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                                                          311
           SECTION 4.6                                                                            Patterns



           Note: This type of shading cannot be used with an Indexed color space.

                        TABLE 4.31 Additional entries specific to a type 3 shading dictionary
KEY              TYPE             VALUE

Coords           array            (Required) An array of six numbers [ x0 y0 r0 x1 y1 r1 ] specifying the centers and
                                  radii of the starting and ending circles, expressed in the shading’s target coor-
                                  dinate space. The radii r0 and r1 must both be greater than or equal to 0. If one
                                  radius is 0, the corresponding circle is treated as a point; if both are 0, nothing is
                                  painted.

Domain           array            (Optional) An array of two numbers [ t0 t1 ] specifying the limiting values of a
                                  parametric variable t. The variable is considered to vary linearly between these
                                  two values as the color gradient varies between the starting and ending circles.
                                  The variable t becomes the input argument to the color function(s). Default
                                  value: [ 0.0 1.0 ].

Function         function         (Required) A 1-in, n-out function or an array of n 1-in, 1-out functions (where n
                                  is the number of color components in the shading dictionary’s color space). The
                                  function(s) are called with values of the parametric variable t in the domain de-
                                  fined by the shading dictionary’s Domain entry. Each function’s domain must be
                                  a superset of that of the shading dictionary. If the value returned by the function
                                  for a given color component is out of range, it is adjusted to the nearest valid val-
                                  ue.

Extend           array            (Optional) An array of two boolean values specifying whether to extend the
                                  shading beyond the starting and ending circles, respectively. Default value:
                                  [ false false ].


           The color blend is based on a family of blend circles interpolated between the
           starting and ending circles that are defined by the shading dictionary’s Coords
           entry. The blend circles are defined in terms of a subsidiary parametric variable
                t – t0
                            -
           s = --------------
               t1 – t0

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