How a soft mask derives its values from its transparency group.
public enum PdfSoftMaskType
ISO 32000-2 §11.6.5.1, Table 142 (S).
kind:method, kind:property, kind:ctor. Press Esc to clear.AlphaAlpha: the group's computed alpha, its colour disregarded (§11.5.2).LuminosityLuminosity: the group's colour, composited over the backdrop BC, as a luminosity (§11.5.3).