CHAPTER 10
912
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) 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).
Formula
Form
(Form) A widget annotation representing an interactive form field (see Section 8.6, “Inter-
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
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-
Alt
is a description of the illustration,
whereas
ActualText
gives the exact text equivalent of a graphical illustration that
has the appearance of text.