How power law gamma calibration can lead to crushed blacks - Page 4 - AVS Forum
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post #91 of 230 Old 07-10-2012, 12:58 AM
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Originally Posted by Light Illusion View Post

I'm not sure this will really help, but LightSpace CMS is one of the main calibration systems in use within the professional film and TV industry, and we (Light Illusion) also provide a lot of direct calibration for many of the major players - be it in training clients to use LightSpace, or as direct calibration for those who prefer to have it done by an external operation.

I can state that the majority (the majority, not all!) of TV based calibration for mastering in our industry uses a Gamma of 2.2, measured by normalising pre-set black/white points.

This means setting black to be 'just' visible, using the Cal Image on the Light Illusion website, and then setting white to be approx 82 cdm2 (23 FtL).

100 or above is not used as most displays distort or clips colour channels at those levels.

The Contrast Cal Image is used to check for clipping.

The room illumination for TV work is set to approx a 'standard' living room environment... what ever that means! But, never a 'dark' room.

(For film grading things are rather different...)

Gamma (and Gamut) is then checked by direct measurement, with 2.2 Gamma as the target, normalising the black and white points the display is capable of.

You can see some of this here: http://www.lightillusion.com/display_calibration.html including the images used as described above.

Hope this helps.


@Light Illusion: thanks for the great info. I have an Eizo here that I'm about to calibrate to Rec. 709. So if I understand your post correctly...

For both mastering & grading: decoding Gamma of 2.2, Rec 709 gamut

For mastering: set black level using a PLUGE pattern, set white to 82 cd/m^2

For grading: set black to minimum possible, but what do I set the white to ? max possible on this display ? That would be around 270 cd/m^2 with the Eizo... Any advise ?

Thanks !

- M

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post #92 of 230 Old 07-10-2012, 01:48 AM
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PC monitors are different than TVs, you don't set brightness with a pluge pattern.

Typically their brightness control is actually the backlight.

If the Eizo does have brightness, contrast and a backlight, then you do have a real brightness control, but you still can't use a pluge pattern due to the fact PC's use the full range all the way down to 0. You can use a pattern like on the AVS disk or the CalPC client to turn it down till something just above black disappears then turn it back up.


White level is usually determined by your viewing environment. Standard overhead lighting, probably 150 cd/m is good, but you could go all the way up to 200 if you feel like you need the brightness. In a dim room something more like 100cd/m is more appropriate.

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post #93 of 230 Old 07-10-2012, 06:35 PM
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Quote:
Originally Posted by sotti View Post

PC monitors are different than TVs, you don't set brightness with a pluge pattern.
Typically their brightness control is actually the backlight.
If the Eizo does have brightness, contrast and a backlight, then you do have a real brightness control, but you still can't use a pluge pattern due to the fact PC's use the full range all the way down to 0. You can use a pattern like on the AVS disk or the CalPC client to turn it down till something just above black disappears then turn it back up.
White level is usually determined by your viewing environment. Standard overhead lighting, probably 150 cd/m is good, but you could go all the way up to 200 if you feel like you need the brightness. In a dim room something more like 100cd/m is more appropriate.

Joel,

as far as I understood Light Illusion's post (please correct me if I misunderstood), he calibrates displays differently depending on if they are used for mastering or grading...

I understood that for a MASTERING display, TV levels are needed, so a Pluge pattern could be used (or any other pattern that displays TV levels)...

I'm trying to get an understanding how to setup displays for mastering and grading to "industry standard", although it appears there is no true standard... rolleyes.gif

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post #94 of 230 Old 07-10-2012, 09:06 PM
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yes but the eizo is a PC monitor, if you are going to be doing video work, then you need to get a VIDEO display.

My point was that most PC monitors simply do not have the correct controls to be setup as a video display.

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post #95 of 230 Old 07-11-2012, 12:09 AM
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Alrighty then ... Third and (hopefully) final version of the BT1886 Calculator/Comparator/Evaluator.

1) Added the sRGB function/curve.
2) Added a third tab/sheet to evaluate "gamma" calibration results with a simple delta luminance calculation (% diff from target value.)

I don't foresee any further additions, anything else would be "mission creep," and the mission has already crept far enough. smile.gif

BT1886CalcV3.zip 19k .zip file
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post #96 of 230 Old 07-11-2012, 10:56 AM
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Quote:
Originally Posted by HDTVChallenged View Post

Alrighty then ... Third and (hopefully) final version of the BT1886 Calculator/Comparator/Evaluator.
1) Added the sRGB function/curve.
2) Added a third tab/sheet to evaluate "gamma" calibration results with a simple delta luminance calculation (% diff from target value.)
I don't foresee any further additions, anything else would be "mission creep," and the mission has already crept far enough. smile.gif
BT1886CalcV3.zip 19k .zip file

thanks, so far I've watched two BD movies with BT.1886 and not only are dark details much more visible in all scenes, dark scenes now have much more contrast and overall depth... in short BT.1886 seems to be very helpful on displays with poor black levels (think of it as a very aggressive black level compensation on such displays)
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post #97 of 230 Old 07-11-2012, 06:25 PM
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Thanks, this is very educational! smile.gif

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post #98 of 230 Old 07-11-2012, 06:37 PM
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Quote:
Originally Posted by PlasmaPZ80U View Post

thanks, so far I've watched two BD movies with BT.1886 and not only are dark details much more visible in all scenes, dark scenes now have much more contrast and overall depth... in short BT.1886 seems to be very helpful on displays with poor black levels (think of it as a very aggressive black level compensation on such displays)

If I'm interpreting your Calman results correctly, it looks like you might have overshot the target a bit at 10%, which may not be all that bad given the stubbornness of 1-9% range, but it might be worth taking another pass.

It would be nice to have an adjustment at 5%, but I guess the 10pt is better than nothing.

PS: BT1886 might backfire on some titles: "Blade Runner: TFC," for example, where it's clear the black level was boosted somewhere in the re-mastering process.
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post #99 of 230 Old 07-11-2012, 07:35 PM
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Quote:
Originally Posted by PlasmaPZ80U View Post

thanks, so far I've watched two BD movies with BT.1886 and not only are dark details much more visible in all scenes, dark scenes now have much more contrast and overall depth... in short BT.1886 seems to be very helpful on displays with poor black levels (think of it as a very aggressive black level compensation on such displays)


Man, do I agree with that. A few months ago, I adjusted the gamma curve on my Samsung 64d7000 to meet the 1886 spec. What a difference. One of the shortcomings of this set was the lack of detail in the near blacks. With the new gamma curve, this is no longer a deficiency.

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post #100 of 230 Old 07-11-2012, 08:22 PM
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Quote:
Originally Posted by HDTVChallenged View Post

If I'm interpreting your Calman results correctly, it looks like you might have overshot the target a bit at 10%, which may not be all that bad given the stubbornness of 1-9% range, but it might be worth taking another pass.
It would be nice to have an adjustment at 5%, but I guess the 10pt is better than nothing.
PS: BT1886 might backfire on some titles: "Blade Runner: TFC," for example, where it's clear the black level was boosted somewhere in the re-mastering process.

I did take another pass today (since I found out the brightness setting needed to go down one click from 52 to 51) and while some fine-tuning might still be possible at a future date, I've basically got it as close as the 10-pt controls will allow. I would like to take 21-pt measurements, though, to see how well gamma and RGB balance hold up between the 10 points of adjustment.

QuickView Grayscale.pdf 52k .pdf file BT1886CalcV3.xls 84k .xls file Picture menu.pdf 97k .pdf file
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post #101 of 230 Old 07-11-2012, 10:59 PM
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Originally Posted by PlasmaPZ80U View Post

I did take another pass today (since I found out the brightness setting needed to go down one click from 52 to 51) and while some fine-tuning might still be possible at a future date, I've basically got it as close as the 10-pt controls will allow. I would like to take 21-pt measurements, though, to see how well gamma and RGB balance hold up between the 10 points of adjustment.
QuickView Grayscale.pdf 52k .pdf file BT1886CalcV3.xls 84k .xls file Picture menu.pdf 97k .pdf file

PlasmaPZ80U,

got a couple of question regarding the docs...

(1.) Did you use a decoding Gamma exponent of 2.2 or 2.4 for your 1886 calibration ?

(2.) What's the origin of the function used to calculate "Effective Gamma Values" in your calculator [ gamma = LN(L / Lw) / LN(stimulus) ] ?

Thanks.

- M

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post #102 of 230 Old 07-11-2012, 11:06 PM
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Quote:
Originally Posted by Iron Mike View Post

PlasmaPZ80U,
got a couple of question regarding the docs...
(1.) Did you use a decoding Gamma exponent of 2.2 or 2.4 for your 1886 calibration ?
(2.) What's the origin of the function used to calculate "Effective Gamma Values" in your calculator [ gamma = LN(L / Lw) / LN(stimulus) ] ?
Thanks.
- M

1. 2.20, though it doesn't affect gamma values for BT.1886

2. it's not my calculator, ask HDTVChallenged
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post #103 of 230 Old 07-12-2012, 03:35 AM
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Originally Posted by PlasmaPZ80U View Post

1. 2.20, though it doesn't affect gamma values for BT.1886

I just realized that, testing it in the calculator... redundant to make it even a variable...
Quote:
Originally Posted by PlasmaPZ80U View Post

2. it's not my calculator, ask HDTVChallenged

HDTVChallenged, can you explain what the origin of the function [ gamma = LN(L / Lw) / LN(stimulus) ] is ?

Thanks.

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post #104 of 230 Old 07-12-2012, 08:31 AM
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Had a go myself today.
Found it fairly difficult to obtain any kind of accuracy since the 100% was essentially a 'slightly' moving target, anyways mines attached, will tweak a little more later when work allows!
Watched a bit of 'Edge of Darkness' certainly the bottom end is a bit more detailed and not crushed as per the power gamma 2.2 setting. BT 1886.png 287k .png file
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File Type: png BT 1886.png (286.9 KB, 76 views)

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post #105 of 230 Old 07-13-2012, 01:53 AM
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Originally Posted by Iron Mike View Post

HDTVChallenged, can you explain what the origin of the function [ gamma = LN(L / Lw) / LN(stimulus) ] is ?

LOL ... good question. That was supposed to be the power law function solved for gamma instead of Luminance @ %Stim. Something might be "off" there ... rechecking my math ...
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post #106 of 230 Old 07-13-2012, 01:58 AM
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Originally Posted by PlasmaPZ80U View Post

I would like to take 21-pt measurements, though, to see how well gamma and RGB balance hold up between the 10 points of adjustment.

From my experience, 10 to 100% should track pretty close. It's the 0 to 10% range where things kind of go off the rails. The LK450 really wants to follow the power law curve down there.
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post #107 of 230 Old 07-13-2012, 02:04 AM
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I just realized that, testing it in the calculator... redundant to make it even a variable...

Not really. The reason it's there as a variable is so you can *compare* the BT1886 curve to the power law curve with various gamma values.

Hint: The BT1886 curve is *very* sensitive to your measured luminance @ "black" (aka. MLL.)
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post #108 of 230 Old 07-13-2012, 02:30 AM
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Originally Posted by HDTVChallenged View Post

Not really. The reason it's there as a variable is so you can *compare* the BT1886 curve to the power law curve with various gamma values.
Hint: The BT1886 curve is *very* sensitive to your measured luminance @ "black" (aka. MLL.)

I meant a variable in the BT.1886 specification... well, it's more of a Rec 709 legacy constant...

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post #109 of 230 Old 07-13-2012, 02:36 AM
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Originally Posted by HDTVChallenged View Post

LOL ... good question. That was supposed to be the power law function solved for gamma instead of Luminance @ %Stim. Something might be "off" there ... rechecking my math ...

well, you use the same function that zoyd uses in his calculator in this thread (only difference: u use the "common" logarithm while he uses the "natural" logarithm - same result)... I found an old thread on AVS that was discussing the effective gamma calculation and most peeps on there were arguing that there are multiple ways and methods...

... just wondering if the function that you used was the one that ended up being generally implemented... ? rolleyes.gif

Thanks !

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post #110 of 230 Old 07-13-2012, 02:58 AM
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Quote:
Originally Posted by Iron Mike View Post

I meant a variable in the BT.1886 specification... well, it's more of a Rec 709 legacy constant...

Right ... it's *not* a variable in the BT1886 function. In the primary/reference BT1886 function, gamma is fixed at 2.4.

The issue here is that we've come to conflate the variable "gamma" with the actual transfer function, which historically is almost always the power law curve. So when I write "effective gamma," what I mean is the gamma of a theoretical power law curve at a specific luminance vs. % stimulus vs. measured luminance of white. i.e. L = Lw * %Stim ^ Gamma. Gamma = ln (L/Lw)/ln(%Stim.)

Due to the magic of logarithms you can substitute Log10 (common) for Ln (natural) here ... but you're just making more work for the computer that way smile.gif.

Again, this is for comparison purposes ... as well helping one decide which power law gamma preset one might want to use as a starting point on a given display.

PS: I'm using the natural log. smile.gif
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post #111 of 230 Old 07-13-2012, 11:37 AM
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Not to mention you can actually go Log Yn(stim)=gamma

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post #112 of 230 Old 07-13-2012, 11:48 AM
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Originally Posted by HDTVChallenged View Post

LOL ... good question. That was supposed to be the power law function solved for gamma instead of Luminance @ %Stim. Something might be "off" there ... rechecking my math ...

so, is anything off with v3 of the calculator?
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post #113 of 230 Old 07-13-2012, 02:07 PM
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Originally Posted by HDTVChallenged View Post

From my experience, 10 to 100% should track pretty close. It's the 0 to 10% range where things kind of go off the rails. The LK450 really wants to follow the power law curve down there.

Do you use the 2-pt controls first or do you use the 10-pt controls only?

I use 10-pt controls only. Also, choosing the 1.9 gamma preset helps bring the 0% to 10% range closer to BT.1886 than with the default 2.2 preset.
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post #114 of 230 Old 07-14-2012, 11:51 PM
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Not to mention you can actually go Log Yn(stim)=gamma

That's just too freaky for my aging grey matter. I actually had to pull out my old calculus textbooks to make sure that I hadn't made a major mistake. smile.gif
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so, is anything off with v3 of the calculator?

Nope. It's fine as far as I can tell.
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post #115 of 230 Old 07-15-2012, 12:01 AM
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Originally Posted by PlasmaPZ80U View Post

Do you use the 2-pt controls first or do you use the 10-pt controls only?
I use 10-pt controls only. Also, choosing the 1.9 gamma preset helps bring the 0% to 10% range closer to BT.1886 than with the default 2.2 preset.

I've been experimenting with using the 2pt controls first, but so far I'm not happy with the results (visually.) I think going with only the 10pt will result in a smoother curve, with the exception of the 1% to 8% stimulus range.

The only concern I have about using the 1.9 preset on the LK450 is that we might sacrifice smoothness in the 10-100% range for the sake of a relatively small improvement in the 8% near black ... I don't know if that's really a smart trade-off. I suppose it would depend on how the 20pt evaluation looks. This is where that missing 5% control would be really handy.
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post #116 of 230 Old 07-15-2012, 11:50 AM
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Originally Posted by sotti View Post

Not to mention you can actually go Log Yn(stim)=gamma

Humm ... I think you might have swapped your log-base and argument there. I'm assuming Yn is the normalized Y value: i.e L/Lw.

I think you probably meant to write:

Gamma = LogStim Yn, or in Excel-speak: gamma = LOG(L/Lw, Stim)

Edit: To be more precise, I changed %Stim to just Stim. %Stim implies values from 0 to 100, instead of 0 to 1 which is what I really meant.
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post #117 of 230 Old 07-15-2012, 01:25 PM
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The only concern I have about using the 1.9 preset on the LK450 is that we might sacrifice smoothness in the 10-100% range for the sake of a relatively small improvement in the 8% near black ... I don't know if that's really a smart trade-off. I suppose it would depend on how the 20pt evaluation looks.

I'll check it out tomorrow if I get a chance. To me, that 1% to 9% gamma is quite important as it affects shadows details greatly (darkest shadows are usually well below the 10% adjustment point). If you happen to be right, I imagine there will be a tedious process of recalibrating grayscale and gamma with the 2.2 preset.
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post #118 of 230 Old 07-16-2012, 12:11 AM
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Quote:
Originally Posted by PlasmaPZ80U View Post

I'll check it out tomorrow if I get a chance. To me, that 1% to 9% gamma is quite important as it affects shadows details greatly (darkest shadows are usually well below the 10% adjustment point). If you happen to be right, I imagine there will be a tedious process of recalibrating grayscale and gamma with the 2.2 preset.

Depends where you start out. My set requires much more work with the 1.9 preset. But yeah ... it's going to be a hassle either way.

Related to that, I have an alternate theory as to why I might be preferring the quasi BT1886 curve on my set. I think I've accidentally created a situation that mimics "black level expansion" from the CRT days. Levels from 10% up to 50% are boosted, while the levels below 9% are sticking much closer to the power-law+BLC curve. So ... it might be working exactly opposite of what the ITU folks had in mind.

Tip: If you can, move your HDMI cable over to an unused input. That will give you two fresh Expert settings that you can adjust without wiping out the work you've already done. Just remember to *not* press the "copy to all inputs" button.
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post #119 of 230 Old 07-16-2012, 09:47 AM
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Originally Posted by HDTVChallenged View Post


Tip: If you can, move your HDMI cable over to an unused input. That will give you two fresh Expert settings that you can adjust without wiping out the work you've already done. Just remember to *not* press the "copy to all inputs" button.

I always record my settings after any (successful) calibration session on my computer, so I can revert to it manually if needed.
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post #120 of 230 Old 07-16-2012, 09:50 AM
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Related to that, I have an alternate theory as to why I might be preferring the quasi BT1886 curve on my set. I think I've accidentally created a situation that mimics "black level expansion" from the CRT days. Levels from 10% up to 50% are boosted, while the levels below 9% are sticking much closer to the power-law+BLC curve. So ... it might be working exactly opposite of what the ITU folks had in mind.

I don't really understand what you're saying here. It seems to me the gamma correction 'boost' is applied mainly to the low end of the grayscale while the mid range and top end are quite consistent with 2.2 power law gamma.
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