10.2 基于 CIE 的颜色到设备颜色¶
CIE-Based Colour to Device Colour
为了在输出设备上渲染基于CIE的颜色，符合标准的阅读器应将指定的CIE颜色空间转换为设备的本机颜色空间（通常为DeviceGray、DeviceRGB或DeviceCMYK），并考虑设备的已知特性。
注意 1
如8.6.5“基于CIE的颜色空间”所述，基于CIE的颜色依赖于人类色彩感知模型。基于CIE的颜色渲染的目标是在设备的本机颜色空间中生成输出，使其尽可能准确地再现人类观察者感知的CIE颜色值。PDF 1.1支持CIE颜色规范和渲染（CalGray、CalRGB和Lab），PDF 1.3支持ICCBased颜色空间。
注意 2
从基于CIE的颜色转换到设备颜色是一个复杂的过程，其理论超出了本规范的范围。该算法包含许多参数，包括一个可选的完整三维颜色查找表。输出的颜色准确性取决于这些参数的正确设置，通常通过某种形式的校准方法来确定。设备可生成的颜色范围由设备描述文件（device profile）表征，该描述文件通常由与设备关联的ICC配置文件指定（且与ICCBased颜色空间中指定的描述文件完全独立）。
注意 3
PDF 没有与PostScript颜色渲染字典等效的功能。设备描述文件与符合标准的阅读器的输出设备的关联方式依赖于具体实现，并未在PDF文件中指定。通常，这一过程由操作系统提供的颜色管理系统（CMS）完成。从PDF 1.4开始，PDF文件可以指定一个或多个输出意图，以提供可能用于处理文件的描述文件（参见14.11.5“输出意图”）。
从基于CIE的颜色值转换为设备颜色值主要包括两个步骤：
a) 应根据基于CIE的色域映射函数调整CIE颜色值。
注意 4
色域（gamut）是某个颜色空间中所有可能颜色的一个子集。页面描述具有一个源色域，由其使用的所有颜色组成。而输出设备具有设备色域，由其能够再现的所有颜色组成。此步骤的作用是将颜色从源色域转换为设备色域，尽量保持色彩外观、视觉对比度或其他明确指定的渲染意图（参见8.6.5.8“渲染意图”）。
b) 应根据基于CIE的颜色映射函数生成相应的设备颜色值。对于给定的CIE颜色值，该函数应计算设备本机颜色空间中的颜色值。
基于CIE的色域映射和颜色映射函数仅适用于CIE颜色空间中的颜色值。设备颜色空间中的颜色值可直接控制设备颜色分量，但可能会受到DefaultGray、DefaultRGB和DefaultCMYK颜色空间资源的影响（参见8.6.5.6“默认颜色空间”）。
源色域应由所选CIE颜色空间的定义信息指定，并且该规范应与设备无关。输出设备的相应特性应包含在与设备关联的设备描述文件中。色域映射和颜色映射函数是符合标准的阅读器实现的一部分。
To render CIE-based colours on an output device, the conforming reader shall convert from the specified CIE-based colour space to the device’s native colour space (typically DeviceGray, DeviceRGB, or DeviceCMYK), taking into account the known properties of the device.
NOTE 1
As discussed in 8.6.5, "CIE-Based Colour Spaces" CIE-based colour is based on a model of human colour perception. The goal of CIE-based colour rendering is to produce output in the device’s native colour space that accurately reproduces the requested CIE-based colour values as perceived by a human observer. CIE-based colour specification and rendering are a feature of PDF 1.1 (CalGray, CalRGB, and Lab) and PDF 1.3 (ICCBased).
NOTE 2
The conversion from CIE-based colour to device colour is complex, and the theory on which it is based is beyond the scope of this specification. The algorithm has many parameters, including an optional, full three- dimensional colour lookup table. The colour fidelity of the output depends on having these parameters properly set, usually by a method that includes some form of calibration. The colours that a device can produce are characterized by a device profile, which is usually specified by an ICC profile associated with the device (and entirely separate from the profile that is specified in an ICCBased colour space).
NOTE 3
PDF has no equivalent of the PostScript colour rendering dictionary. The means by which a device profile is associated with a conforming reader’s output device are implementation-dependent and not specified in a PDF file. Typically, this is done through a colour management system (CMS) that is provided by the operating system. Beginning with PDF 1.4, a PDF file can also specify one or more output intents providing possible profiles that may be used to process the file (see 14.11.5, "Output Intents").
Conversion from a CIE-based colour value to a device colour value requires two main operations:
a) The CIE-based colour value shall be adjusted according to a CIE-based gamut mapping function.
NOTE 4
A gamut is a subset of all possible colours in some colour space. A page description has a source gamut consisting of all the colours it uses. An output device has a device gamut consisting of all the colours it can reproduce. This step transforms colours from the source gamut to the device gamut in a way that attempts to preserve colour appearance, visual contrast, or some other explicitly specified rendering intent (see 8.6.5.8, "Rendering Intents").
b) A corresponding device colour value shall be generated according to a CIE-based colour mapping function. For a given CIE-based colour value, this function shall compute a colour value in the device’s native colour space.
The CIE-based gamut and colour mapping functions shall be applied only to colour values presented in a CIE- based colour space. Colour values in device colour spaces directly control the device colour components though this may be altered by the DefaultGray, DefaultRGB, and DefaultCMYK colour space resources (see 8.6.5.6, "Default Colour Spaces").
The source gamut shall be specified by the information contained in the definition of the CIE-based colour space when selected. This specification shall be device-independent. The corresponding properties of the output device shall be given in the device profile associated with the device. The gamut mapping and colour mapping functions are part of the implementation of the conforming reader.