SECTION 2.1
35
Imaging Model
2.1.2 Adobe Imaging Model
The Adobe imaging model is a simple and unified view of two-dimensional
graphics borrowed from the graphic arts. In this model, “paint” is placed on a
page in selected areas:
•
The painted figures can be in the form of character shapes
(glyphs),
geometric
shapes, lines, or sampled images such as digital representations of photographs.
•
The paint may be in color or in black, white, or any shade of gray. It may also
take the form of a repeating
pattern (PDF 1.2)
or a smooth transition between
colors
(PDF 1.3).
•
Any of these elements may be
clipped
to appear within other shapes as they are
placed onto the page.
A page’s content stream contains
operands
and
operators
describing a sequence of
graphics objects. A PDF consumer application maintains an implicit
current page
that accumulates the marks made by the painting operators. Initially, the current
page is completely blank. For each graphics object encountered in the content
stream, the application places marks on the current page, which replace or com-
bine with any previous marks they may overlay. Once the page has been com-
pletely composed, the accumulated marks are rendered on the output medium
and the current page is cleared to blank again.
PDF 1.3 and earlier versions use an
opaque imaging model
in which each new
graphics object painted onto a page completely obscures the previous contents of
the page at those locations (subject to the effects of certain optional parameters
that may modify this behavior; see Section 4.5.6, “Overprint Control”). No mat-
ter what color an object has—white, black, gray, or color—it is placed on the page
as if it were applied with opaque paint. PDF 1.4 introduces a
transparent imaging
model
in which objects painted on the page are not required to be fully opaque.
Instead, newly painted objects are
composited
with the previously existing con-
tents of the page, producing results that combine the colors of the object and its
backdrop according to their respective opacity characteristics. The transparent
imaging model is described in Chapter 7.
The principal graphics objects (among others) are as follows:
•
A
path object
consists of a sequence of connected and disconnected points,
lines, and curves that together describe shapes and their positions. It is built up