色域

色域是对一种颜色进行编码的方法，也指一个技术系统能够产生的颜色的总和。

在计算机图形处理中，色域是颜色的某个完全的子集。颜色子集最常见的应用是用来精确地代表一种给定的情况。例如一个给定的色空间或是某个输出装置的呈色范围。

Another sense, less frequently used but not less correct, refers to the complete set of colors found within an image at a given time. In this context, digitizing a photograph, converting a digitized image to a different color space, or outputting it to a given medium using a certain output device generally alters its gamut, in the sense that some of the colors in the original are lost in the process.

In color theory, the gamut of a device or process is that portion of the visible color space that can be represented, detected, or reproduced. Generally, the color gamut is specified in the hue-saturation plane, as many systems can produce colors with a wide range intensity within their color gamut; in addition, for subtractive color systems, such as printing, the range of intensity available in the system is for the most part meaningless outside the context of its illumination.

When certain colors cannot be displayed within a particular color model, those colors are said to be out of gamut. For example, pure red which is contained in the RGB color model gamut is out of gamut in the CMYK model.

A device which is able to reproduce the entire visible color space is somewhat of a holy grail in the engineering of color displays and printing processes. While modern techniques allow increasingly good approximations, the complexity of these systems often makes them impractical. It should be noted that limitations of human perception dictate what is "good enough".

While processing a digital image, the most convenient color model used is the RGB model. Printing the image requires transforming the image from the original RGB color space to the printer's CMYK color space. During this process, the colors from the RGB which are out of gamut must be somehow converted to approximate values within the CMYK space gamut. Simply trimming only the colors which are out of gamut to the closest colors in the destination space would "burn" the image. There are several algorithms approximating this transformation, but none of them can be truly perfect, since those colors are simply out of the target device's capabilities. This is why identifying the colors in an image which are out of gamut in the target color space as soon as possible during processing is critical for the quality of the final product.

Owners of digital capture devices (i.e. digital cameras) should note that the generic traditional photography and film term for the complete range of colors which can be captured on a particular medium is dynamic range. This term traditionally refers to the maximum range of brightness (or "lightness", as in the HLS color space) which can be captured on a particular medium. That's because traditional film is more "honest" than its digital counterpart, in that it performs more or less the same in capturing all three primary colors (the digital capture devices generally try to "cheat" on the user by storing as little color information as needed in order to satisfy its given sale characteristics, while also saving on sensors and storage space; this "cheating" is possible due to the characteristics of the human retina). The practical result is that differentiating between films means differentiating between their capability to record different brightness ranges, as opposed to fully defined gamuts, due to their uniformity in recording all colors in the spectrum at a given brightness. It is also worth mentioning that specific films do differentiate between colors, in that they induce a given tint or highlight given colors, thus acting as photographic filters in effect. This behavior is expected (and often desired) by experienced traditional photographers who choose one film over another based on these characteristics.

[编辑] Representation of gamuts

A typical CRT gamut.
The grayed-out horseshoe shape is the entire range of possible colors. The colored triangle is the gamut available to a typical computer monitor; it does not cover the entire space. The corners of the triangle are the primaries for this gamut; in the case of a CRT, they depend on the emittance of the phosphors of the monitor.

Gamuts are commonly represented as areas in the CIE 1931 chromaticity diagram as shown at right, with the curved edge representing the monochromatic colors. Gamut areas typically have triangular shapes because most color reproduction is done with three primaries.

However, the accessible gamut depends on the brightness; a full gamut must therefore be represented in 3D space, as below:

The pictures at left show the gamuts of RGB color space (top), such as on computer monitors, and of reflective colors in nature (bottom). The cone drawn in grey corresponds roughly to the CIE diagram at right, with the added dimension of brightness.

The axes in these diagrams are the responses of the short-wavelength, middle-wavelength, and long-wavelength cones in the human eye. The other letters indicate black, red, green, blue, cyan, magenta, yellow, and white colors. (Note: These pictures are not exactly to scale.)

The left diagram shows that the shape of the RGB gamut is a triangle between red, green, and blue at lower luminosities; a triangle between cyan, magenta, and yellow at higher luminosities, and a single white point at maximum luminosity. The exact positions of the apexes depends on the emission spectra of the phosphors in the computer monitor, and on the ratio between the maximum luminosities of the three phosphors (i.e., the color balance).

The gamut of the CMYK color space is, ideally, approximately the same as that for RGB, with slightly different apexes, depending on both the exact properties of the dyes and the light source. In practice, due to the way raster-printed colors interact with each other and the paper and due to their non-ideal absorption spectra, the gamut is smaller and has rounded corners.

The gamut of reflective colors in nature has a similar, though more rounded, shape. An object that reflects only a narrow band of wavelengths will have a color close to the edge of the CIE diagram, but it will have a very low luminosity at the same time. At higher luminosities, the accessible area in the CIE diagram becomes smaller and smaller, up to a single point of white, where all wavelengths are reflected exactly 100 per cent. The exact coordinates of white are of course determined by the color of the light source.

[编辑] 色彩表现的局限性

绝大多数系统的色域都是由于很难生成单色（单波长）的光线所导致的。最好的接近单色光的技术就是激光，对于大多数系统来说这种方法过于昂贵，不太现实。随着激光技术的进步，成本进一步降低，这种方法也逐渐有所应用。除了激光之外，大多数系统都是用大致近似的方法表示高度饱和的颜色，这些光线通常包含所期望的颜色之外多种颜色。This may be more pronounced for some hues than others.

使用加性色彩处理的系统通常在色域饱和平面上大致是一个凸多边形。多边形的顶点是系统能够产生的最饱和的颜色。在减性色彩系统中，色域经常是不规则区域。

[编辑] 各种颜色系统的比较

下面是大致按照从大到小的色域排列的色彩系统：

• 如今使用三束激光的激光视频投影机已经步入实用阶段。其理论依据是激光是真正的单元色。激光视频投影机使用三束激光在如今实用的显示设备中产生较宽色域。这种系统象电子束在 CRT 上扫描那样逐个扫描图像上的每个点，然后在较高的频率直接对激光进行调制，或者是对激光进行光学扩展、调制、每次扫描一行，就象 DLP 中的方式调制扫描线。
• 底片是最好的检测、重现色彩的系统之一。常去看电影的人对于电影院中的电影与家庭影院之间的色彩质量都深有感触。这是因为电影胶片的色域要远大于电视的色域。
• 激光放映使用激光产生非常接近单色的光线，这样就可以产生远远超出其它系统的色饱和度。但是，这种方法很难通过色域的合成产生饱和度较低的其它颜色。另外，这样的系统非常复杂、昂贵、不适于通常的视频放映。
• CRT 及类似的显示器都有一个大致为三角形的能够覆盖可见色彩空间大部分的色域。CRT 显示器的色域受限于产生红色、绿色、蓝色光线的荧光物质。除了显示器本身之外，显示实际的图像的时候，通常还受限于如数码相机、扫描仪等设备中的色彩传感器的质量相关。索尼公司最近引进了一种四色（RGB加上母绿）色彩传感器系统以提高视频显示的质量以及更大的色域，但是这种技术的成效还有待时间检验。
• 液晶显示器（LCD）的屏幕通过对背光进行过滤进行显示。因此 LCD 的色域完全取决于背光的光谱。通常 LCD 显示器使用荧光灯作为背光，而荧光灯的色域通常比 CRT 显示器要小很多。一些使用发光二极管（LED）的 LCD 显示器则比 CRT 的色域更加宽广。
• 电视通常使用 CRT 显示器，但是由于广播系统的限制，电视系统并没有充分利用 CRT 显示的优点。高清电视相对来说效果要远远好于普通的电视，但是仍然比使用同样显示技术的计算机显示稍逊一筹。
• Paint mixing, both artistic and for commercial applications, achieves a reasonably large color gamut by starting with a larger palette than the red, green, and blue of CRTs or cyan, magenta, and yellow of printing. Paint may reproduce some highly saturated colors that can not be reproduced well by CRTs (particularly violet), but overall the color gamut is smaller.
• 印刷过程中通常使用CMYK色彩空间（青色 C、品红 M、黄色 Y 与黑色 K）。有极少数印刷系统中不使用黑色，但是在表现低饱和度、低亮度颜色的时候效果不好。通过添加基本颜色之外的其它颜色来扩展印刷过程的色域。
• 单色显示器的色域是色彩空间中的一条一维曲线。