SECTION 7.6
561
Color Space and Rendering Issues
of objects painted with the
Normal
blend mode, this behavior can be optimized
by treating the pattern cell as if it were an isolated group. Since in this case the
results depend only on the color, shape, and opacity of the pattern cell and not on
those of the backdrop, the pattern cell can be evaluated once and then replicated,
just as in opaque painting.
Note:
In a raster-based implementation of tiling, it is important that all tiles togeth-
er be treated as a single transparency group. This avoids artifacts due to multiple
marking of pixels along the boundaries between adjacent tiles.
The foregoing discussion applies to both colored (
PaintType
1) and uncolored
(
PaintType
2) tiling patterns. In the latter case, the restriction that an uncolored
pattern’s definition may not specify colors extends as well to any transparency
group that the definition may include. There are no corresponding restrictions,
however, on specifying transparency-related parameters in the graphics state.
7.6 Color Space and Rendering Issues
This section describes the interactions between transparency and other aspects of
color specification and rendering in the Adobe imaging model.
7.6.1 Color Spaces for Transparency Groups
As discussed in Section 7.5.5, “Transparency Group XObjects,” a transparency
group can either have an explicitly declared color space of its own or inherit that
of its parent group. In either case, the colors of source objects within the group
are converted to the group’s color space, if necessary, and all blending and com-
positing computations are done in that space (see Section 7.2.3, “Blending Color
group is subsequently composited with its backdrop.
Under this arrangement, it is envisioned that all or most of a given piece of art-
work will be created in a single color space—most likely, the working color space
of the application generating it. The use of multiple color spaces typically will
arise only when assembling independently produced artwork onto a page. After
all the artwork has been placed on the page, the conversion from the group’s color
space to the page’s device color space will be done as the last step, without any
further transparency compositing. The transparent imaging model does not re-
quire that this convention be followed, however; the reason for adopting it is to