CHAPTER 7
7
Transparency
PDF 1.4 extends the Adobe imaging model to include the notion of
transparency.
Transparent objects do not necessarily obey a strict opaque painting model but
can blend
(composite)
in interesting ways with other overlapping objects. This
chapter describes the general transparency model but does not cover how it is im-
plemented. Implementation-like descriptions are used at various points to de-
scribe how things work, for the purpose of elucidating the behavior of the model.
The actual implementation will almost certainly be different from what these de-
scriptions might imply.
The chapter is organized as follows:
Section 7.1, “Overview of Transparency,” introduces the basic concepts of the
transparency model and its associated terminology.
Section 7.2, “Basic Compositing Computations,” describes the mathematics
involved in compositing a single object with its backdrop.
Section 7.3, “Transparency Groups,” introduces the concept of
transparency
groups
and describes their properties and behavior.
Section 7.4, “Soft Masks,” covers the creation and use of masks to specify
position-dependent shape and opacity.
Section 7.5, “Specifying Transparency in PDF,” describes how transparency
properties are represented in a PDF document.
Section 7.6, “Color Space and Rendering Issues,” deals with some specific inter-
actions between transparency and other aspects of color specification and
rendering.
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