What's the difference between Rec. 709 Gamma 2.4 and HDTV (Rec. 709) in AE? Which one should I use for advanced VFX when working in linear color space?

You can use any of those or even sRGB in a linearized workspace, as the gamma in the profile is not used, only the color coordinates of the primaries and white point.

When you are working in LINEARIZED space, your gamma is 1.0

 sRGB and Rec709 use the same primary coordinates, so there is no difference between them when linearizing your workspace.

None of those profiles should be used for output though. When you are converting from linearized space to render out to a Rec709 video, you want to use the correct gamma for video files, and the gamma of such a profile is closer to 1.9-2.0, and not 2.4.

"HDTV (Rec709)" uses a 2.2 gamma curve (and appears no different than sRGB) with no offset as is specified in the actual ITU-R.

The profile I am now recommending is: Rec709-elle-V4-rec709.icc

Downloadable here:

https://github.com/ellelstone/elles_icc_profiles/blob/master/profiles/Rec709-elle-V4-rec709.icc

It is the only one I've found that appears to have the correct linear "knee" in the near black area. It seems many other profiles, including those that Adobe distributes, are using an incorrect gamma curve.

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