Represents the "ca" entry of the annotation dictionary object.

It is an optional entry defined as part of the PDF 2.0 specification.

It contains a double representing a PDF numeric object.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 164, page 383.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 166, page 466.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 252, page 467.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 255, page 565.

The non-stroking constant alpha sets the opacity of the annotation's fill or background, participating in the transparency model in the same way as the graphics state non-stroking alpha parameter.