(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 specified by the CS entry in the group attributes dictionary. Default value: the color space’s initial value, representing black.

(Required) A transparency group XObject to be used as the source of alpha or color values for deriving 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.

(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. Luminosity: Convert the group’s computed color to a single-component luminosity value.

(Optional) A function object specifying the transfer function to be used in deriving the mask values. The function accepts 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.

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