CHAPTER 4
356
Graphics
Form XObjects have various uses:
•
As its name suggests, a form XObject can serve as the template for an entire
page. For example, a program that prints filled-in tax forms can first paint the
fixed template as a form XObject and then paint the variable information on
top of it.
•
Any graphical element that is to be used repeatedly, such as a company logo or
a standard component in the output from a computer-aided design system, can
be defined as a form XObject.
•
Certain document elements that are not part of a page’s contents, such as
annotation appearances (see Section 8.4.4, “Appearance Streams”), are repre-
sented as form XObjects.
•
A specialized type of form XObject, called a
group XObject (PDF 1.4),
can be
used to group graphical elements together as a unit for various purposes (see
fine transparency groups and soft masks for use in the transparent imaging
model (see “Soft-Mask Dictionaries” on page 552 and Section 7.5.5, “Transpar-
•
Another specialized type of form XObject, a
reference XObject (PDF 1.4),
can
be used to import content from one PDF document into another (see Section
The use of form XObjects requires two steps:
1.
Define the appearance of the form XObject.
A form XObject is a PDF content
stream. The dictionary portion of the stream (called the
form dictionary)
contains descriptive information about the form XObject; the body of the
stream describes the graphics objects that produce its appearance. The con-
tents of the form dictionary are described in Section 4.9.1, “Form Diction-
2.
Paint the form XObject.
The
Do
operator (see Section 4.7, “External Objects”)
paints a form XObject whose name is supplied as an operand. (The name is
defined in the
XObject
subdictionary of the current resource dictionary.)
Before invoking this operator, the content stream in which it appears should
set appropriate parameters in the graphics state. In particular, it should alter
the current transformation matrix to control the position, size, and orientation
of the form XObject in user space.