2.5 is display gamma, NOT 2.2 - Page 6 - AVS Forum | Home Theater Discussions And Reviews

Forum Jump: 
 1Likes
Reply
 
Thread Tools
post #151 of 161 Old 11-13-2012, 12:39 PM
AVS Special Member
 
Elix's Avatar
 
Join Date: Jun 2011
Location: Dungeon, Pillar of Eyes
Posts: 1,542
Mentioned: 4 Post(s)
Tagged: 0 Thread(s)
Quoted: 215 Post(s)
Liked: 98
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.
Elix is offline  
Sponsored Links
Advertisement
 
post #152 of 161 Old 11-13-2012, 04:01 PM
AVS Forum Special Member
 
Chronoptimist's Avatar
 
Join Date: Sep 2009
Posts: 3,073
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Liked: 500
Quote:
Originally Posted by Elix View Post

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.
Chronoptimist is offline  
post #153 of 161 Old 11-14-2012, 05:10 AM
AVS Special Member
 
Light Illusion's Avatar
 
Join Date: Aug 2010
Posts: 1,025
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 280 Post(s)
Liked: 573
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

Steve Shaw
LIGHT ILLUSION

Light Illusion is online now  
post #154 of 161 Old 11-14-2012, 11:28 AM
AVS Special Member
 
Doug Blackburn's Avatar
 
Join Date: May 2008
Location: San Francisco - East Bay area
Posts: 3,650
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 187 Post(s)
Liked: 352
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.

"Movies is magic..." Van Dyke Parks
THX -- ISF -- HAA
Doug Blackburn is offline  
post #155 of 161 Old 11-14-2012, 11:41 AM
AVS Forum Special Member
 
sotti's Avatar
 
Join Date: Aug 2004
Location: Seattle, WA
Posts: 7,103
Mentioned: 7 Post(s)
Tagged: 1 Thread(s)
Quoted: 329 Post(s)
Liked: 328
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.

Joel Barsotti
SpectraCal
CalMAN Lead Developer
sotti is offline  
post #156 of 161 Old 11-14-2012, 12:19 PM
AVS Special Member
 
ConnecTEDDD's Avatar
 
Join Date: Sep 2010
Location: Athens, Greece
Posts: 3,889
Mentioned: 6 Post(s)
Tagged: 1 Thread(s)
Quoted: 755 Post(s)
Liked: 1213
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.

Ted's LightSpace CMS Calibration Disk Free Version for Free Calibration Software: LightSpace DPS / CalMAN ColorChecker / HCFR
S/W: LightSpace CMS, SpaceMan ICC, SpaceMatch DCM, CalMAN 5, CalMAN RGB, ChromaPure, ControlCAL
V/P: eeColor 3D LUT Box - P/G: DVDO AVLab TPG
Meters: JETI Specbos 1211, Klein K-10A, i1PRO2, i1PRO, SpectraCAL C6, i1D3, C5
ConnecTEDDD is offline  
post #157 of 161 Old 11-14-2012, 07:50 PM
AVS Forum Special Member
 
Chronoptimist's Avatar
 
Join Date: Sep 2009
Posts: 3,073
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Liked: 500
Quote:
Originally Posted by sotti View Post

The point of BT.1886 is that it emulates a CRT gamma curve, which is what everything was mastered on.
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)
Chronoptimist is offline  
post #158 of 161 Old 11-15-2012, 01:42 AM
AVS Special Member
 
Light Illusion's Avatar
 
Join Date: Aug 2010
Posts: 1,025
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 280 Post(s)
Liked: 573
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
bonafied likes this.

Steve Shaw
LIGHT ILLUSION

Light Illusion is online now  
post #159 of 161 Old 11-20-2012, 03:46 AM
Advanced Member
 
Iron Mike's Avatar
 
Join Date: Oct 2006
Posts: 882
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 61 Post(s)
Liked: 407
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)...

calibration & profiling solutions: Lightspace, Spaceman, Calman, Argyll, ColorNavigator, basICColor
profiling & calibration workflow tools: Display Calibration Tools
meter: CR-250, Klein K-10 A, i1Pro, i1D3
AVS thread: Lightspace & Custom Color Patch Set & Gamma Calibration on Panasonic 65VT60
Iron Mike is offline  
post #160 of 161 Old 11-20-2012, 06:20 AM
AVS Forum Special Member
 
Chronoptimist's Avatar
 
Join Date: Sep 2009
Posts: 3,073
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Liked: 500
Quote:
Originally Posted by Light Illusion View Post

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.
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.
Quote:
Originally Posted by Iron Mike View Post

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)...
Rec 709 is 1.96 not 2.2
Chronoptimist is offline  
post #161 of 161 Old 11-20-2012, 07:23 AM
Advanced Member
 
Iron Mike's Avatar
 
Join Date: Oct 2006
Posts: 882
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 61 Post(s)
Liked: 407
Quote:
Originally Posted by Chronoptimist View Post

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...

calibration & profiling solutions: Lightspace, Spaceman, Calman, Argyll, ColorNavigator, basICColor
profiling & calibration workflow tools: Display Calibration Tools
meter: CR-250, Klein K-10 A, i1Pro, i1D3
AVS thread: Lightspace & Custom Color Patch Set & Gamma Calibration on Panasonic 65VT60
Iron Mike is offline  
Sponsored Links
Advertisement
 
Reply Display Calibration

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page


Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off