- TUTORIALS: BIT DEPTH -

-指南：位深-

Bit depth quantifies how many unique colors are available in an image's color palette in terms of the number of 0's and 1's, or "bits," which are used to specify each color.  This does not mean that the image necessarily uses all of these colors, but that it can instead specify colors with that level of precision.  For a grayscale image, the bit depth quantifies how many unique shades are available.  Images with higher bit depths can encode more shades or colors since there are more combinations of 0's and 1's available.

位深是对图像调色板中可用颜色的一种量化，******或者是用来指定每种颜色的位数。这并不意味着图像必须使用所有的这些颜色，而是它能够替代来指定那种精度级别的颜色。对于一个灰度图像，位深量化了可用的唯一阴影的数量。更高位深的图像能够对更多的阴影和颜色进行编码，因为它有更多可用的0和1数据组合。

TERMINOLOGY  术语

Every color pixel in a digital image is created through some combination of the three primary colors: red, green, and blue.  Each primary color is often referred to as a "color channel" and can have any range of intensity values specified by its bit depth.  The bit depth for each primary color is termed the "bits per channel."  The "bits per pixel" (bpp) refers to the sum of the bits in all three color channels and represents the total colors available at each pixel.  Confusion arises frequently with color images because it may be unclear whether a posted number refers to the bits per pixel or bits per channel.  Using "bpp" as a suffix helps distinguish these two terms.

数字图像中的每一个颜色像素是由三原色的组合创建的：红色、绿色和蓝色。每一种原色常常被称为“颜色通道”，并且具有任意的、由位深指定的强度值范围。每种原色的位深对应的术语是“位 / 通道”(每通道的位数)。“位 / 像素”（bpp）是指三种颜色通道的位数之和，它表示每一像素的全部可用颜色。在彩色图像中，经常会混淆这些概念，因为可能不清楚提供的数据是指“位/像素”还是“位/通道”。使用“bpp”（bits per pixel, 位/像素）后缀可以帮助区分这两个术语。

EXAMPLE

Most color images from digital cameras have 8-bits per channel and so they can use a total of eight 0's and 1's.  This allows for 2⁸ or 256 different combinations—translating into 256 different intensity values for each primary color.  When all three primary colors are combined at each pixel, this allows for as many as 2^8*3 or 16,777,216 different colors, or "true color."  This is referred to as 24 bits per pixel since each pixel is composed of three 8-bit color channels.  The number of colors available for any X-bit image is just 2^X if X refers to the bits per pixel and 2^3X if X refers to the bits per channel.

举例

大部分来自数码摄像机的图像具有8位/通道，因此它们可使用0和1共八位数据。这样就允许 2⁸或者256种不同的组合-----每种原色可以转换成256种不同的强度值。当每个像素的所有三原色组合在一起时，就允许有多达2^8*3或者16,777,216 种不同的颜色，或者称为“真彩”。

COMPARISON

The following table illustrates different image types in terms of bits (bit depth), total colors available, and common names.

BIT DEPTH VISUALIZATION

By moving your mouse over any of the labels below, the image will be re-displayed using the chosen amount of colors.  The difference between 24 bpp and 16 bpp is subtle, but will be clearly visible if you have your display set to true color or higher (24 or 32 bpp).

USEFUL TIPS

• The human eye can only discern about 10 million different colors, so saving an image in any more than 24 bpp is excessive if the only intended purpose is for viewing.  On the other hand, images with more than 24 bpp are still quite useful since they hold up better under post-processing (see "Posterization Tutorial").
• Color gradations in images with less than 8-bits per color channel can be clearly seen in the image histogram.
• The available bit depth settings depend on the file type.  Standard JPEG and TIFF files can only use 8-bits and 16-bits per channel, respectively.

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