CHAPTER 4
328
Graphics
Although the Coons patch is more concise and easier to use, the tensor-product
patch affords greater control over color mapping.
Note:
The data format for type 7 shadings (as for types 4 through 6) is the same in
PDF as it is in PostScript. However, the numbering and order of control points was
described incorrectly in the first printing of the
PostScript Language Reference,
Third Edition. That description has been corrected here.
Like the Coons patch mapping, the tensor-product patch mapping is controlled
by the location and shape of four cubic Bézier curves marking the boundaries of
the patch. However, the tensor-product patch has four additional, “internal”
control points to adjust the mapping. The 16 control points can be arranged in a
4-by-4 array indexed by row and column, as follows (see Figure 4.24):
p
03
p
02
p
01
p
00
p
13
p
12
p
11
p
10
p
23
p
22
p
21
p
20
p
33
p
32
p
31
p
30
p
13
p
03
p
23
p
12
p
11
p
01
p
00
p
20
p
10
p
22
p
21
p
31
p
30
p
33
p
32
p
02
FIGURE 4.24
Control points in a tensor-product patch