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CHAPTER 6 Rendering
Halftone cells at nonzero angles can be difficult to specify because they may not
line up well with scan lines and because it may be difficult to determine where a
given sampled point goes. The type 10 halftone addresses these difficulties by
dividing the halftone cell into a pair of squares that line up at zero angles with the
output device’s pixel grid. The squares contain the same information as the origi-
nal cell but are much easier to store and manipulate. In addition, they can be
mapped easily into the internal representation used for all rendering.
Figure 6.2 shows a halftone cell with a frequency of 38.4 cells per inch and an
angle of 50.2 degrees, represented graphically in device space at a resolution of
300 dots per inch. Each asterisk in the figure represents a location in device space
that is mapped to a specific location in the threshold array.
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FIGURE 6.2 Halftone cell with a nonzero angle
Figure 6.3 shows how the halftone cell can be divided into two squares. If the
squares and the original cell are tiled across device space, the area to the right of
the upper square maps exactly into the empty area of the lower square, and vice
versa (see Figure 6.4). The last row in the first square is immediately adjacent to
the first row in the second square and starts in the same column.
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