Spears & Munsil HD Benchmark Blu-Ray 2nd Edition - Page 19 - AVS Forum
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post #541 of 722 Old 12-20-2013, 02:11 AM
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Whan you guys say tv manufacturers need to be behind it, do you mean a tv needs to have it as a selectable preset, vamma 1886? Because most calibration/hometheater enthusiasts will try to get a tv with 10 point gamma or an external videoprocessor, so for those people it's possible to achive bt1886 right?
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post #542 of 722 Old 12-20-2013, 02:24 AM
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Quote:
Originally Posted by spacediver View Post

What do you mean by decoding function? Are you talking about a display end luminance function? As far as I understand, there's only an encoding function in Rec 709 (camera gamma).

when u ENcode something, u have to then DEcode it...

using the original Rec 709 decoding function (w/ the slope / offset) and a 2.2 exponent or 1/0.45 (2.22222) exponent comes very close to straight 2.0 Gamma... that's what most TV's and older TV's equal to from the manufacturer...

they changed that, so using now a 2.4 exponent in the Rec 709 decoding function (w/ the slope / offset) comes very close to a straight 2.2 Gamma...

so just imagine post grades on BT.1886 2.4 and the viewer watches on a standard uncalibrated display with straight 2.0 gamma....
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post #543 of 722 Old 12-20-2013, 02:30 AM
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Quote:
Originally Posted by Wouter73 View Post

Whan you guys say tv manufacturers need to be behind it, do you mean a tv needs to have it as a selectable preset, vamma 1886? Because most calibration/hometheater enthusiasts will try to get a tv with 10 point gamma or an external videoprocessor, so for those people it's possible to achive bt1886 right?

Yes, and if you have an external video processor/LUT box, it should be a simple matter to achieve bt.1886
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post #544 of 722 Old 12-20-2013, 02:44 AM
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Quote:
Originally Posted by Iron Mike View Post

when u ENcode something, u have to then DEcode it...

using the original Rec 709 decoding function (w/ the slope / offset) and a 2.2 exponent or 1/0.45 (2.22222) exponent comes very close to straight 2.0 Gamma... that's what most TV's and older TV's equal to from the manufacturer...

they changed that, so using now a 2.4 exponent in the Rec 709 decoding function (w/ the slope / offset) comes very close to a straight 2.2 Gamma...

I'm sorry, I'm still not following. You keep mentioning a REC 709 decoding function. I'm not aware of any such thing.

An encoding gamma of 0.45 (which is specified in Rec 709) combined with the inverse decoding gamma (and a decoding gamma wasn't specified until BT.1886) will preserve the original luminance information of the captured scene (except in dim surround conditions: see below).

The defacto display gamma (~2.4) combined with the 0.45 encoding gamma means that the end -to-end exponent is about 1.1-1.2, which actually results in a better image when you have a dim surround compared to using a display gamma of 2.2, which is the exact inverse.
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post #545 of 722 Old 12-20-2013, 02:48 AM
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Quote:
Originally Posted by Iron Mike View Post


so just imagine post grades on BT.1886 2.4 and the viewer watches on a standard uncalibrated display with straight 2.0 gamma....

Yes that would be awful, which is why we calibrate our displays...

Also, how is the situation worse than if the reference display was using a pure 2.4 gamma function?
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post #546 of 722 Old 12-20-2013, 02:52 AM
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Originally Posted by spacediver View Post

I'm sorry, I'm still not following. You keep mentioning a REC 709 decoding function. I'm not aware of any such thing.

An encoding gamma of 0.45 (which is specified in Rec 709) combined with the inverse decoding gamma (and a decoding gamma wasn't specified until BT.1886) will preserve the original luminance information of the captured scene (except in dim surround conditions: see below).

The defacto display gamma (~2.4) combined with the 0.45 encoding gamma means that the end -to-end exponent is about 1.1-1.2, which actually results in a better image when you have a dim surround compared to using a display gamma of 2.2, which is the exact inverse.

so - traveling back in time here - when the camera back in the day encoded the signal, it then had to be decoded... preferably by the exact inverse function.

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post #547 of 722 Old 12-20-2013, 02:58 AM
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Yes, it was naturally decoded by the CRT. It was never explicitly formulated into a recommendation, and was never "programmed" into the displays.

The recommended encoding function for video was, as I understand it, defined precisely because of the way CRTs worked.
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post #548 of 722 Old 12-20-2013, 03:31 AM
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The electron guns were the "decoders" on CRTs. BT.1886 is an idealized electron gun voltage to luminance transfer.

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post #549 of 722 Old 12-20-2013, 08:27 AM
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Quote:
Originally Posted by ConnecTEDDD View Post

THX CineSpace/CineCube, a big player in Color Management Solutions that pro industry was using for a lot of years (5-6 years already) for profiling broadcasting flat panel displays had REC.709 option with fixed value of 2.2 gamma only.

CineSpace's Private Secured FTP where I have access, there no support for BT1886, just at August 2013 they added the Target Profiling File for REC.709 2.4 Power Gamma, before August 2013 it were available only REC709 2.2 option.

THX Cinespace has 2 worklows: ITU-R_709_g2.2 + ITU-R_709_g2.4, no BT1886.

And there were many post-production falicities with THX CineCube installed that before August 2013, the 2.4 REC.709 has not possible to be used.

One of the reasons most post-production facilities are switching to a system that DOES have BT1886 and other advanced features.

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post #550 of 722 Old 12-20-2013, 09:09 AM
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Wow, this thread has taken off in turbo mode, but unfortunately for me, I don't understand most of it. cool.gif

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post #551 of 722 Old 12-20-2013, 09:25 AM
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Quote:
Originally Posted by Iron Mike View Post

when u ENcode something, u have to then DEcode it...

using the original Rec 709 decoding function (w/ the slope / offset) and a 2.2 exponent or 1/0.45 (2.22222) exponent comes very close to straight 2.0 Gamma... that's what most TV's and older TV's equal to from the manufacturer...

they changed that, so using now a 2.4 exponent in the Rec 709 decoding function (w/ the slope / offset) comes very close to a straight 2.2 Gamma...

so just imagine post grades on BT.1886 2.4 and the viewer watches on a standard uncalibrated display with straight 2.0 gamma....

Rec 709 never specified a decoding gamma, just an encoding gamma. There was never an expectation that the TV's gamma would be an inverse of the encoding gamma. In fact, they explicitly did not try to make the encoding gamma an exact inverse because they found that when TV had a final end-to-end gamma of around 1.2 the picture looked better. You got a little extra contrast which looked good. Keep in mind that video studios had to be really brightly lit with flat shadowless lighting. A little bit of positive gamma (making the picture slightly darker) added some snap. So the encoding gamma is close to 1/2 (not 1/2.2) and the decoding gamma was expected to be around 2.4-2.5. BT.1886 doesn't change that, it just codifies existing practice, or at least codifies practice in the CRT era.

It's not unlike film - the end-to-end gamma of most photographs is above 1, because people tend to take photos in different lighting conditions than they look at them. You can think of it as a very simple attempt at an appearance model.

TVs never had 2.0 gamma in the CRT era - they had the native CRT gamma, which is close to 2.4 normally, but if the brightness was turned up they had an effective gamma closer to 2.2 or 2.0, as we've seen. This worked out nicely. If people were watching TV in a bright room, they'd tend to turn the brightness up, which lowered the effective gamma.

I don't know of any current flat panel that comes from the factory with 2.0 gamma, but it wouldn't surprise me to find out that are some. It seems to me that lots of current flat panels are taking advantage of the fact that they have a LUT to do wacky things with the gamma, like adding an S-curve to add pop to the image and other stuff that doesn't fit the normal model of a power curve with offsets.

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post #552 of 722 Old 12-20-2013, 09:41 AM
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Quote:
Originally Posted by Iron Mike View Post

when u ENcode something, u have to then DEcode it...

using the original Rec 709 decoding function (w/ the slope / offset) and a 2.2 exponent or 1/0.45 (2.22222) exponent comes very close to straight 2.0 Gamma... that's what most TV's and older TV's equal to from the manufacturer...

they changed that, so using now a 2.4 exponent in the Rec 709 decoding function (w/ the slope / offset) comes very close to a straight 2.2 Gamma...

so just imagine post grades on BT.1886 2.4 and the viewer watches on a standard uncalibrated display with straight 2.0 gamma....

Rec.709 only specified a camera gamma function, it is very explicitly for encoding.

So go read some of Poynton's texts on gamma. You'll see he advises to view content with an end to end gamma of 1.1-1.2. The Camera encode gamma is extremely shallow and looks nothing like flat 2.2, it has a linear tail and an offset that make it completely unsuitable for viewing. If you throw an exponent around the full function and raise it to 1.2, you'll see something that looks very similiar to BT.1886.

There has never, ever been a HDTV display gamma standard util BT.1886.

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post #553 of 722 Old 12-20-2013, 10:54 AM - Thread Starter
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Quote:
Originally Posted by derekjsmith View Post

Quote:
Originally Posted by ConnecTEDDD View Post

THX CineSpace/CineCube, a big player in Color Management Solutions that pro industry was using for a lot of years (5-6 years already) for profiling broadcasting flat panel displays had REC.709 option with fixed value of 2.2 gamma only.

CineSpace's Private Secured FTP where I have access, there no support for BT1886, just at August 2013 they added the Target Profiling File for REC.709 2.4 Power Gamma, before August 2013 it were available only REC709 2.2 option.

THX Cinespace has 2 worklows: ITU-R_709_g2.2 + ITU-R_709_g2.4, no BT1886.

And there were many post-production falicities with THX CineCube installed that before August 2013, the 2.4 REC.709 has not possible to be used.

One of the reasons most post-production facilities are switching to a system that DOES have BT1886 and other advanced features.

Most of them have installed LightSpace CMS, and those who don't have LS, they use TrueLight and THX CineCube. as far I have seen by looking pro-related forums (Red/ARRI etc.) and others that colorists / directors of photography are talking etc.


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post #554 of 722 Old 12-20-2013, 11:31 AM
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I've found that on many displays 2.2 gamma simply looks more natural in terms of shadow detail and overall mid tone brightness. This is with simple power gamma, not bt.1886.


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post #555 of 722 Old 12-20-2013, 12:37 PM
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This is ridiculous.

Do we have a "standard" or not?


2.22 has always looked correct to me.
2.4 is to contrasty to me
I have not tried bt.1886 yet.


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post #556 of 722 Old 12-20-2013, 12:58 PM
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Just to be realistic may I suggest that the mastering houses are not in the business of making things easy for the 1% of people who actually calibrate their displays. I think they, the display manufacturers and 99% of the viewing public have no dollar interest in standardisation.
They realise that the public will simply adjust the brightness if they think things look too dark / bright (as Don eloquently posted).

Derek is having some success in getting BT.1886 accepted by at least one studio but frankly perhaps because it is unimportant to them they maybe can't remember what gamma they mastered it to otherwise it would be clearly displayed on the Blu ray disc given the same prominence as the anti copying notice.
We hear Ted telling us how easy it is to produce an LUT in another gamma so perhaps we should just accept that until this capability is locked down to a standard at the mastering studios there is no control of gamma.

With this uncertainty in mind, I am reasonably happy to accept BT.1886 as the standard I should calibrate to being midway between the possible values of 2.2 to 2.6.
Not very scientific I know but presumably much better (from a viewing point of view) than guessing at one of the two extremes.

My worry is that the mastering houses will see a financial opportunity to guarantee any gamma they master to.

Incidentally, is there any simple way to discover the mastered gamma of my Blu Ray collection other than saying it looks a bit dark?
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post #557 of 722 Old 12-20-2013, 01:06 PM - Thread Starter
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If you take 3 different displays, lets say, one plasma, one LED-backlit LCD and one CCFL-LCD backlighting and you calibrate them using the same gamma target value, if you display the same material after that the same time from that 3 panels, it will be different at shadow detai/mid-tones etc. etc.. to your eyes....

An idea of having 2 different memories to your setup and swap after your movie starts, to the one that it gives you the best performace using your eyes is the best solution right now.

We can't do anything about it, and we don't know if the monitors that are calibrated at studios are calibrated so tight to the specs..... or how frequently they are calibrated... a mystery.....


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post #558 of 722 Old 12-20-2013, 01:13 PM
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Quote:
Originally Posted by dmunsil View Post

Rec 709 never specified a decoding gamma, just an encoding gamma. There was never an expectation that the TV's gamma would be an inverse of the encoding gamma. In fact, they explicitly did not try to make the encoding gamma an exact inverse because they found that when TV had a final end-to-end gamma of around 1.2 the picture looked better. You got a little extra contrast which looked good. Keep in mind that video studios had to be really brightly lit with flat shadowless lighting. A little bit of positive gamma (making the picture slightly darker) added some snap. So the encoding gamma is close to 1/2 (not 1/2.2) and the decoding gamma was expected to be around 2.4-2.5. BT.1886 doesn't change that, it just codifies existing practice, or at least codifies practice in the CRT era.

It's not unlike film - the end-to-end gamma of most photographs is above 1, because people tend to take photos in different lighting conditions than they look at them. You can think of it as a very simple attempt at an appearance model.

TVs never had 2.0 gamma in the CRT era - they had the native CRT gamma, which is close to 2.4 normally, but if the brightness was turned up they had an effective gamma closer to 2.2 or 2.0, as we've seen. This worked out nicely. If people were watching TV in a bright room, they'd tend to turn the brightness up, which lowered the effective gamma.

I don't know of any current flat panel that comes from the factory with 2.0 gamma, but it wouldn't surprise me to find out that are some. It seems to me that lots of current flat panels are taking advantage of the fact that they have a LUT to do wacky things with the gamma, like adding an S-curve to add pop to the image and other stuff that doesn't fit the normal model of a power curve with offsets.

Don,

first of all, I'm prob a couple of years younger than you so when I say "old TV's" I don't mean CRT's ! biggrin.gifwink.gif

Had at least 4-5 RPTV's or Plasmas that came straight out of the box with around 2.0 / 2.1 Gamma... obviously u can adjust that, IF u choose to do so.

0.5 en with 2.4 de will get you the desired final image gamma of 1.2... and using 2.4 in the inverse Rec 709 encoding function will approximate a straight 2.2 gamma curve...

just because the specification has always been a mess, does not mean that post (--> esp. VFX) has not needed to decode camera footage... and (for the lack of a direct specification) a lot of people used the exact inverse encoding function...

and IIRC - for the sake of history - 1.0/2.2 - ca. 0.4545 - was the standard for television camera encoding before the advent of color TVs and was formalized in 1953 with the NTSC broadcast television standards... smile.gif
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post #559 of 722 Old 12-20-2013, 01:27 PM
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Thanks Ted, what is the purpose of selecting the type of display a meter is calibrating prior to calibration?
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post #560 of 722 Old 12-20-2013, 02:08 PM
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Quote:
Originally Posted by Iron Mike View Post

Don,


0.5 en with 2.4 de will get you the desired final image gamma of 1.2... and using 2.4 in the inverse Rec 709 encoding function will approximate a straight 2.2 gamma curve...

I don't know what you mean when you say a 2.4 decoding function will approximate a straight 2.2 gamma curve.

A 2.4 decoding function IS a 2.4 gamma curve.
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post #561 of 722 Old 12-20-2013, 02:15 PM
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Originally Posted by spacediver View Post

I don't know what you mean when you say a 2.4 decoding function will approximate a straight 2.2 gamma curve.

A 2.4 decoding function IS a 2.4 gamma curve.

read the specs... there is an OFFSET... implemented with the intention to minimize noise in the camera...

I said using 2.4 as the exponent in the inverse function... I shorthanded that in my other reply as I now stated this 10 times...

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post #562 of 722 Old 12-20-2013, 02:32 PM
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You keep referencing a Rec.709 decoding function with a slope and offset.

Which function are you referring to in particular. Can you write it out here?
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post #563 of 722 Old 12-20-2013, 02:39 PM
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Quote:
Originally Posted by spacediver View Post

You keep referencing a Rec.709 decoding function with a slope and offset.

Which function are you referring to in particular. Can you write it out here?

spacediver, as others have stated here, there was no decoding function specified... but if you needed to decode camera footage, a very common process was to INVERSE the encoding function... I know lots of people in post facilities who simply decoded using a straight 2.2 Gamma, they didn't even care about the offsets...

regarding the math, that'll be your homework ! biggrin.gif

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post #564 of 722 Old 12-20-2013, 02:40 PM
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Quote:
Originally Posted by Iron Mike View Post

0.5 en with 2.4 de will get you the desired final image gamma of 1.2... and using 2.4 in the inverse Rec 709 encoding function will approximate a straight 2.2 gamma curve...

just because the specification has always been a mess, does not mean that post (--> esp. VFX) has not needed to decode camera footage... and (for the lack of a direct specification) a lot of people used the exact inverse encoding function...

and IIRC - for the sake of history - 1.0/2.2 - ca. 0.4545 - was the standard for television camera encoding before the advent of color TVs and was formalized in 1953 with the NTSC broadcast television standards... smile.gif

Yes, though I don't know anyone advocating using an inverted 709 gamma curve with 2.4 substituted for the 1/0.45 exponent. That would be odd, though I guess without a standard people will do all kinds of wacky stuff.

And yes, until BT.709, the NTSC standards specified 1/2.2 for the camera encoding. The fact that the current standard includes a scaled power curve with a roughly 1/2.2 exponent is yet another source of confusion, because it's not obvious that 709 is intended to be approximately 1/2, i.e. it changed the standard from 1/2.2 to ~1/2.

Poynton used to talk about scene-referenced gamma and screen-referenced gamma, which was confusing. I think the breakthrough was when he and others realized that once something has been recorded, it's always screen-referenced from that point out. The camera gamma curve is interesting, but the important curve is the one for the screen. Once people got away from trying to figure out where the inverse camera gamma fit into the whole scheme (the answer: nowhere), things fell into place pretty rapidly.

If you look at Poynton's gamma FAQ today, it's kind of all over the map. It could really use an edit to bring it up to his current thinking and add references to BT.1886. He talks about the 2.5 gamma of the electron gun and how TVs should be modeled as L = (V + e) ^ 2.5, but he also talks about the inverse of 709 as being relevant for decoding camera material, which I think he would probably repudiate today. But I'm not sure - maybe I'm misunderstanding him.

In the end, I think for the folks that are calibrating to 2.2, the actual difference between absolute 2.2 (with whatever random black point compensation) and BT.1886 is not going to be that far apart in the midtones and low-midtones. Where it's going to help a lot is in smoothing out the shadow detail. Right now when you calibrate any real-world monitor to 2.2, you have to make some decisions about how to handle the fact that an LCD monitor can't actually display the levels near black at the levels implied by straight 2.2 (or even inverted 709 for that matter). So all the calibration software has a choice of clipping the black detail or putting some kind of knee on the low range, and exactly what knee you use is going to affect the shadows a lot. Switching to BT.1886 should make that situation better. Of course, it'll also mean that the midtones will shift up and down on different LCD monitors with different absolute black levels, and people will complain about that. People will complain: of this I am sure. smile.gif

I'm going to Disney World next week, so I may drop out of the conversation for a while. Hope everyone has a great holiday season and New Year!

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post #565 of 722 Old 12-20-2013, 02:47 PM
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Right now when you calibrate any real-world monitor to 2.2, you have to make some decisions about how to handle the fact that an LCD monitor can't actually display the levels near black at the levels implied by straight 2.2 (or even inverted 709 for that matter). So all the calibration software has a choice of clipping the black detail or putting some kind of knee on the low range, and exactly what knee you use is going to affect the shadows a lot. !

absolutely agree, and that's why it is so important to triple check after calibration that you're not crushing / lifting blacks... which is what some sw solutions do, depending on the screen your calibrating...

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for real world reference footage, we use (amongst others)... u might know it wink.gif

http://www.amazon.com/Lighthouses-Pacific-Northwest-Blu-ray-Stacey/dp/B0043TY5GO/ref=sr_1_6?ie=UTF8&qid=1387579782&sr=8-6&keywords=lighthouses+of+the+pacific
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for real world reference footage, we use (amongst others)... u might know it wink.gif

http://www.amazon.com/Lighthouses-Pacific-Northwest-Blu-ray-Stacey/dp/B0043TY5GO/ref=sr_1_6?ie=UTF8&qid=1387579782&sr=8-6&keywords=lighthouses+of+the+pacific

I'm using it too wink.gif


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post #568 of 722 Old 12-20-2013, 04:06 PM
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for real world reference footage, we use (amongst others)... u might know it wink.gif

http://www.amazon.com/Lighthouses-Pacific-Northwest-Blu-ray-Stacey/dp/B0043TY5GO/ref=sr_1_6?ie=UTF8&qid=1387579782&sr=8-6&keywords=lighthouses+of+the+pacific

I know I've seen that somewhere before... smile.gif

I'm kind of hoping the montage on the new disc will become the de facto standard footage for checking 3D. It's really some of the best 3D I've seen (and it was all shot by Stacey, so I'm not tooting my own horn).

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post #569 of 722 Old 12-20-2013, 04:33 PM
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Poynton used to talk about scene-referenced gamma and screen-referenced gamma, which was confusing. I think the breakthrough was when he and others realized that once something has been recorded, it's always screen-referenced from that point out. The camera gamma curve is interesting, but the important curve is the one for the screen. Once people got away from trying to figure out where the inverse camera gamma fit into the whole scheme (the answer: nowhere), things fell into place pretty rapidly.

My understanding is that camera gamma combined with display gamma will tell you how the luminance information in the (real world) scene will be rendered on the display. If you don't know the camera gamma, you have no way of knowing this information.

Knowing only the display gamma (or display encoding function if you prefer) will tell you something about how perceptually uniform the display is.

I'll have to re-read his 1993 paper and think about it more carefully, but this is my take on the issue right now (this is all new to me).
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Originally Posted by dmunsil View Post

If you look at Poynton's gamma FAQ today, it's kind of all over the map. It could really use an edit to bring it up to his current thinking and add references to BT.1886. He talks about the 2.5 gamma of the electron gun and how TVs should be modeled as L = (V + e) ^ 2.5, but he also talks about the inverse of 709 as being relevant for decoding camera material, which I think he would probably repudiate today. But I'm not sure - maybe I'm misunderstanding him.

I'm gonna read his latest paper this wknd and will report back. He's also offered to chat some time so hopefully if I meet up with him I'll have some decent questions I can put forward.
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post #570 of 722 Old 12-20-2013, 05:09 PM
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My understanding is that camera gamma combined with display gamma will tell you how the luminance information in the (real world) scene will be rendered on the display. If you don't know the camera gamma, you have no way of knowing this information.

The camera encoding gamma is relevant, and it should be in some way related to the target screen gamma if your camera is going to produce good images. But you never need the inverse camera gamma - it's not relevant to anything. Once something has been recorded, from that point on you should think of it as display-referenced and just apply the screen gamma to it.

We got to this world of confusion at least partially because of people thinking that there were certain instances where applying the inverse of the 709 gamma would be a good idea. And it turns out that's not true; there's no good reason to ever use the inverse of BT.709 for anything.

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