| A PDFShader fills a given region with a shading, such as a gradient.
Shading Dictionaries (section 4.6)
A shading dictionary specifies details of a particular gradient fill,
including the type of shading to be used, the geometry of the area to
be shaded, and the geometry of the gradient fill. Various shading types
are available, depending on the value of the dictionary’s ShadingType entry:
Function-based shadings (type 1) define the color of every point in the
domain using a mathematical function (not necessarily smooth or continuous).
Axial shadings (type 2) define a color blend along a line between two
points, optionally extended beyond the boundary points by continuing
the boundary colors.
Radial shadings (type 3) define a blend between two circles,
optionally extended beyond the boundary circles by continuing the
boundary colors. This type of shading is commonly used to represent
three-dimensional spheres and cones.
Free-form Gouraud-shaded triangle meshes (type 4) define a
common construct used by many three-dimensional applications to
represent complex colored and shaded shapes. Vertices are specified
in free-form geometry.
Lattice-form Gouraud-shaded triangle meshes (type 5) are based on
the same geometrical construct as type 4 but with vertices specified
as a pseudorectangular lattice.
Coons patch meshes (type 6) construct a shading from one or more
color patches, each bounded by four cubic Bézier curves.
Tensor-product patch meshes (type 7) are similar to type 6 but
with additional control points in each patch, affording greater
control over color mapping.
Table 4.28 shows the entries that all shading dictionaries share
in common; entries specific to particular shading types are
described in the relevant sections below.
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