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SECTION 6.4 Halftones
The Xsquare and Ysquare entries specify the dimensions of the two squares in
device pixels. The stream must contain Xsquare 2 + Ysquare 2 bytes, each repre-
senting a single threshold value. The contents of square X are specified first,
followed by those of square Y. Threshold values within each square are defined in
device space in the same order as image samples in image space (see Figure 4.26
on page 338), with the first value at device coordinates (0, 0) and horizontal coor-
dinates changing faster than vertical coordinates.
Type 16 Halftones
Like type 10, a type 16 halftone (PDF 1.3) defines a halftone screen with a thresh-
old array and allows nonzero screen angles. In type 16, however, each element of
the threshold array is 16 bits wide instead of 8. This allows the threshold array to
distinguish 65,536 levels of color rather than only 256 levels. The threshold array
can consist of either one rectangle or two rectangles. If two rectangles are speci-
fied, they tile the device space as shown in Figure 6.5. The last row in the first
rectangle is immediately adjacent to the first row in the second and starts in the
same column.
Width
Height Width ´ Height
Height2 Width2 ´
Height2
Width2
FIGURE 6.5 Tiling of device space in a type 16 halftone
A type 16 halftone, like type 6 and type 10, is represented as a stream containing
the threshold values, with the parameters defining the halftone specified by en-
tries in the stream dictionary. This dictionary can contain the entries shown in
Table 6.6 in addition to the usual entries common to all streams (see Table 3.4 on
page 62). The dictionary’s Width and Height entries define the dimensions of the
first (or only) rectangle. The dimensions of the second, optional rectangle are de-
fined by the optional entries Width2 and Height2. Each threshold value is repre-
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