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CHAPTER 10                                                      Document Interchange



The model is deliberately defined loosely to allow reasonable latitude in the interpre-
tation of structure elements and attributes when converting to other document for-
mats. Some degree of variation in the resulting layout from one format to another is
to be expected.

The basic layout model begins with the notion of a reference area. This is a rect-
angular region used by the layout application as a frame or guide in which to
place the document’s content. Some of the standard structure attributes, such as
StartIndent and EndIndent (see “Layout Attributes for BLSEs” on page 922), are
measured from the boundaries of the reference area. Reference areas are not
specified explicitly but are inferred from context. Those of interest are generally
the column area or areas in a general text layout, the outer bounding box of a
table and those of its component cells, and the bounding box of an illustration or
other floating element.

The standard structure types are divided into four main categories according to
the roles they play in page layout:

• Grouping elements (see “Grouping Elements” on page 899) group other ele-
  ments into sequences or hierarchies but hold no content directly and have no
  direct effect on layout.
• Block-level structure elements (BLSEs) (see “Block-Level Structure Elements” on
  page 901) describe the overall layout of content on the page, proceeding in the
  block-progression direction.
• Inline-level structure elements (ILSEs) (see “Inline-Level Structure Elements” on
  page 905) describe the layout of content within a BLSE, proceeding in the in-
  line-progression direction.
• Illustration elements (see “Illustration Elements” on page 911) are compact se-
  quences of content, in page content order, that are considered to be unitary ob-
  jects with respect to page layout. An illustration can be treated as either a BLSE
  or an ILSE.

The meaning of the terms block-progression direction and inline-progression
direction depends on the writing system in use, as specified by the standard
attribute WritingMode (see “General Layout Attributes” on page 917). In Western
writing systems, the block direction is from top to bottom and the inline direc-
tion is from left to right. Other writing systems use different directions for laying
out content.

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