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                                                     305
            SECTION 4.6                                                                     Patterns




                           TABLE 4.28 Entries common to all shading dictionaries
KEY              TYPE            VALUE

ShadingType      integer         (Required) The shading type:
                                    1    Function-based shading
                                    2    Axial shading
                                    3    Radial shading
                                    4    Free-form Gouraud-shaded triangle mesh
                                    5    Lattice-form Gouraud-shaded triangle mesh
                                    6    Coons patch mesh
                                    7    Tensor-product patch mesh

ColorSpace       name or         (Required) The color space in which color values are expressed. This may be
                 array           any device, CIE-based, or special color space except a Pattern space. See
                                 “Color Space: Special Considerations” on page 306 for further information.

Background       array           (Optional) An array of color components appropriate to the color space,
                                 specifying a single background color value. If present, this color is used, be-
                                 fore any painting operation involving the shading, to fill those portions of the
                                 area to be painted that lie outside the bounds of the shading object. In the
                                 opaque imaging model, the effect is as if the painting operation were
                                 performed twice: first with the background color and then with the shading.
                                 Note: The background color is applied only when the shading is used as part of
                                 a shading pattern, not when it is painted directly with the sh operator.

BBox             rectangle       (Optional) An array of four numbers giving the left, bottom, right, and top
                                 coordinates, respectively, of the shading’s bounding box. The coordinates are
                                 interpreted in the shading’s target coordinate space. If present, this bounding
                                 box is applied as a temporary clipping boundary when the shading is painted,
                                 in addition to the current clipping path and any other clipping boundaries in
                                 effect at that time.

AntiAlias        boolean         (Optional) A flag indicating whether to filter the shading function to prevent
                                 aliasing artifacts. The shading operators sample shading functions at a rate
                                 determined by the resolution of the output device. Aliasing can occur if the
                                 function is not smooth—that is, if it has a high spatial frequency relative to
                                 the sampling rate. Anti-aliasing can be computationally expensive and is usu-
                                 ally unnecessary, since most shading functions are smooth enough or are
                                 sampled at a high enough frequency to avoid aliasing effects. Anti-aliasing
                                 may not be implemented on some output devices, in which case this flag is
                                 ignored. Default value: false.

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