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       SECTION 7.5                                            Specifying Transparency in PDF




                          TABLE 7.10 Entries in a soft-mask dictionary
KEY    TYPE                 VALUE

Type   name                 (Optional) The type of PDF object that this dictionary describes; if present,
                            must be Mask for a soft-mask dictionary.

S      name                 (Required) A subtype specifying the method to be used in deriving the mask
                            values from the transparency group specified by the G entry:
                               Alpha         Use the group’s computed alpha, disregarding its color (see
                                             Section 7.4.1, “Deriving a Soft Mask from Group Alpha”).
                               Luminosity    Convert the group’s computed color to a single-component
                                             luminosity value (see Section 7.4.2, “Deriving a Soft Mask
                                             from Group Luminosity”).

G      stream               (Required) A transparency group XObject (see Section 7.5.5, “Transparency
                            Group XObjects”) to be used as the source of alpha or color values for deriv-
                            ing the mask. If the subtype S is Luminosity, the group attributes dictionary
                            must contain a CS entry defining the color space in which the compositing
                            computation is to be performed.

BC     array                (Optional) An array of component values specifying the color to be used as
                            the backdrop against which to composite the transparency group XObject G.
                            This entry is consulted only if the subtype S is Luminosity. The array consists
                            of n numbers, where n is the number of components in the color space speci-
                            fied by the CS entry in the group attributes dictionary (see Section 7.5.5,
                            “Transparency Group XObjects”). Default value: the color space’s initial
                            value, representing black.

TR     function or name     (Optional) A function object (see Section 3.9, “Functions”) specifying the
                            transfer function to be used in deriving the mask values. The function ac-
                            cepts one input, the computed group alpha or luminosity (depending on the
                            value of the subtype S), and returns one output, the resulting mask value.
                            Both the input and output must be in the range 0.0 to 1.0; if the computed
                            output falls outside this range, it is forced to the nearest valid value. The
                            name Identity may be specified in place of a function object to designate the
                            identity function. Default value: Identity.


       Soft-Mask Images

       The second way to define a soft mask is by associating a soft-mask image with an
       image XObject. This is a subsidiary image XObject specified in the SMask entry

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