How power law gamma calibration can lead to crushed blacks - Page 8 - AVS Forum
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post #211 of 230 Old 12-03-2012, 04:15 AM
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In a calibration system where profiling is separate from calibration there is no reason for Gamma to affect Gamut...

The problem is when the two are interlinked, so that altering gamma causes a global change to the calibration - which it shouldn't.

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post #212 of 230 Old 12-03-2012, 04:54 AM
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I had actually missed page 8 of this thread where there is an explanation. Except, judging from the explanation, gamma DOES affect the hue. Check out post #182 by Sotti. This going to sound either understandable or stupid:-):

Changing gamma does only not only change Y(luminance) for white, which we check by measuring the greyscale, but for all the colours (I found out you are British hehe).
Changing luminance is actually nothing more than altering the ratio of R, G and B at a certain point of stimulus (adding/removing green will have the biggest effect on Y).

The above tells me gamma will always affect gamut. Since I know you are a very clever person in the matter, either the above is wrong or I'm missing a crucial bit of information?

edit: I'm reading the previous page but it will take a few more reads to really get it...

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post #213 of 230 Old 12-03-2012, 05:37 AM
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Quote:
Originally Posted by Light Illusion View Post

We have done a lot of work with ICC's (CMM's), and have found time and again just how inaccurate they can be...
Let's face it, any calibration process that has a 'Use Black Point Compensation' as part of its options isn't really looking like an accurate calibration method.
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Seriously, we have been called into facilities time and again to rescue them from issues caused by trying to use ICC profiles for accurate calibration.

For video? I wasn't aware that videos were capable of embedding an ICC profile and that there were CMMs capable of using them (yet). I understood video to be stuck in the intransigent world of hard coding calibration which of course is annoying if you regularly switch between SD and HD media (and your display can't adapt to the changing content). ICC profiling is very effective in graphics (non video) world. As for BPC, it makes immense sense to scale luminance for a difference in black point.

(Of course also we need to distinguish between calibration and profiling.)

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post #214 of 230 Old 12-03-2012, 06:36 AM
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Jerone, see my comment above your last post - if profiling is separated from calibration there is no interlinking.

With LightSpace you can do a profile and then set a Rec709 target with 2.2 Gamma, or 2.4 Gamma, and the changes in gamma will not alter the Gamut calibration...

Steve, Adobe attempted to force ICC calibration (and we do indeed understand profiling vs. calibration) on to their video users - it didn't work.

And there is no sense at all for 'Use Black Point Compensation' within an accurate calibration workflow.

Switching between colour spaces is very, very easy.
The professional video world does this all the time.

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post #215 of 230 Old 12-03-2012, 08:26 AM
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Quote:
Originally Posted by Light Illusion View Post

Jerone, see my comment above your last post - if profiling is separated from calibration there is no interlinking.
With LightSpace you can do a profile and then set a Rec709 target with 2.2 Gamma, or 2.4 Gamma, and the changes in gamma will not alter the Gamut calibration...
Steve, Adobe attempted to force ICC calibration (and we do indeed understand profiling vs. calibration) on to their video users - it didn't work.
And there is no sense at all for 'Use Black Point Compensation' within an accurate calibration workflow.
Switching between colour spaces is very, very easy.
The professional video world does this all the time.
Steve

How do you transfer your profile to a TV's settings if there is no LUT box involved?

Also how do you implement BT.1886 in your software since it does indeed use the black point as part of formula.

Interesting that you condem black offset, yet the only approved standard for HDTV playback includes it.
http://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.1886-0-201103-I!!PDF-E.pdf

Adobe RGB also has many provisions for the fact that displays have non-zero black level
http://www.adobe.com/digitalimag/pdfs/AdobeRGB1998.pdf

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post #216 of 230 Old 12-03-2012, 08:32 AM
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Quote:
Originally Posted by sotti View Post

How do you transfer your profile to a TV's settings if there is no LUT box involved?
Also how do you implement BT.1886 in your software since it does indeed use the black point as part of formula.
Interesting that you condem black offset, yet the only approved standard for HDTV playback includes it.
http://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.1886-0-201103-I!!PDF-E.pdf
Adobe RGB also has many provisions for the fact that displays have non-zero black level
http://www.adobe.com/digitalimag/pdfs/AdobeRGB1998.pdf

Umm, we are ONLY interested in LUT based calibration... As is says on the website - turn off all the internal rubbish, as it does more harm than good.

We can and do implement ANY colour space as we will have profiled the display in advance... that's kind'a obvious, isn't it?

'Use Black Point Compensation' is not black offset... very, very different animals.

It is impossible to 'add' black to a display, so the calibration needs to be 'float' based. Again that has to be obvious, doesn't it?

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post #217 of 230 Old 04-17-2013, 07:57 AM - Thread Starter
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Quote:
Originally Posted by Jeroen1000 View Post

sorry for kicking this up when the dust has settled. Why is it indeed, that "gamma" affects the gamut?

Not a direct answer (it's been covered in this thread already) but here are some statistics on what type of errors can be generated across the gamut when your assumption of source gamma does not match display gamma. I also threw in a color space mismatch (601 vs. 709) for comparison. These are measured values (2500 colors) on my display with dE calculated based on the assumptions in the legend.

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post #218 of 230 Old 04-20-2013, 12:33 PM
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^ Interesting.

 

The 601/709 mismatch is pretty small. I've often wondered if the difference between 601 and 709 would be big enough to catch in a simple visual color-bar decoder test.

 

I wonder what a Rec. 709 vs. Rec. 2020 (wide gamut) mismatch would look like btw? (Pretty substantial I'm guessin.)


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post #219 of 230 Old 04-21-2013, 05:52 AM - Thread Starter
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Quote:
The 601/709 mismatch is pretty small. I've often wondered if the difference between 601 and 709 would be big enough to catch in a simple visual color-bar decoder test.

I can see it in color bars when switching between 709/601 calibrated primaries but picking it out stand-alone is pretty tough.
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Originally Posted by ADU View Post

I wonder what a Rec. 709 vs. Rec. 2020 (wide gamut) mismatch would look like btw? (Pretty substantial I'm guessin.)

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post #220 of 230 Old 04-21-2013, 10:32 AM
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Quote:
Originally Posted by ADU View Post

^ Interesting.

The 601/709 mismatch is pretty small. I've often wondered if the difference between 601 and 709 would be big enough to catch in a simple visual color-bar decoder test.

From my personal experience:

REC601 sources through a REC709 decoder is tolerable.

REC709 sources through a REC601 decoder will drive you completely nuts even on normal programming (blue is boosted significantly.)

Yes, you can detect it through a "simple" color decoder/filter test, if you know what to look for ... smile.gif
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post #221 of 230 Old 04-21-2013, 03:31 PM
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Quote:
Originally Posted by ADU View Post

The 601/709 mismatch is pretty small. I've often wondered if the difference between 601 and 709 would be big enough to catch in a simple visual color-bar decoder test.

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post #222 of 230 Old 04-21-2013, 08:09 PM
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^ Nice. Thanks for posting those. They illustrate the difference quite well.


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post #223 of 230 Old 04-21-2013, 08:10 PM
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Quote:
Originally Posted by zoyd View Post

 

Very cool! Thanks for posting this zoyd.

 

This graphically illustrates why it can be so difficult to get the color on some wide gamut displays close to Rec. 709 spec.


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post #224 of 230 Old 06-19-2013, 01:25 PM
 
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Could anyone tell how does Calman 5 calculate gamma when non-zero black level is applied? It doesn't look like simple black offset (Vin = Vout^gamma + black lvl) because the value is somehow scaled across the dynamic range (for example 1% IRE target is raised by almost 100% value of the black level, while 99% IRE is almost not affected at all)... Is it a standard way of calculating gamma?
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post #225 of 230 Old 06-19-2013, 02:38 PM
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Quote:
Originally Posted by Koperfild View Post

Could anyone tell how does Calman 5 calculate gamma when non-zero black level is applied? It doesn't look like simple black offset (Vin = Vout^gamma + black lvl) because the value is somehow scaled across the dynamic range (for example 1% IRE target is raised by almost 100% value of the black level, while 99% IRE is almost not affected at all)... Is it a standard way of calculating gamma?

It depends on the gamma formula. Some (like BT.1886) use the black level as part of the input value for the function. For those that don't we do a simple compression of the range.

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post #226 of 230 Old 06-20-2013, 12:17 AM
 
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Originally Posted by sotti View Post

For those that don't we do a simple compression of the range.

Thanks for your answer Joel. Is, what you do, a common procedure, or is it your own solution?

Poynton in his "Gamma Rehabilitation" doc. mentioned following equation: L= (V’ +ε)^gamma, where ε is black offset. On the internet more popular equation is Luminance = contrast * value^gamma + black level. None of them is similar to yours as far as I understand.
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post #227 of 230 Old 06-20-2013, 09:34 AM
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Don't know what Joel's using, but in my spreadsheet I believe I used:

L = Lb + (Lw-Lb)* V^gamma

In other words, apply power law over the reduced range between your measured white and black levels then add the black level in as a linear offset.

BT.1886 does this much more intelligently (not to mention aggressively.)

Or you could just follow power law down as far as you can, and let your brightness control setup handle the compression near black.

There's probably a "right" answer somewhere in all of this, but the truth is probably that there's no way to know what method was used on the mastering/editing monitor, until you see something that looks really wrong ... like "black" holes in the back of peoples heads where there should be blondish hair (e.g.: "Black Swan".)
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post #228 of 230 Old 06-20-2013, 11:03 AM
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Originally Posted by HDTVChallenged View Post

There's probably a "right" answer somewhere in all of this, but the truth is probably that there's no way to know what method was used on the mastering/editing monitor

Exactly. Studio's aren't all conforming to any particular standard, which means that there is actually no right answer. Quite a few just run standard 2.2 with no offset for black. Some are starting to use BT.1886.

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post #229 of 230 Old 06-20-2013, 11:25 AM
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I've read through the whole topic (quite a long time ago, actually) and have been conducting my own experiments on this for a while. Nobody seemed to ask some questions which seem quite obvious to me. When pursuing for ultimate shadow detail what level would you want to be able to see in the actual video content? Is it 1 or maybe 2 (on a 0-255 scale)? And when we say 'see' how distinctly that would be? Must we use this or that pattern to judge the visibility of low level signals? I believe that's practical questions.
A theoretical question will be: "Will every display calibrated to BT.1886 be able to distinctly reproduce these low level signals?".
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post #230 of 230 Old 06-20-2013, 10:15 PM
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I've read through the whole topic (quite a long time ago, actually) and have been conducting my own experiments on this for a while. Nobody seemed to ask some questions which seem quite obvious to me. When pursuing for ultimate shadow detail what level would you want to be able to see in the actual video content? Is it 1 or maybe 2 (on a 0-255 scale)? And when we say 'see' how distinctly that would be?

Perhaps the reason that nobody asked that question is this: You should be able to see anything between 8 bit digital level 16 and 255 for video content. IOW: 17 to 254. By definition, black =16 and reference-white = 235. 236 through 254 is the overshoot (aka superwhite) range.
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Must we use this or that pattern to judge the visibility of low level signals? I believe that's practical questions.

????
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A theoretical question will be: "Will every display calibrated to BT.1886 be able to distinctly reproduce these low level signals?".

Yep ... assuming the display can actually pull off BT.1886.
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