Represents the "TransferFunction" entry of the type 1 halftone dictionary object.

It is an optional entry defined as part of the PDF 1.0 specification.

This property may contain one of two different types:.

1) A string representing a PDF name object.

If provided, this item must always take the value "Identity".

2) A FunctionElement.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 130, page 310.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 128, page 371.