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                                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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