Table 144 – Entries in a soft-mask dictionary

Key

Type

Value

Type

name

(Optional) The type of PDF object that this dictionary describes; if present, shall 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:

AlphaThe group’s computed alpha shall be used, disregarding its colour (see “Deriving a Soft Mask from Group Alpha” ).

LuminosityThe group’s computed colour shall be converted to a single-component luminosity value (see “Deriving a Soft Mask from Group Luminosity” ).

G

stream

(Required) A transparency group XObject (see “Transparency Group XObjects” ) to be used as the source of alpha or colour values for deriving the mask. If the subtype S is Luminosity, the group attributes dictionary shall contain a CS entry defining the colour space in which the compositing computation is to be performed.

BC

array

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

TR

function or name

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