Channel (digital image) 通道 色彩深度 Color_depth

en.wikipedia.org/wiki/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.

通道，是数字图像中存储不同类型信息的灰度图像。一个图像最多可以有数十个个通道，常用的RGB和Lab图像默认有三个通道，而CMYK图像则默认有四个通道。

【下采样 subsampling】

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.^{[citation needed]} 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.

【视网膜是红色通道区分细节】

Among other techniques, lossy video compression uses Chroma subsampling to reduce the bit depth in color channels (Hue and Saturation), while keeping all brightnessinformation (Value in HSV).

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

What Are Alpha Channels?

https://www.howtogeek.com/howto/42393/rgb-cmyk-alpha-what-are-image-channels-and-what-do-they-mean/

【贡献透明度，非色彩信息】

Simply to make things one step more confusing, some image files (notably, like PNG) contain more image information in an extra channel. This channel is essentially no different than an RGB or CMYK channel, with 256 shades of gray representing the image areas. However, Alpha channels have on crucial difference: they denote transparency, not color information.

zh.wikipedia.org/wiki/色彩深度

Color depth or colour depth (see spelling differences), also known as bit depth, is either the number of bits used to indicate the color of a single pixel, in a bitmapped image or video frame buffer, or the number of bits used for each color component of a single pixel.

色彩深度计算机图形学领域表示在位图或者视频帧缓冲区中储存1像素的颜色所用的位数，它也称为位/像素（bpp）。色彩深度越高，可用的颜色就越多。

色彩深度是用“n位颜色”（n-bit colour）来说明的。若色彩深度是n位，即有2ⁿ种颜色选择，而储存每像素所用的位数就是n。常见的有：

• 1位：2种颜色，单色光，黑白二色，用于compact Macintoshes。
• 2位：4种颜色，CGA，用于gray-scale早期的NeXTstation及color Macintoshes。
• 3位：8种颜色，用于大部分早期的电脑显示器。
• 4位：16种颜色，用于EGA及不常见及在更高的分辨率的VGA标准，color Macintoshes。

色彩深度计算机图形学领域表示在位图或者视频帧缓冲区中储存1像素的颜色所用的位数，它也称为位/像素（bpp）。色彩深度越高，可用的颜色就越多。

With the relatively low color depth, the stored value is typically a number representing the index into a color map or palette (a form of vector quantization). The colors available in the palette itself may be fixed by the hardware or modifiable within the limits of the hardware (for instance, both color Macintosh systems and VGA-equipped IBM-PCs typically ran at 8-bit due to limited VRAM, but while the best VGA systems only offered an 18-bit (262,144 color) palette from which colors could be chosen, all color Macintosh video hardware offered a 24-bit (16 million color) palette). Modifiable palettes are sometimes referred to as pseudocolor palettes.

• 1-bit color (2¹ = 2 colors): monochrome, often black and white, compact Macintoshes, Atari ST.
• 2-bit color (2² = 4 colors): CGA, gray-scale early NeXTstation, color Macintoshes, Atari ST.
• 3-bit color (2³ = 8 colors): many early home computers with TV displays, including the ZX Spectrum and BBC Micro