Color Metadata

Gamma

Gamma is an extension that must be used in conjunction with an nclc extension. The nclc transfer function must be set to 2 = "unknown".
The standard nclc gamma amounts to 2.0 on a computer screen (QuickTime display) and 2.2 (sRGB) on TV.
Setting gamma to 2.2 makes the movie darker on the computer screen. Some people prefer this look.
RGB based movies often have gamma 1.8. Some older QuickTime movies also have gamma 1.8.

Nclc

Currently the offcial values for nclc are limited as follows:

primaries 1-5-6 (HD, PAL, NTSC)
transferFunction 1 (rec.709)
matrix 1 (HD), 6 (PAL/NTSC).

In addition the value 2 means "unknown".

In actual practice the difference between the various nclc options is only visible in strongly saturated colors. For example look at this test image: source ntsc.png
Rec.709 is like gamma = 2.0 (transferFunction = 1). In most cases you don't need a separate gamma extension.