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                                          568
CHAPTER 7                                                                 Transparency



The value of the blend function B (cb , cs ) in the CompatibleOverprint mode is
either cb or cs , depending on the setting of the overprint mode parameter, the
current and group color spaces, and the source color value cs :

• If the overprint mode is 1 (nonzero overprint mode) and the current color
  space and group color space are both DeviceCMYK, then only process color
  components with nonzero values replace the corresponding component values
  of the backdrop. All other component values leave the existing backdrop value
  unchanged. That is, the value of the blend function B (cb , cs ) is the source com-
  ponent cs for any process (DeviceCMYK) color component whose (subtractive)
  color value is nonzero; otherwise it is the backdrop component cb . For spot col-
  or components, the value is always cb .
• In all other cases, the value of B (cb , cs ) is cs for all color components specified
  in the current color space, otherwise cb . For instance, if the current color space
  is DeviceCMYK or CalRGB, the value of the blend function is cs for process color
  components and cb for spot components. On the other hand, if the current col-
  or space is a Separation space representing a spot color component, the value is
  cs for that spot component and cb for all process components and all other spot
  components.

Note: In the descriptions above, the term current color space refers to the color
space used for a painting operation. This may be specified by the current color space
parameter in the graphics state (see Section 4.5.1, “Color Values”), implicitly by col-
or operators such as rg (Section 4.5.7, “Color Operators”), or by the ColorSpace en-
try of an image XObject (Section 4.8.4, “Image Dictionaries”). In the case of an
Indexed space, it refers to the base color space (see “Indexed Color Spaces” on page
262); likewise for Separation and DeviceN spaces that revert to their alternate color
space, as described under “Separation Color Spaces” on page 264 and “DeviceN
Color Spaces” on page 268.

If the current blend mode when CompatibleOverprint is invoked is any mode
other than Normal, the object being painted is implicitly treated as if it were
defined in a non-isolated, non-knockout transparency group and painted using
the CompatibleOverprint blend mode. The group’s results are then painted using
the current blend mode in the graphics state.

Note: It is not necessary to create such an implicit transparency group if the current
blend mode is Normal; simply substituting the CompatibleOverprint blend mode
while painting the object produces equivalent results. There are some additional
cases in which the implicit transparency group can be optimized out.

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