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CHAPTER 4                                                                  Graphics



Shading Dictionaries

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 geome-
try of the gradient fill. Various shading types are available, depending on the val-
ue 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 ex-
  tended 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 pseudo-
  rectangular 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.

Note: The term target coordinate space, used in many of the following descriptions,
refers to the coordinate space into which a shading is painted. For shadings used
with a type 2 pattern dictionary, this is the pattern coordinate space, discussed in
Section 4.6.1, “General Properties of Patterns.” For shadings used directly with the
sh operator, it is the current user space.

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