SECTION 10.7
887
Tagged PDF
•
Page artifacts.
Production aids extraneous to the document itself, such as cut
marks and color bars.
•
Background artifacts.
Images, patterns or colored blocks that either run the en-
tire length and/or width of the page or the entire dimensions of a structural el-
ement. Background artifacts typically serve as a background for content shown
either on top of or placed adjacent to that background.
A background artifact can further be classified as visual content that serves to en-
hance the user experience, that lies under the actual content, and that is not re-
quired except to retain visual fidelity. Examples of this include a colored
background, pattern, blend, or image that resides under main body text. In the
case of white text on a black background, the black background is absolutely nec-
essary to be able to read the white text; however, the background itself is merely
there to enhance the visual experience. However, a draft or other identifying wa-
termark is classified as a pagination artifact because it does not serve to enhance
the experience; rather, it serves as a running artifact typically used on every page
in the document. As a further example, a Figure is distinguishable from a back-
ground artifact in that removal of the graphics objects from a Figure would de-
tract from the overall contextual understanding of the Figure as an entity.
Tagged PDF consumer applications may have their own ideas about what page
content to consider relevant. A text-to-speech engine, for instance, probably
should not speak running heads or page numbers when the page is turned. In
general, consumer applications can do any of the following:
•
Disregard elements of page content (for example, specific types of artifacts)
that are not of interest
•
Treat some page elements as
terminals
that are not to be examined further (for
example, to treat an illustration as a unit for reflow purposes)
•
Replace an element with alternate text (see Section 10.8.2, “Alternate Descrip-
Depending on their goals, different consumer applications can make different de-
cisions in this regard. The purpose of Tagged PDF is not to prescribe what the
consumer application should do, but to provide sufficient declarative and de-
scriptive information to allow it to make appropriate choices about how to pro-
cess the content.