CHAPTER 4
304
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.
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
sh
operator, it is the current user space.