Graphics Programs Reference
In-Depth Information
Transparency
PDF has a sophisticated but complicated transparency mechanism which works in
multiple color spaces, allows different types of blending, and supports grouped trans-
parencies. We only consider simple transparency here.
There are no specific transparency operators so we use the
gs
operator to load the fill
transparency level from the
/ca
entry in the
/ExtGState
entry in the page's resources.
The
/ExtGState
entry is a dictionary of collections of
external graphics state
, which we
can load in using the
gs
operator.
For our example, the resources consist of just the
/ExtGState
entry, with a single
collection of state, called
/gs1
. It contains just the
/ca
entry for fill transparency:
<< /ExtGState
<< /gs1
<< /ca 0.5 >>
Half transparent
>>
>>
Here is the corresponding content stream:
2.0 w
Select 2pt line width
/gs1 gs
Select /gs1 from external graphics state
0.75 g
Select light Gray
200 250 m 300 350 400 450 500 250 c
400 250 300 200 y h B
1 0 0 1 100 100 cm
200 250 m 300 350 400 450 500 250 c
400 250 300 200 y h B
The result is shown in
Figure 5-11
. The transparency is defined so that
0
means wholly
transparent, and
1
wholly opaque. The stroke transparency may be altered with
/CA
in
place of (or in addition to)
/ca
.
Shadings and Patterns
As well as plain colors, PDF allows various
patterns
to be used to fill and stroke objects:
•
Tiling patterns
, where a
pattern cell
is replicated over the page.
•
Shading patterns
, where a gradient between colors is used to fill an object. There
are many types, with many options and settings:
Function-based
Axial
Radial
Free-form Gouraud-shaded triangle mesh
Lattice-form Gouraud-shaded triangle mesh
Coons patch mesh
Tensor-product patch mesh
