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                                                    152
           CHAPTER 3                                                                        Syntax



           erator must immediately precede that operator. Operators do not return results, and
           operands cannot be left over when an operator finishes execution.

           Most operators have to do with painting graphical elements on the page or with
           specifying parameters that affect subsequent painting operations. The individual
           operators are described in the chapters devoted to their functions:

           • Chapter 4 describes operators that paint general graphics, such as filled areas,
             strokes, and sampled images, and that specify device-independent graphical
             parameters, such as color.
           • Chapter 5 describes operators that paint text using character glyphs defined in
             fonts.
           • Chapter 6 describes operators that specify device-dependent rendering param-
             eters.
           • Chapter 10 describes the marked-content operators that associate higher-level
             logical information with objects in the content stream. These operators do not
             affect the rendered appearance of the content; they specify information useful
             to applications that use PDF for document interchange.

           Ordinarily, when an application encounters an operator in a content stream that
           it does not recognize, an error occurs. (See implementation note 39 in Appendix
           H.) A pair of compatibility operators, BX and EX (PDF 1.1), modify this behavior
           (see Table 3.29). These operators must occur in pairs and may be nested. They
           bracket a compatibility section, a portion of a content stream within which
           unrecognized operators are to be ignored without error. This mechanism enables
           a PDF document to use operators defined in later versions of PDF without
           sacrificing compatibility with older applications. It should be used only in cases
           where ignoring such newer operators is the appropriate thing to do. The BX and
           EX operators are not themselves part of any graphics object (see Section 4.1,
           “Graphics Objects”) or of the graphics state (Section 4.3, “Graphics State”).

                                   TABLE 3.29 Compatibility operators
OPERANDS     OPERATOR      DESCRIPTION

—            BX            (PDF 1.1) Begin a compatibility section. Unrecognized operators (along with their
                           operands) are ignored without error until the balancing EX operator is encountered.

—            EX            (PDF 1.1) End a compatibility section begun by a balancing BX operator.

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