Channel (digital image)
Color digital images are made of pixels, and pixels are made of combinations of primary colors represented by a series of code. 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 geographic information systems, channels are often referred to as raster bands.[1] Another closely related concept is feature maps, which are used in convolutional neural networks.
A 24-bit RGB image
The red channel, displayed as grayscale
The green channel, displayed as grayscale
The blue channel, displayed as grayscale
CA Tak[edit]
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.
The GIF and PNG image formats use alpha channels on the World Wide Web to merge images on web pages so that they appear to have an arbitrary shape even on a non-uniform background.
Other channels[edit]
In 3D computer graphics, multiple channels are used for additional control over material rendering; e.g., controlling specularity and so on.
Optimized channel sizes[edit]
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. This type of "preferential" compression is the result of studies which show that the human retina 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.