What is BT.1886 gamma? - Page 3 - AVS Forum
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post #61 of 96 Old 07-14-2014, 02:05 AM
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In HCFR you just tick "use black level compensation". Been doing that since the beginning. If you don't do that, you may end up with black crush.

I say "may", because most calibrators will check the Brightness test pattern after doing gamma, which if there is crush, Brightness will be raised, which pushes the low end up, achieving basically the same result anyway.

i.e you'll only get black crush if you haven't set Brightness properly after doing the gamma calibration.

btw my goal is perceptual uniformity first, then "what the director saw on their monitor" second.
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post #62 of 96 Old 07-17-2014, 09:33 PM
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Originally Posted by spacediver View Post
BT.1886 and PL@2.2 (output offset) are, by definition, multiple EOTF's. Furthermore, why would you choose 2.2 with an output offset over 2.2 with an input offset??
Yes they are. Why? Because we have seven years or so of BD and other broadcast video libraries that were probably created with PL@2.2. ... Then there's the plasma issue. I personally have both BT1886 and PL2.2 setup on my display ...even though I never actually use the PL setting.

All *I* care about here is getting the major movie studios and the broadcast industry to adopt an (as in one) actual EOTF *standard* for use in the mastering booths and control rooms. And frankly there seems to be a great deal of resistance to this. If you want to put your expensive "reference level plasma" in the solarium, that's your business ... go with what ever higher power you believe in.

But, on further consideration, if I were to find myself in the your original hypothetical situation, I wouldn't even bother with trying to do an actual "daylight" calibration. If I were to tinker with the gamma, I'd just use my regular "reference" BT1886 curve and bump the display's gamma control. At the point that this would be necessary, I'd just be wanting a picture that's actually visible ... forget about "accurate."

PS: By "multiple" I mean "more than two" ... as in "BT1886 2.4", "BT1886 2.2", "BT1886 2.0" plus any number of the usual power law variations ... where does that stop and how do we choose the "correct" EOTF for any given title and lighting conditions? That way lies madness.

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post #63 of 96 Old 07-17-2014, 09:46 PM
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Originally Posted by HDTVChallenged View Post

All *I* care about here is getting the major movie studios and the broadcast industry to adopt an actual EOTF *standard* for use in the mastering booths and control rooms. And frankly there seems to be a great deal of resistance to this.
That's the whole point of BT.1886, and why Joel Silver wanted to be on Home Theater Geeks—to promote the idea.

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post #64 of 96 Old 07-17-2014, 09:47 PM
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Quote:
Originally Posted by HDTVChallenged View Post
All *I* care about here is getting the major movie studios and the broadcast industry to adopt an (as in one) actual EOTF *standard* for use in the mastering booths and control rooms. And frankly there seems to be a great deal of resistance to this.
As far as I know, Technicolor, Deluxe, CO3, Encore, Modern, Sony, Universal, and the other major video mastering companies in LA are all basing their current HD monitor adjustments on BT1886. It's not a gigantic difference from Rec709.

What they're more concerned are all the new problems coming right over the horizon with HDR, 4K, high-frame rate, Rec2020, and all that other stuff. If you thought things were screwy at the moment, just wait until 5 years from now.

Look at it this way: there have been good standards in place for more than 15 years on sound, and they still can't reliably mix movies & TV shows with a reasonable dynamic range for home video. I constantly run into Blu-rays that have way, way too much dynamic range, where they've clearly been mixed on stages intended only for an 85dB theatrical release. I don't believe masters like this translate well for home use, and I'm a big fan of having the sound mixers go back in and remix the show for a normal 79dB TV mix, where the dynamic range is constricted somewhat and the dialogue is brought up so that the music and explosions don't blow you out of the room. And as I said, audio is comparatively easy next to audio... and they often get the audio wrong.
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post #65 of 96 Old 07-17-2014, 10:37 PM
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Quote:
Originally Posted by HDTVChallenged View Post
But, on further consideration, if I were to find myself in the your original hypothetical situation, I wouldn't even bother with trying to do an actual "daylight" calibration. If I were to tinker with the gamma, I'd just use my regular "reference" BT1886 curve and bump the display's gamma control. At the point that this would be necessary, I'd just be wanting a picture that's actually visible ... forget about "accurate."
Well, there are people who actually do care about perceptual accuracy in lighting compromised environments, and having such a care in no way jeopardizes the adoption of a standard EOTF (which everyone in this thread wants). Remember, this isn't about creating new addendums to the official recommendations - this is about informed enthusiasts making intelligent adjustments to adapt to non ideal situations. Nothing changes whatsoever on the ITU end. I think you might have deeply misinterpreted something along the way here...


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Yes they are. Why? Because we have seven years or so of BD and other broadcast video libraries that were probably created with PL@2.2.
Most studios have deep blacks. Deeper than most consumer displays. If you watch content that was mastered in a studio with PL 2.2 on a display that has a higher black level with PL 2.2 you're gonna crush your blacks. It's probably a better bet to use an input offset with 2.2 in that case.

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post #66 of 96 Old 07-18-2014, 01:30 AM
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With any calibration, using any target formula, there is no reason for any image crushing at all, if the correct calibration approach/workflow is used.

Crushing (or clipping for that matter) usually means the calibration is based on an 'absolute' approach, when a 'relative' approach is actully the correct way to calibrate.

This is obviously required as no two displays have the same absolute black level.

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post #67 of 96 Old 07-18-2014, 02:08 AM
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Quote:
Originally Posted by Light Illusion View Post
With any calibration, using any target formula, there is no reason for any image crushing at all, if the correct calibration approach/workflow is used.

Crushing (or clipping for that matter) usually means the calibration is based on an 'absolute' approach, when a 'relative' approach is actully the correct way to calibrate.

This is obviously required as no two displays have the same absolute black level.

Steve
Not to beat a dead horse, but this discussion is way beyond the issue of absolute vs relative approach. Both "power-law" and "input offset (e.g. BT.1886)" approaches are relative. But content mastered on a display with deep blacks, and rendered on a display with a raised black level that has a "power law" will produce perceptual crushing, as has been patiently explained many times on this forum.

The issue is perceptual crushing here (crushing of lightness values), not luminance crushing. Luminance crushing would occur if you used an absolute approach with a raised black level, since the first few video values would have identical luminances. But we're not talking about this form of crushing

Unless your workflow includes adjustments based on visual assessment (which it isn't if you're using a target formula), then there's no guarantee against lightness crushing.

If you want proof of this, try implementing a 2.2 power law gamma on a display with a ridiculously high black level, and load up a grayscale ramp. Notice anything strange?

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post #68 of 96 Old 07-18-2014, 02:28 AM
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Quote:
and rendered on a display with a raised black level that has a "power law" will produce perceptual crushing
If calibrated correctly all you will see is lifted blacks, based on the absolute minimum the display can generate.
The relative grey scale will be the same - no crushing.

Simples

But actually, I think what you are trying to describe is the effect of a reduced contrast ratio, where the limited range from min black to max white has been reduced to the point where there is not enough available range for all steps in a greyscale (220 for an 8 bit TV legal signal) to provide a visible change.

With such a scenario there are really just 2 options.

Reject the display as being not fit for purpose, or raise the max white to compensate for the loss of CR.
I obviously always chooses the former.

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post #69 of 96 Old 07-18-2014, 03:22 AM
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Quote:
Originally Posted by Light Illusion View Post
If calibrated correctly all you will see is lifted blacks, based on the absolute minimum the display can generate.
The relative grey scale will be the same - no crushing.
No, the relative luminance scale will be the same, but the relative lightness scale will not. JND's in luminance (just noticeable differences) grow rather rapidly as luminance rises. If you start your relative gamma function from a raised black level, not only will everything be lifted, but the luminance change at each successive video level won't hold the same relationship to the JNDs as they would if u started from a lower black level. This is why you need to increase the gain of the function when black level increases. Think of JNDs as a unit of how humans perceive luminance. For example, between 0 and 1 cd/m2 we might fit 100 JNDs, but between 1 and 2 cd/m2, we might only fit 30 JNDs (meaning we're more sensitive in the region between 0 and 1 nits).

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Originally Posted by Light Illusion View Post
But actually, I think what you are trying to describe is the effect of a reduced contrast ratio, where the limited range from min black to max white has been reduced to the point where there is not enough available range for all steps in a greyscale (220 for an 8 bit TV legal signal) to provide a visible change.
This has nothing to do with quantization artifacts - the issue would exist with an arbitrary bit depth, nor to do with reduced contrast. Sure, if you reduce your contrast enough, then any function is gonna have perceptual crushing. However, the issue being discussed here would exist even if you lifted both your min and max luminance.

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post #70 of 96 Old 07-19-2014, 08:47 AM
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That's the whole point of BT.1886, and why Joel Silver wanted to be on Home Theater Geeks—to promote the idea.
And also why I think mucking around with a perfectly good technical specification is a very, very, very bad idea. Especially when simply raising the black-level of the display, while maintaining the proper, specified behavior of BT1886 would accomplish the same goals without screwing with the definition of BT1886.

IOW, with BT1886, raising the display's black-level gives one more "mileage" than changing the exponent to an "undefined" value. This also has the added benefit of being the proper way to deal with less than "reference" level ambient lighting levels. (One might also want to adjust the reference white level upward as well.)

Yes, I know that this "proper" way ultimately means more work for the calibrator ... but if we are really concerned with 'doing things the right way,' this shouldn't be a major burden.
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post #71 of 96 Old 07-19-2014, 08:55 AM
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Originally Posted by spacediver View Post
Well, there are people who actually do care about perceptual accuracy in lighting compromised environments, and having such a care in no way jeopardizes the adoption of a standard EOTF (which everyone in this thread wants). Remember, this isn't about creating new addendums to the official recommendations - this is about informed enthusiasts making intelligent adjustments to adapt to non ideal situations. Nothing changes whatsoever on the ITU end. I think you might have deeply misinterpreted something along the way here...
The problem is that your "solution" to this "problem" is not really correct ... furthermore, it opens a brand-new Pandora's box ... if you can't see that, then there's not much more I can say to convince you.

As Bones McCoy would say, 'Good god man, drilling holes in his head isn't the answer! ... The artery must be repaired!'
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post #72 of 96 Old 07-19-2014, 10:54 AM
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Quote:
Originally Posted by HDTVChallenged View Post
And also why I think mucking around with a perfectly good technical specification is a very, very, very bad idea. Especially when simply raising the black-level of the display, while maintaining the proper, specified behavior of BT1886 would accomplish the same goals without screwing with the definition of BT1886.

IOW, with BT1886, raising the display's black-level gives one more "mileage" than changing the exponent to an "undefined" value. This also has the added benefit of being the proper way to deal with less than "reference" level ambient lighting levels. (One might also want to adjust the reference white level upward as well.)
I'm not sure where you read that this is the proper way to do this. It's certainly not specified in the documents.

You might be correct: raising the black level will certainly increase the gain of the function (which is needed when adapting luminance increases), so it might be a good way to deal with higher ambient lighting conditions. But you have to realize that the compensatory gain increase with increased black level (in the case of an input offset function), and the compensatory gain increase required to deal with raised ambient levels are two separate issues. By raising the black level of the display while using BT.1886 all you are really doing is ensuring a consistent perceptual experience relative to how that experience would be with a lowered black level, all else being equal. If ambient lighting conditions are high, then raising the black level while using BT.1886 will simply ensure that the perceptual experience will be the same as it was with a lowered black level, with that raised ambient lighting. In other words, crushed. Perceptual theory suggests that lowering the exponent of the input offset function is the correct way to deal with raised ambient lighting. Psychophysical data is what is required to test this idea.

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post #73 of 96 Old 07-19-2014, 01:28 PM
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Poor ambient lighting has the indirect effect of lifting blacks, which means that is the last thing you want to do to try to compensate for the poor viewing environment.


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post #74 of 96 Old 08-11-2014, 11:27 AM
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@spacediver

Ok ... so how about not actually changing the physical black level of the display, but instead "pretend" that you have for the purposes of calculating BT1886.

IOW, leave your display black level at 0.03Nits but use 0.13Nits as the "Lb" for BT1886 ... now compare the results from BT1886@2.4 to the results of power-law @ gamma=2.2.

-----
@lightillusion

My experience has always been that the higher the ambient light level, ... the less the impact of the actual black level of the display. IOW, in "bright conditions" it doesn't really matter if your physical black level is 0.03nits or 0.13nits ... you can't tell the difference anyway.

-----

To all ... I've run the (luminance) numbers a few dozen different ways ... I don't see *any* benefit to "quasi-BT1886@2.2." That being said, I don't object, in theory, to changing the exponent, what I object to is calling the resulting "experimental" EOTF by the name "BT1886."

PS: Sorry for the delayed response ... been busy of late.
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post #75 of 96 Old 08-11-2014, 12:12 PM
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CalMAN did implement a version with an exposed exponent.

I labeled it Sliding Power, to reference the method the black offset is applied. BT.1886 effectively slides the range along the curve till the black level meets the curve. That is compared to the standard power function that compresses the curve on the Y axis till the 0% value meets the desired Y output.

But I didn't put the name BT.1886 anywhere near this, because BT.1886 is a bold faced standard and as such any modification would invalidate it.

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post #76 of 96 Old 08-11-2014, 03:51 PM
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Tangentially related question: I'm doing some interesting JND pilot experiments (will report full results once I've done the proper psychophysics), and one thing I've noticed is that with a peak luminance of around 85 nits, a min luminance of below 0.005 nits, and a perfect gamma of 2.4, I can just detect differences between each successive video level (8 bit PC RGB). And that's only in a very dark room. As soon as I turn on my bias light, there's some perceptual crushing.

So here's the question. For those who are working in PC RGB (256 levels) on a well calibrated display, are you able to make out differences between each successive gray level?

Here's a zip file that has 255 test patterns created in matlab - each pattern comprises a circular test patch whose gray level is one higher than the background.
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post #77 of 96 Old 08-11-2014, 04:45 PM
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Spacediver, I think you're working under a false assumption.

The goal of a correctly setup gamma formula would be that each step is below our ability to perceive a difference. Ideally we should NOT be able to make out the step of a grayscale ramp, it would blend smoothly if we had enough bits, and an ideal transfer function.

Many monitors will crush the first few bit coming out of black, especially if they have a CMS as you need to convert from signal space to linear space to run the values through a matrix and come back out. In order to persist the 1st 8bit step in linear space, you need something like 18 bits, which most display processing pipelines don't have so the first step or two is crushed to black.

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post #78 of 96 Old 08-11-2014, 05:24 PM
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makes sense re my false assumption.

In practice, however, I think being able to perceive a JND between each successive level under extreme conditions (i.e. dark adapated, large test patch, large background patch, as in my test patterns) doesn't translate to being able to perceive them in the context of real images, where there is a lot of adapting to higher luminances. But I do see your point.

Also starting to learn a bit about the DICOM Grayscale Standard Display Function (GSDF) - a standard used in medical imaging that emphasizes perceptual uniformity. I think it emphasizes JNDs between each successive gray level (not sure yet), clearly to optimize the visual information available for diagnosis.
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post #79 of 96 Old 08-11-2014, 07:18 PM
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Quote:
Originally Posted by spacediver View Post
makes sense re my false assumption.

In practice, however, I think being able to perceive a JND between each successive level under extreme conditions (i.e. dark adapated, large test patch, large background patch, as in my test patterns) doesn't translate to being able to perceive them in the context of real images, where there is a lot of adapting to higher luminances. But I do see your point.

Also starting to learn a bit about the DICOM Grayscale Standard Display Function (GSDF) - a standard used in medical imaging that emphasizes perceptual uniformity. I think it emphasizes JNDs between each successive gray level (not sure yet), clearly to optimize the visual information available for diagnosis.
It does, but that is also an entirely different context than video reproduction.

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post #80 of 96 Old 08-11-2014, 08:18 PM
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yep, I appreciate that - being able to detect the boundaries of healthy vs tumorous tissue is more critical than smooth gradients.

I do wonder, though. With any given transfer function, let's say BT.1886, what is the maximum peak luminance (given a nice deep black level) that can be enjoyed without creating grayscale banding in an 8 bit context, due to the stretch?
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post #81 of 96 Old 08-11-2014, 10:21 PM
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Quote:
Originally Posted by spacediver View Post
yep, I appreciate that - being able to detect the boundaries of healthy vs tumorous tissue is more critical than smooth gradients.

I do wonder, though. With any given transfer function, let's say BT.1886, what is the maximum peak luminance (given a nice deep black level) that can be enjoyed without creating grayscale banding in an 8 bit context, due to the stretch?
Using JND would tell us ~15cd/m^2 for 255 steps.

1023 JNDs is equivalent to 3993 cd/m^2.

511 JNDs is 130cd/m^2.

All of this basically points out that 8bits really isn't enough for end user color, but 10bits probably is, especially if 10bits are distributed more intelligently than a simple power function.

But in terms of video calibration, you really just want to replicate the mastering environment, full stop.
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post #82 of 96 Old 08-14-2014, 08:25 AM
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Originally Posted by sotti View Post
CalMAN did implement a version with an exposed exponent.

I labeled it Sliding Power, to reference the method the black offset is applied. BT.1886 effectively slides the range along the curve till the black level meets the curve. That is compared to the standard power function that compresses the curve on the Y axis till the 0% value meets the desired Y output.

But I didn't put the name BT.1886 anywhere near this, because BT.1886 is a bold faced standard and as such any modification would invalidate it.
Is sliding power like BT.1886 but with an adjustable exponent?
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post #83 of 96 Old 08-14-2014, 12:37 PM
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Originally Posted by PlasmaPZ80U View Post
Is sliding power like BT.1886 but with an adjustable exponent?
Yes, that is exactly what it is.
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post #84 of 96 Old 08-15-2014, 09:47 AM
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Yes, that is exactly what it is.
thanks, seems like a very useful function for anyone who is not quite happy with BT.1886 gamma but does want better shadow detail than basic power law gamma with a black offset
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post #85 of 96 Old 08-15-2014, 01:34 PM
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thanks, seems like a very useful function for anyone who is not quite happy with BT.1886 gamma but does want better shadow detail than basic power law gamma with a black offset
But also potentially harmful for the establishment of an actual standard.

I consider it a research tool, but not something that actually gets used in practice.

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post #86 of 96 Old 08-17-2014, 10:54 AM
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Originally Posted by sotti View Post
But also potentially harmful for the establishment of an actual standard.

I consider it a research tool, but not something that actually gets used in practice.
you might be right... but BT.1886 never looked right on my Samsung F5300 PDP (still missing some shadow detail and too dim midtones on an already dim set... 35 fL max 100% white with medium window patterns)

Regular power 2.2 was pretty good but still missing some shadow detail (and regular power 2.3 was only worse in this department)

Right now I'm using sliding power 2.24 (2.24 because it brings the top end in line with point gamma 2.20) and I prefer it visually to BT.1886 and 2.2 power
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post #87 of 96 Old 08-17-2014, 10:57 AM
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could be BT.1886 looks wrong because of the ABL on my plasma and also limited light output (combined with slightly higher than desirable black levels)
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post #88 of 96 Old 08-28-2014, 05:52 AM
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Are live sporting events such as college football done in power law 2.2 gamma or BT1886?

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post #89 of 96 Old 08-28-2014, 09:20 AM
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Originally Posted by JimP View Post
Are live sporting events such as college football done in power law 2.2 gamma or BT1886?
Or 2.3, or 2.4 or 2.35?

Unless you're in the truck and you calibrated the displays, you wouldn't know.

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post #90 of 96 Old 08-30-2014, 01:19 AM
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I have to settled on the so called "Sliding Gamma" but with ArgyllCMS.

BT.1886 (0% output offset = full input offset) gave me too elevated blacks.
Power law (100% output offset) clipped the first 3-4 steps of black to 0.

ArgyllCMS gave me an option to choose a custom Black Offset value in which I found the sweet spot of my gamma sweet spot.
I found the perfect settings where the blacks are compressed (not clipped) just right for my viewing environment (w/ bias lighting) but above that a flat 2.2 gamma curve.
I always use Relative gamma curve so that at least some of the curve reaches the set gamma of 2.2. Absolute (bt.1886) could not reach 2.2 because of my low contrast display (850:1).

For my display (Dell U2410) BT.1886 just sucked, it gave me extremely elevated blacks and did not reach 2.2 on any place of the curve resulting in a flat washed out picture.
But custom "Black Compensated Power Law" was just perfect, pure 2.2 above 5% and compressed first few steps of black so that I can see them all and they differ in luminance.


IMO:
BT.1886 is completely useless for mastering studios because the WHOLE resulting gamma is variable and dependent on the Contrast Ratio of the display.
A random gamma curve ranging from 2.0 to 2.4 based on the Contrast Ratio is a ridiculous laughing matter for mastering/grading studios.
Like "Light Illusion" pointed, BT.1886 is a recommendation NOT a standard, it should in no way be used in the grading/mastering studio.

What we should be debating is the degree of compression to the Black Region with different Contrast Ratio in the sub 5% luminance region.
Above 5% luminance (which even a poor sub 800:1 CR display can give) should be a hard standard like 2.2 or 2.4.


The mastering studios should stick to a certain standardized high Contrast Ratio like 10,000:1 and a black compensated power law of 2.2 or 2.4.
The consumer should compress the black level depending on his display contrast ratio.
Both should agree that ABOVE certain low luminance % (CR dependent) the gamma should stick to the standard of 2.2 or 2.4.

For example:
On a 10,000:1 display only below 1% luminance should be compressed, everything above 1% can easily reach pure 2.2 power curve.
On a 800:1 display below 5% should be compressed, above 5% can easily be pure 2.2.

The correct formula should take into account from what point and ABOVE the low CR display can give us the Pure Power Gamma, and only compress/compensate BELOW that.
The BT.1886 formula compresses the whole gamma curve which is just a stupid thing to do, and can't be called a Standard nor should it be Recommended to anyone.

Last edited by James Freeman; 08-30-2014 at 02:00 AM.
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