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CHAPTER 3                                                                    Syntax



• A premultiplied opacity channel provides samples that have been multiplied
  into the color samples of those channels with which it is associated.

Opacity and premultiplied opacity channels are associated with specific color
channels. There is never more than one opacity channel (of either type)
associated with a given color channel. For example, it is possible for one opacity
channel to apply to the red samples and another to apply to the green and blue
color channels of an RGB image.

Note: The method by which the opacity information is to be used is explicitly not
specified, although one possible method shows a normal blending mode.

In addition to using opacity channels for describing transparency, JPX files also
have the ability to specify chroma-key transparency. A single color is specified by
giving an array of values, one value for each color channel. Any image location
that matches this color is considered to be completely transparent.

Images in JPX files can have one of the following color spaces:
• A predefined color space, chosen from a list of enumerated color spaces. (Two of
  these are actually families of spaces and parameters are included.)
• A “restricted ICC profile.” (These are the only sorts of ICC profiles that are al-
  lowed in JP2 files.)
• An input ICC profile of any sort defined by ICC-1.
• A vendor-defined color space.
More than one color space may be specified for an image, with each space being
tagged with a precedence and an approximation value that indicates how well it
represents the preferred color space. In addition, the image’s color space may
serve as the foundation for a palette of colors that are selected using samples
coming from the image’s data channels: the equivalent of an Indexed color space
in PDF.

There are other features in the JPX format beyond describing a simple image.
These include provisions for describing layering and giving instructions on
composition, specifying simple animation, and including generic XML metadata
(along with JPEG2000-specific schemas for such data). It is recommended, but
not required, that relevant metadata be replicated in the image dictionary’s
Metadata stream in XMP format (see Section 10.2.2, “Metadata Streams).

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