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                                                   572
         CHAPTER 7                                                                  Transparency




                        AFFECTED COLOR             VALUE OF BLEND FUNCTION B (cb , cs ) EXPRESSED AS TINT
SOURCE COLOR SPACE      COMPONENT OF
                        GROUP COLOR SPACE
                                                   OP FALSE        OP TRUE, OPM 0          OP TRUE, OPM 1

A group (not an         All color             cs                   cs                 cs
elementary object)      components


         Color component values are represented in these tables as subtractive tint values
         because overprinting is typically applied to subtractive colorants such as inks
         rather than to additive ones such as phosphors on a display screen. The Compati-
         bleOverprint blend mode is therefore described as if it took subtractive argu-
         ments and returned subtractive results. In reality, however, CompatibleOverprint
         (like all blend modes) treats color components as additive values; subtractive
         components must be complemented before and after application of the blend
         function.

         Note an important difference between the two tables. In Table 7.14, the process
         color components being discussed are the actual device colorants—the color
         components of the output device’s native color space (DeviceGray, DeviceRGB, or
         DeviceCMYK). In Table 7.15, the process color components are those of the
         group’s color space, which is not necessarily the same as that of the output device
         (and can even be something like CalRGB or ICCBased). For this reason, the pro-
         cess color components of the group color space cannot be treated as if they were
         spot colors in a Separation or DeviceN color space (see Section 7.6.2, “Spot
         Colors and Transparency”). This difference between opaque and transparent
         overprinting and erasing rules arises only within a transparency group (including
         the page group, if its color space is different from the native color space of the
         output device). There is no difference in the treatment of spot color components.

         Table 7.15 has one additional row at the bottom. It applies when painting an ob-
         ject that is a transparency group rather than an elementary object (fill, stroke,
         text, image, or shading). As stated in Section 7.6.2, “Spot Colors and Transparen-
         cy,” a group is considered to paint all color components, both process and spot.
         Color components that were not explicitly painted by any object in the group
         have an additive color value of 1.0 (subtractive tint 0.0). Since no information is
         retained about which components were actually painted within the group, com-
         patible overprinting is not possible in this case; the CompatibleOverprint blend
         mode reverts to Normal, with no consideration of the overprint and overprint
         mode parameters. (A transparency-aware application can choose a more suitable
         blend mode, such as Darken, to produce an effect similar to overprinting.)

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