Previous Next

                                        562
CHAPTER 7                                                              Transparency



avoid the loss of color information and the introduction of errors resulting from
unnecessary color space conversions.

Only an isolated group may have an explicitly declared color space of its own.
Non-isolated groups must inherit their color space from the parent group (sub-
ject to special treatment for the page group, as described in Section 7.3.6, “Page
Group”). This is because the use of an explicit color space in a non-isolated group
would require converting colors from the backdrop’s color space to that of the
group in order to perform the compositing computations. Such conversion may
not be possible (since some color conversions can be performed only in one
direction), and even if possible, it would entail an excessive number of color con-
versions.

The choice of a group color space has significant effects on the results that are
produced:

• As noted in Section 7.2.3, “Blending Color Space,” the results of compositing in
  a device color space is device-dependent. For the compositing computations to
  work in a device-independent way, the group’s color space must be CIE-based.
• A consequence of choosing a CIE-based group color space is that only CIE-
  based spaces can be used to specify the colors of objects within the group. This
  is because conversion from device to CIE-based colors is not possible in gener-
  al; the defined conversions work only in the opposite direction. See below for
  further discussion.
• The compositing computations and blend functions generally compute linear
  combinations of color component values, on the assumption that the compo-
  nent values themselves are linear. For this reason, it is usually best to choose a
  group color space that has a linear gamma function. If a nonlinear color space
  is chosen, the results are still well-defined, but the appearance may not match
  the user’s expectations. Note, in particular, that the CIE-based sRGB color
  space (see page 256) is nonlinear and hence may be unsuitable for use as a
  group color space.

Note: Implementations of the transparent imaging model are advised to use as
much precision as possible in representing colors during compositing computations
and in the accumulated group results. To minimize the accumulation of roundoff er-
rors and avoid additional errors arising from the use of linear group color spaces,
more precision is needed for intermediate results than is typically used to represent
either the original source data or the final rasterized results.

Previous Next