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CHAPTER 4                                                                 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):

  p03   p13   p23   p33
  p02   p12   p22   p32
  p01   p11   p21   p31
  p00   p10   p20   p30



                                              p13
                                p03                          p33

                                                     p23            p32
                          p02
                                        p12            p22


                                        p11            p21
                                  p01
                                               p10           p31

                                                                   p30
                                p00
                                                     p20




                FIGURE 4.24 Control points in a tensor-product patch

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