Represents the "Gamma" entry of the calgray colour space dictionary object.

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

It contains a double representing a PDF numeric object.

It should have a positive value.

The PDF specification states that this item assumes a value of 1 if no value has been provided.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 63, page 145.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 62, page 183.

The gamma value defines the exponent applied to the encoded gray component before it is scaled by the white point's Y value. A value of 1.0 means the encoding is linear; values greater than 1.0 apply a toe curve that darkens the midtones relative to linear encoding.

Typical display gammas are around 2.2. Setting gamma to match the encoding of the source material ensures that the decoded values are linear light values suitable for CIE colorimetric calculations.