SECTION 6.4
487
Halftones
tions, it sacrifices portability. Associated with every output device is a default half-
tone definition that is appropriate for most purposes. Only relatively sophisticated
documents need to define their own halftones to achieve special effects.
All halftones are defined in device space, unaffected by the current transforma-
tion matrix. For correct results, a PDF document that defines a new halftone
must make assumptions about the resolution and orientation of device space. The
best choice of halftone parameters often depends on specific physical properties
of the output device, such as pixel shape, overlap between pixels, and the effects of
electronic or mechanical noise.
6.4.1 Halftone Screens
In general, halftoning methods are based on the notion of a
halftone screen,
which
divides the array of device pixels into
cells
that can be modified to produce the
desired halftone effects. A screen is defined by conceptually laying a uniform
rectangular grid over the device pixel array. Each pixel belongs to one cell of the
grid; a single cell typically contains many pixels. The screen grid is defined entire-
ly in device space and is unaffected by modifications to the current transforma-
tion matrix. This property is essential to ensure that adjacent areas colored by
halftones are properly stitched together without visible seams.
On a bilevel (black-and-white) device, each cell of a screen can be made to ap-
proximate a shade of gray by painting some of the cell’s pixels black and some
white. Numerically, the gray level produced within a cell is the ratio of white pix-
els to the total number of pixels in the cell. A cell containing
n
pixels can render
n
+
1 different gray levels, ranging from all pixels black to all pixels white. A gray
value
g
in the range 0.0 to 1.0 is produced by making
i
pixels white, where
i
=
floor (g
×
n).
The foregoing description also applies to color output devices whose pixels con-
sist of primary colors that are either completely on or completely off. Most color
printers, but not color displays, work this way. Halftoning is applied to each color
component independently, producing shades of that color.
Color components are presented to the halftoning machinery in additive form,
regardless of whether they were originally specified additively (RGB or gray) or
subtractively (CMYK or tint). Larger values of a color component represent light-
er colors—greater intensity in an additive device such as a display or less ink in a