2.5 is display gamma, NOT 2.2 - Page 6

This is such a great topic I think it deserves to be necro'ed! sotti and andrewfee never gave us a formula for their reverse BT.709 curve. I wish I knew it so I could use it in an excel spreadsheet to calculate target values for my display.

We now have a standard for gamma, the BT.1886 function, which takes your display contrast into account and calculates a curve tailored to its limitations. If the display can exceed 10,000:1 contrast, then it should be set to a 2.4 power curve.

The BT.709 curve is not intended to be used on displays, and while it might be interesting to see how the "compensated" BT.709 curve they suggested compares with BT.1886 on certain displays (I suspect the "1.2" version could end up close) there's no need to debate anything now, unless you want to debate whether the BT.1886 spec should actually be applied or not.

Please note that all the DVDs and Blu-Rays you will be watching will have been professionally graded and mastered on displays calibrated to Rec709 with a gamma of 2.2 to 2.35.

We do a lot of the calibration for the professional grading and mastering rooms, so know this well.
(LightSpace CMS is one of the most used calibration systems in the professional market)
And 2.2 is the more common 'standard' used.

BT.1886, with a gamma of 2.4, will get adopted in time, but that time is not yet.

Steve

This is what I've been saying all along (my background goes back to the scanners used in some of the early digital restorations, like Snow White)... everything we are watching on Blu-ray was mastered on displays running 2.2-2.3 gamma (in my experience, I didn't see 2.35 being used, but you've probably seen more mastering setups than I have). This also explains why setting your display to 2.4 or 2.5 NOW will make everything look WAY too dark.

Trying to shift the whole industry to a different gamma now is going to have to be done carefully. The difference between 2.2-2.3 and 2.4 is fairly significant (especially 2.2 vs 2.4). Not everybody is going to change overnight and there will be legacy inventory out there for a LONG LONG LONG time with the "old" gamma encoded on them. There will have to be some kind of "trigger" embedded in newly mastered content that switches compatible video displays to 2.4 (or whatever the final standard is) for that content and allows the display to drop back to 2.2 for legacy content that is not mastered to the new, darker, gamma standard. This switchover could become a real annoyance if there's not some relatively seamless way to accommodate both legacy content and "new" content mastered to 1884 or whatever the "final" standard is -- it will be really annoying if content is always either too bright or too dim because the display gamma and content gamma don't match.

Here's the deal BT.1886 isn't a 2.4 gamma unless you're running an OLED (or some other tech with a 0 black level).

On normal displays with normal black levels, BT.1886 is closer to 2.25 than it is to 2.4.

The point of BT.1886 is that it emulates a CRT gamma curve, which is what everything was mastered on.

There is not any BT.1886 Workflow from my experience running Light Illusion LightSpace CMS & THX CineSpace, the 2 world-leader software that is used at blu-ray post-production houses around the world.

THX Cinespace has 2 worklows: ITU-R_709_g2.2 + ITU-R_709_g2.4. I don't know if it's possible to request a specific workflow from THX.

In Light Illusion LightSpace CMS you can set the gamma by entering the value you want based at power curve.

I have no info about FilmLight Truelight color management system BT.1886 support because it's a Linux/Mac OS X software.

Currently i don't believe there is any reason to follow BT.1886 curve as the blu-ray movies are not mastered to this standard.

* LightSpace has added BT.1886 at October 2012, i don't think any studio has used it yet for any blu-ray release.
Edited by ConnecTEDDD - 11/15/12 at 12:55am

Which was basically 2.4 gamma, at least for the Sony models with a correctly set-up black level. (raise the black level and you will approach 2.2)

On all the Sony CRTs we have profiled the ALL measured closer to 2.2... and I mean ALL (and we have profiled MANY over the years,,,,).

We have calibrated displays to Rec709 and BT.1886, and Rec709 is always closer to the traditional Sony CRT.

Steve

a simple 2.2 exponent curve will come very close to a reversed Rec 709 (gamma decoding) function - which is a hybrid - using a black level offset (slope) and a 2.5 exponent...

it might be that some peeps confuse the 2.5 exponent (they've heard of) in the Rec 709 hybrid decoding function (not considering the effect of the slope / offset) with a simple function with just an exponent...

so a 2.5 exponent in that decoding hybrid will result in a 2.2 exponent curve (not fully identical, but very close)...

Reading through a lot of the posts here which link to technical papers from various groups, it seems like they are measuring 2.35-2.40 gamma for their Sony CRTs?

With CRT, when you adjust the black level, you are also shifting the entire gamma curve. A lot of Sony's later CRTs also have an issue where they raise the G2 voltage to correct for tube aging, but are far too aggressive on it and end up raising the brightness. (similar to the problem Panasonic have with their Plasmas)

Black level should be set at 0.01 nits for accurate reproduction.
Rec 709 is 1.96 not 2.2

if use 1/0.45 (2.22) as the exponent in gamma decoding - exchange that with a 2.5 exponent and it will approximate a 2.2 curve...