.
RGB
An RGBRGB color model
The RGB color model is an additive color model in which red, green, and blue light is added together in various ways to reproduce a broad array of colors...
image has three channels: red, green, and blue. RGB channels roughly follow the color receptors in the human eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
, and are used in computer display
Computer display
A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...
s and image scanner
Image scanner
In computing, an image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting, or an object, and converts it to a digital image. Common examples found in offices are variations of the desktop scanner where the document is placed on a glass...
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 resolution), each channel is made of 16-bit images.
RGB color sample
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.CMYK
A CMYKCMYK color model
The CMYK color model is a subtractive color model, used in color printing, and is also used to describe the printing process itself. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key...
image has four channels: cyan, magenta, yellow, and black. CMYK is the standard for print, where subtractive color
Subtractive color
A subtractive color model explains the mixing of paints, dyes, inks, and natural colorants to create a full range of colors, each caused by subtracting some wavelengths of light and reflecting the others...
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 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.
CMYK color sample
- , the 32-bit CMYK image won't 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.
HSV
HSV, or HueHue
Hue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"...
Saturation
Saturation (color theory)
In colorimetry and color theory, colorfulness, chroma, and saturation are related but distinct concepts referring to the perceived intensity of a specific color. Colorfulness is the degree of difference between a color and gray. Chroma is the colorfulness relative to the brightness of another color...
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 CMYK Black channel, or its negative
Negative (photography)
In photography, a negative may refer to three different things, although they are all related.-A negative:Film for 35 mm cameras comes in long narrow strips of chemical-coated plastic or cellulose acetate. As each image is captured by the camera onto the film strip, the film strip advances so that...
.
HSV is especially useful in lossy
Lossy data compression
In information technology, "lossy" compression is a data encoding method that compresses data by discarding some of it. The procedure aims to minimize the amount of data that need to be held, handled, and/or transmitted by a computer...
video compression, where loss of color information is less noticeable to the human eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
. See Optimized channel sizes below.
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 compositingCompositing
Compositing is the combining of visual elements from separate sources into single images, often to create the illusion that all those elements are parts of the same scene. Live-action shooting for compositing is variously called "chroma key", "blue screen", "green screen" and other names. Today,...
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.
The GIF and PNG image formats use alpha channels on the World Wide Web
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
to merge images on web page
Web page
A web page or webpage is a document or information resource that is suitable for the World Wide Web and can be accessed through a web browser and displayed on a monitor or mobile device. This information is usually in HTML or XHTML format, and may provide navigation to other web pages via hypertext...
s so that they appear to have an arbitrary shape even on a non-uniform background.
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.Optimized channel sizes
Since the brain doesn't 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. This type of "preferential" compression is the result of studies which show that the human retinaRetina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
actually uses the red channel to distinguish detail, 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
Chroma subsampling
Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance....
to reduce the bit depth in color channels (Hue
Hue
Hue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"...
and Saturation
Saturation (color theory)
In colorimetry and color theory, colorfulness, chroma, and saturation are related but distinct concepts referring to the perceived intensity of a specific color. Colorfulness is the degree of difference between a color and gray. Chroma is the colorfulness relative to the brightness of another color...
), while keeping all brightness
Brightness
Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target...
information (Value in HSV).
16-bit HiColor stores red and blue in 5 bits, and green in 6 bits.