11.1 概述

11.1 General

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PDF 图像模型包含透明度的概念。透明对象不一定遵循严格的不透明绘制模型，而是可能以有趣的方式与其他重叠对象进行混合（合成）。本节描述了一般的透明度模型，但不涉及其具体实现方式。在多个地方，它使用类似实现的描述来说明工作原理，目的是阐明模型的行为。然而，实际实现方式几乎肯定与这些描述不同。

注意

透明度功能在 PDF 1.4 版本中引入。

本节的组织结构如下：

• 11.2 透明度概述：介绍透明度模型的基本概念及相关术语。
• 11.3 基本合成计算：描述将单个对象与其背景合成时涉及的数学计算。
• 11.4 透明度组：介绍透明度组的概念，并描述其属性和行为。
• 11.5 软遮罩：介绍如何创建和使用遮罩，以指定与位置相关的形状和不透明度。
• 11.6 在 PDF 中指定透明度：描述如何在 PDF 文档中表示透明度属性。
• 11.7 颜色空间与渲染问题：讨论透明度与颜色指定及渲染之间的一些特定交互。

The PDF imaging model includes the notion of transparency. Transparent objects do not necessarily obey a strict opaque painting model but may blend (composite) in interesting ways with other overlapping objects. This clause describes the general transparency model but does not cover how it is implemented. At various points it uses implementation-like descriptions to describe how things work, for the purpose of elucidating the behaviour of the model. The actual implementation will almost certainly be different from what these descriptions might imply.

NOTE

Transparency was added to PDF in version 1.4

The clause is organized as follows:

• 11.2, "Overview of Transparency," introduces the basic concepts of the transparency model and its associated terminology.
• 11.3, "Basic Compositing Computations," describes the mathematics involved in compositing a single object with its backdrop.
• 11.4, "Transparency Groups," introduces the concept of transparency groups and describes their properties and behaviour.
• 11.5, "Soft Masks," covers the creation and use of masks to specify position-dependent shape and opacity.
• 11.6, "Specifying Transparency in PDF," describes how transparency properties are represented in a PDF document.
• 11.7, "Colour Space and Rendering Issues," deals with some specific interactions between transparency and other aspects of colour specification and rendering.