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                                                      912
          CHAPTER 10                                                           Document Interchange




                      TABLE 10.27 Standard structure types for illustration elements
STRUCTURE TYPE      DESCRIPTION

Figure              (Figure) An item of graphical content. Its placement may be specified with the Placement
                    layout attribute (see “General Layout Attributes” on page 917).

Formula             (Formula) A mathematical formula.
                    Note: This structure type is useful only for identifying an entire content element as a formula.
                    No standard structure types are defined for identifying individual components within the for-
                    mula. From a formatting standpoint, the formula is treated similarly to a figure (structure
                    type Figure; see above).

Form                (Form) A widget annotation representing an interactive form field (see Section 8.6, “Inter-
                    active Forms”). If the element contains a Role attribute, it may contain content items that
                    represent the value of the (non-interactive) form field. If the element omits a Role attribute
                    (see Table 10.35 on page 934), its only child is an object reference (see “PDF Objects as
                    Content Items” on page 868) identifying the widget annotation. The annotations’ appear-
                    ance stream (see “Appearance Streams” on page 612) defines the rendering of the form ele-
                    ment.

          An illustration may have logical substructure, including other illustrations. For
          purposes of reflow, however, it is moved (and perhaps resized) as a unit, without
          examining its internal contents. To be useful for reflow, it must have a BBox
          attribute. It may also have Placement, Width, Height, and BaselineShift attributes
          (see “Layout Attributes” on page 916).

          Often an illustration is logically part of, or at least attached to, a paragraph or oth-
          er element of a document. Any such containment or attachment is represented
          through the use of the Figure structure type. The Figure element indicates the
          point of attachment, and its Placement attribute describes the nature of the at-
          tachment. An illustration element without a Placement attribute is treated as an
          ILSE and laid out inline.

          Note: For accessibility to users with disabilities and other text extraction purposes,
          an illustration element should always have an Alt entry or an ActualText entry (or
          both) in its structure element dictionary (see Sections 10.8.2, “Alternate Descrip-
          tions,” and 10.8.3, “Replacement Text”). Alt is a description of the illustration,
          whereas ActualText gives the exact text equivalent of a graphical illustration that
          has the appearance of text.

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