CHAPTER 7
574
Transparency
•
The current blend mode in the graphics state is
Normal
(or
Compatible
, which
is treated as equivalent to
Normal
).
•
The current soft mask in the graphics state is
None
. If the object is an image
XObject, there is no
SMask
entry in its image dictionary.
•
The foregoing three conditions were also true at the time the
Do
operator was
invoked for the group containing the object, as well as for any direct ancestor
groups.
•
If the current color is a tiling pattern, all objects in the definition of its pattern
cell also satisfy the foregoing conditions.
Together, these conditions ensure that only the object itself contributes to the col-
or at the given point, completely obscuring the backdrop. For portions of the page
whose topmost object is not fully opaque or that are never painted at all, the de-
fault halftone and transfer function for the page are used.
Note:
If a graphics object is painted with overprinting enabled—that is, if the applic-
able (stroking or nonstroking) overprint parameter in the graphics state is
true
—the
halftone and transfer function to use at a given point must be determined indepen-
dently for each color component. Overprinting implicitly invokes the Compatible-
Overprint blend mode (see “Compatibility with Opaque Overprinting” on page
Overprint yields the source color (not the backdrop color) for that component.
Rendering Intent and Color Conversions
The rendering intent, black-generation, and undercolor-removal parameters
need to be handled somewhat differently. The rendering intent influences the
conversion from a CIE-based color space to a target color space, taking into ac-
count the target space’s color gamut (the range of colors it can reproduce).
Whereas in the opaque imaging model the target space is always the native color
space of the output device, in the transparent model it may instead be the group
color space of a transparency group into which an object is being painted.
The rendering intent is needed at the moment such a conversion must be per-
formed—that is, when painting an elementary or group object specified in a CIE-
based color space into a parent group having a different color space. This differs
from the current halftone and transfer function, whose values are used only when
all color compositing has been completed and rasterization is being performed.