CHAPTER 4
290
Graphics
Patterns come in two varieties:
•
Tiling patterns
consist of a small graphical figure (called a
pattern cell)
that is
replicated at fixed horizontal and vertical intervals to fill the area to be painted.
The graphics objects to use for tiling are described by a content stream.
•
Shading patterns
define a
gradient fill
that produces a smooth transition
between colors across the area. The color to use is specified as a function of
position using any of a variety of methods.
Note:
The ability to paint with patterns is a feature of PDF 1.2 (tiling patterns) and
PDF 1.3 (shading patterns). With some effort, it is possible to achieve a limited form
of tiling patterns in PDF 1.1 by defining them as character glyphs in a special font
and painting them repeatedly with the
Tj
operator. Another technique, defining
patterns as halftone screens, is not recommended because the effects produced are
device-dependent.
Patterns are specified in a special family of color spaces named
Pattern
. These
spaces use
pattern objects
as the equivalent of color values instead of the numeric
component values used with other spaces. A pattern object may be a dictionary
or a stream, depending on the type of pattern; the term
pattern dictionary
is used
generically throughout this section to refer to either a dictionary object or the
dictionary portion of a stream object. (Those pattern objects that are streams are
specifically identified as such in the descriptions of particular pattern types; un-
less otherwise stated, they are understood to be simple dictionaries instead.) This
section describes
Pattern
color spaces and the specification of color values within
them. See Section 4.5, “Color Spaces,” for information about color spaces and col-
or values in general and Section 7.5.6, “Patterns and Transparency,” for further
discussion of the treatment of patterns in the transparent imaging model.
4.6.1 General Properties of Patterns
A pattern dictionary contains descriptive information defining the appearance
and properties of a pattern. All pattern dictionaries contain an entry named
PatternType
, whose value identifies the kind of pattern the dictionary describes:
type 1 for a tiling pattern or type 2 for a shading pattern. The remaining contents
of the dictionary depend on the pattern type and are detailed below in the sec-
tions on individual pattern types.