To be honest, I don't think it really matters exactly what display gamma is used for mastering or home viewing, as long as it's roughly the inverse of lightness in an average surround. What's more critical, in my opinion, is the relationship between the display's brightness and it's surroundings, which will vary depending on the application.
Display gamma is an important part of this equation though. If you're eyeballing the video correction in a room with gray walls and 16 lux illumination on a 100 cd/m^2 display, you won't get the same result with both a 2.20 and 2.40 display gamma. The correction/encode done with a 2.20 display gamma will almost certainly be darker.
To get around this problem, my suggestion is to use the display as a reference for adjusting either the display's white level or the surround levels, depending on which is easier to tweak in the setup. For home video content (ie DVD, Blu-ray, broadcast), IMO the surround reference should be close to the APL of video over time, and typical light levels used for home viewing (which may vary depending on the time of day). For "prime-time" video content in the US, I recommend a surround reference that's somewhere between a 12.5% to 20% stimulus gray, or nominally about 15% stimulus gray. Other parts of the world, and other video applications (such as web content) may differ.
In 8-bit consumer video, a 15% stimulus gray would be around R'G'B' or Y' = 49...
(49-16)/(235-16) = .15
^ Note this is in compressed 16 to 235 "studio swing" levels. In 0 to 255 "full swing" levels (before the palette is compressed to video levels), the value would be R'G'B' = 38.
38/255 = .15
IMO though, this is the essential paradigm that our current video system is based on...
- ENCODING GAMMA = roughly a square root, or ~.50
- DECODING/DISPLAY GAMMA = roughly the geometric mean of a cube and square, or ~2.45
All of the recent authoritative references I've run across on this subject support a display gamma for video production of ~2.40 (which is pretty close to the 2.45 value above). This includes Rec. 1886, EBU Tech 3320 (see Annex A: Gamma considerations), Poynton, and Motta. They all seem to agree that the value for display gamma should be around 2.40 (not 2.2).
This 2.40 reference value is based on both the gamma of legacy CRTs (per Motta and studies by the EBU/BBC), and on perception of lightness in an average surround (per Poynton), even though mastering and prime-time viewing is usually done in a dimmer surround. IMHO, using substantially lower values for mastering than this 2.40 value will result in poorer perceptual performance. (More about this issue here.)
Here's what I believe is actually happening though in current video mastering in the US...
- TYPICAL ENCODING GAMMA = .50 to .55 (nominally about .525)
- TYPICAL DECODING/DISPLAY GAMMA = 2.20 to 2.50 (nominally about 2.35)
For the record, I have no specific recommendation for display gamma on the users' end, except to stay in the neighborhood of the reference or typical values cited above, and to use whatever is most comfortable and appealing to your eyes in your typical veiwing arrangement.
For mastering US video content my recommendation would be to stay pretty close to the Rec. 1886 reference display value of 2.40 and/or the nominal value of 2.35 for best results. The only justification I can see for using lower values on the order of 2.2 would be ergonomic considerations (to reduce eye-strain).
For encoding prime-time US video content (for DVDs, Blu-rays, etc.) I suggest using whatever encoding value looks best with an ~15% stimulus surround reference as described above. IMO that should implicitely result in encodes close to the typical range of .50 to .55 above, or about .520 to .525 on average.
If you look at the graph below, you'll notice that Rec. 709 has an effective exponent of .520-.525 right around 40% stimulus, which would be roughly the range of brightness for a perceptually middle gray in the dimmer surrounds used for mastering and home viewing. IMO this is no coincidence.
YMMV of course, on all of the above. And other applications may work better with different values.
Edited by ADU - 7/18/12 at 4:58pm