Creates a soft-mask dictionary whose mask values come from a transparency group XObject, or returns the one this document already has for the same values.
public PdfSoftMask CreateSoftMask(PdfSoftMaskType type, PdfFormXObject group, IReadOnlyList<double>? backdrop = null, PdfFunction? transferFunction = null)
How the values are derived: the group's alpha, or its luminosity over the backdrop.
A transparency group XObject of this document (CreateTransparencyGroup()); for a luminosity mask its
group colour space shall be set. Its bounding box bounds the mask; its matrix maps it into the coordinate system of the
gs that sets the mask.
For a luminosity mask, the backdrop colour (BC) the group composites over, one component per component of the group
colour space; null, the default, is that space's initial colour, black. Not allowed for an alpha mask.
The transfer function (TR) applied to the alpha or luminosity, a function of this document from 1 input to 1 output;
null, the default, is the identity.
<< /Type /Mask /S ... /G ... >>, to set with SetSoftMask(PdfSoftMask?).ISO 32000-2 §11.6.5.1, Table 142 (PDF 1.4), §11.5.2 and §11.5.3. Outside the group's bounding box the mask value is the transfer function of 0 (alpha) or of the backdrop's luminosity (luminosity). Records PDF 1.4 and the function's version. A mutation.