QUESTION: Why doesn't Rec709 have a specific gamma?

Rec709 has always specified a gamma, it is:

OETC It details the scene referred encoding gamma for cameras and Rec709 video signals. It is scene referred, not output referred.

In the initial Rec709 spec (1993) output referred gamma assumed to be the "known quantity" of CRT type displays that were in use at the time. Output gamma was defined as Rec1886 in 2011for the display itself, and Rec2035 in 2013 to define the associated viewing environment.

A principle reason a scene referred profile was defined for the SIGNAL (i.e. how you OUTPUT from AE for example) was due to the unknown variations in viewing environment and how the monitor was adjusted for a particular environment. The correct output gamma for a given display will vary based on the viewing environment. Thus "scene referred" is the best practice for the signal.

When you *export* from AE using the "BT-709-5" profile or "HD 709-A" profile, it uses an approximate 1.9 Gamma Curve as defined in the standard (it is approximate because the standard defines a linearized portion near black that offsets the gamma curve slightly). There are however other profiles floating around that are "named" Rec709 that are using other gamma curves, for instance the profile named "HDTV (Rec709)" seems to be using a 2.2 gamma curve. While this may be the appropriate one to use for WEB video, it is technically incorrect as far as the Rec709 standard is concerned.

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