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      SECTION 7.6                                           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
      Space”). The resulting colors are then interpreted in that color space when the
      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

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