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post #1 of 54 Old 09-28-2013, 04:50 PM - Thread Starter
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Not looked at it myself..but might be good for new to calibration...:-)
Sorry if it was posted before...:-)
https://www.youtube.com/watch?v=eqWd8qs9YAk&feature=youtube_gdata_player
Cheers!/H

Sent from my Oppo X909 CM10.1
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post #2 of 54 Old 09-28-2013, 09:20 PM
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oh wow this is exactly what I needed. Thanks for sharing this. I'm 20 min in and loving it. It's nice having all this knowledge put into a coherent context. Scott and Robert are awesome.
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post #3 of 54 Old 09-29-2013, 12:41 AM
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well that was a really fascinating watch. Does bring up a question though. They mentioned that, when adjusting RGB offset and gain, it's better to reduce the levels when striving for a balance, rather than boosting them (they made the analogy to audio EQ in which you should supposedly adopt the same heuristic).

I wonder if the decisions you make about whether to decrease or increase these levels, have an impact on your gamma, and if so, wouldn't that be a good way to fine tune gamma?

For example, on a two point greyscale adjustment, when adjusting at 30%, there may be several combinations of RGB values that yield excellent delta E values, and these combinations mainly differ in the overall magnitude of the signal. Is it possible, for instance, that the reason that Scott and Robert ended up with some black crush is precisely because they took this "always use decrements when fine tuning greyscale" heuristic too far, and ended up with a (balanced) combination that had too low a magnitude?
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post #4 of 54 Old 09-29-2013, 10:27 AM
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This vid is great for a noob like me.
Great insight into the calibration.
Loved the part where they explained what the graphs meant and what the white balance controls really do, Excellent!

For white balance controls very good explanation and demonstration starting from around 1:00:00 but it's important to see the initial measurement results just before to get the context of grayscale calibration.
But the whole video is very informative.

For two point white balance controls:

"Offset is what controls the low end of the brightness scale so that's what we'll use for 30(%), Gain is what controls the high end of the brightness range so that's what we'll use for 80(%)."

I was ignorant of this as I didn't know where to look for to get the right information.
Thanks for sharing!
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post #5 of 54 Old 09-29-2013, 02:16 PM
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That was a great video.
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post #6 of 54 Old 09-29-2013, 02:29 PM
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love watching this smile.gif

Sometimes you are 1 click away from pulling your hair out and bang your head against the wall
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post #7 of 54 Old 09-29-2013, 02:54 PM
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Quote:
Originally Posted by spacediver View Post

well that was a really fascinating watch. Does bring up a question though. They mentioned that, when adjusting RGB offset and gain, it's better to reduce the levels when striving for a balance, rather than boosting them (they made the analogy to audio EQ in which you should supposedly adopt the same heuristic).

I wonder if the decisions you make about whether to decrease or increase these levels, have an impact on your gamma, and if so, wouldn't that be a good way to fine tune gamma?

For example, on a two point greyscale adjustment, when adjusting at 30%, there may be several combinations of RGB values that yield excellent delta E values, and these combinations mainly differ in the overall magnitude of the signal. Is it possible, for instance, that the reason that Scott and Robert ended up with some black crush is precisely because they took this "always use decrements when fine tuning greyscale" heuristic too far, and ended up with a (balanced) combination that had too low a magnitude?

That statement makes zero sense. There is no "rule" about raising or lowering offsets or gains. You simply must be observant to detect any odd-ball behavior on the part of the TV when you make ANY adjustments including gain and offset adjustments. Some models respond poorly to making offsets lower or to making gains higher, but it is NOT even remotely universal. In some cases, raising offsets will raise the black level. If you aren't paying attention to that during calibration, you could end up with much worse black level performance than the TV is capable of. On the other hand, if you are trying to find the darkest black level, you may find that lowering the offsets does not affect black level at all. Sometimes you MUST adjust gains or offsets in ways they would leave you to believe you should not do.

Their explanation of how you use an audio equalizer is unrelated to adjusting gains and offsets in every possible way. When using an audio equalizer, it is simply IMPOSSIBLE to remove a dip in response in any bass frequency because those dips exist because of room cancellation effects. Some combination of dimensions of the room results in cancelling a narrow band of frequencies entirely. If you try to boost energy at that frequency, the cancellation remains and the dip in response remains. However, higher up in the frequency spectrum (say higher than 250 Hz), cancellations due to reflections become increasingly minute and boosting a range to get more linear in-room response is certainly possible and useful. In the bass however, if you have a 20 dB suckout centered on 60 Hz, if you pump 60 Hz up 20 dB trying to get linear response, you're still going to be 20 dB down at 60 Hz because the room will still do the cancelling. To stop the cancellation effects you have to do some combination of: changing the room dimensions (13x 15 x 11 is much better than 12 x 16 x 8 where all numbers are divisible by 2 and 4 while there are no numbers that divide into 13 x 15 x 11 as whole numbers) or moving the speakers or moving the seat. So using audio equalization requires understanding the underlying issues just as much as using calibration controls effectively requires understanding the underlying issues. In the case of video calibration, there are so many TV models and so many of them respond differently to various controls that there's no one set of rules that you must always follow. You have to understand the concepts, then figure out how those concepts relate to the TV at hand. And what do you do with your calibration controls if there are 10 or 20 adjustments, each with a red, green, blue level setting (no gain or bias controls at all)? If you don't understand that you don't need gain and bias controls to calibrate a TV, you're missing a huge chunk of information. In fact, today, I wouldn't even buy a TV if it only had gain and bias controls. I'd want 10 adjustment points at a minimum (30 individual adjustments) and 20 would be preferred. Properly implemented, those controls should allow you to make graycale and gamma adjustments far more accurately than you can hope to get from just gain and offset controls.

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post #8 of 54 Old 09-29-2013, 03:35 PM
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interesting, I wonder why they made that statement then. I also wonder why they didn't have in mind the target luminances at those two points so they could preserve the proper gamma while making grayscale adjustments (and I do understand that a 10 or 20 point adjustment is better than a 2 point adjustment, and that gain and offset refer to 80% and 30% respectively, and that with a 10 point adjustment those terms lose their meaning - at least I think I understand!).
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post #9 of 54 Old 09-29-2013, 06:12 PM
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Alot of basic rules of thumb proceedures are expanded distorted and extensions of some reality from legacy designs.

Some methods apply to some display types, some don't, many there is no hard cast rules.

Reality is, each person must adopt a proceedure for themselves, much based upon the teachings of those experienced, but then if those teachings become 2nd or 3rd hand without being put to the test now and again they can be just method clutter.

Examples of no real hard and fast rules.

1q. CMS before White balance or White balance before CMS

2q. Setting colour and tint before white balance and CMS or after for that matter

3q. Only cut dont add white balance controls and never touch green


1a. It depends, but if the display has independant controls for white balance and CMS it doesn't matter what you start and end with. If you have interactive white balance and primaries(RGB) as some do you have real problems, adjusting one without checking other can lead you to many wasted hours.

2a. Personally I have a problem with setting colour and tint before examining the complete package. Secondly, if the CMS and white balance/gamma are all good then there is no need for colour and tint.

3a. All legacy, but sometimes it applies, here you have to look for clipping and run out or roll off at the top end. The bottom end can be hard without decent tools. Adjusting green, in some cases can be the only usefull way to balance the things. Most projection systems could not be balanced if you didn't adjust green. Usually you have to leave red alone in projectors.


Watching a video of these likely lads calibrating is a learning tool even for those experienced, because a wise calibrator will compare what they do and put their own practices to the test. The student will see things differently and will take everything on board. But as I alluded to above, put your methods to the test, try things in different orders, see why things work and dont work in a certain sequence.

The only real constant, if I dare state it, is the constant of references, in either and both generators and measurement devices. Without them anyone would only be guessing.
Many AVS users have realised the reality of a quality tool set.

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post #10 of 54 Old 09-29-2013, 07:22 PM
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I wonder why he likes gamma 2.2 for a room that dark. My room is a little similar to that but more of an orange light. Gamma 2.2 is quite cloudy in a room as dark as that. 2.3 looks sweet. Though I suppose it can't be compared since different tv's.

Yet he said the blacks still looked crushed a little at 2.2. I wonder if that tv would look better at 2.0 gamma the way he described the blacks. Or is it what one is used to viewing, what appears crushed to one looks better to another?
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post #11 of 54 Old 09-29-2013, 08:19 PM
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It by all accounts is a display type that can create low video levels, lower than they have witnessed before.
This could this be a subjective statement due to the new technology?
Maybe the source material is crushed?
Maybe the display is crushing everything to generate low levels?

It is why viewing known material at the end is important, the room and lighting does impact our perceptions.

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post #12 of 54 Old 09-30-2013, 10:46 AM
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It's a shame they didn't have Tron Legacy and The Dark Knight Rises to test. The first 13 minutes of The Hobbit is another good one for black levels.
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post #13 of 54 Old 09-30-2013, 04:00 PM
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Quote:
Originally Posted by spacediver View Post

interesting, I wonder why they made that statement then. I also wonder why they didn't have in mind the target luminances at those two points so they could preserve the proper gamma while making grayscale adjustments (and I do understand that a 10 or 20 point adjustment is better than a 2 point adjustment, and that gain and offset refer to 80% and 30% respectively, and that with a 10 point adjustment those terms lose their meaning - at least I think I understand!).

You can't really establish a target luminance for black... most meters will not measure the black of a really good flat panel or of most projection setups. If you get a measurement, it's likely to not be accurate anyway. And the room has to be darker than the TV or projection system for the measurement to be accurate. I've had sensitive light meters in my room (not calibration meters, light meters) I was experimenting with and the meter would pickup the presence or absence of a single LED on a piece of equipment even when the equipment was behind an opaque screen (screen free-standing in the room on "legs", in front of the equipment racks). So unless you are going to make the room pitch black including covering every LED with opaque tape, you will never get a accurate black level measurement. For 100% white, yes, you can decide your target luminance level is going to be 33 fL and measure 100% white repeatedly so that when you adjust gains, you maintain the luminance level you want. But it can be tricky because what we perceive as white or gray is 72% green light, 21% red light, and 7% blue light. So if you want to make a +3 adjustment to the green gain control, you'll make 100% white a fair bit brighter while a +3 blue gain adjustment will barely influence how bright 100% white is after the adjustment. So your +3 green gain adjustment might be better done as +1 on green and -2 red and -3 blue or some variation of that where you increase some controls and decrease others to maintain your desired 100% white level... but since you can't measure the black accurately, you have to use your powers of observation -- which is also necessary because at, say, 5% white, leep in mind that there will be so little blue light that almost no colorimeter or spectroradiometer within the price range of pro calibrators (let alone home calibrators who won't spend that much) will be accurate measuring the blue light at 5% white and measuring blue at 0% white is even harder... remember, you could easily be measuring .001 fL or less down in those dark ranges and only 7% of those tiny amounts of light will be blue light. So you always have to rely on your eyes to tell you what's going on at black or near-black. I've seen strong magenta, and blue tints at very los light levels even though the meter was saying the measurement was neutral gray. You HAVE to believe your eyes at that point because the meter just gets increasingly unreliable as the light levels decrease. So your meter can tell you that your darkest step is good, but you could have a red tint to star-fields and errors like that are VERY obvious and very difficult to live with.

So, again, being observant about how the TV behaves when you move offset or gain controls is critical. That's not something you can probably learn from a video because there are so many different scenarios you can run into with different models, different years, and different brands of TV or projection setup.

Gain and offset do NOT refer to 80% and 30% respectively. Your goal with the gain and offset controls is to get the ENTIRE grayscale as accurate as it can be with just those 2 controls (3 settings for each, red, green, and blue). It is my experience that you almost NEVER get the best result using 80% and 30% with gain and offset controls. I look at the ENTIRE grayscale after making an adjustment because the offset controls typically affect everything from 0% to 70% or so while the gain controls usually control the entire range from 30% to 100%. Meaning there's a fair bit of overlap in the middle between 30% and 70% and you'd never see the effects of that overlap if you were only looking at 80 and 30.

"They" (essentially everybody) makes statements without thinking through the logic or perhaps from not understanding the underlying concepts completely or simply by not thinking far enough through what they are saying and what they are intending to say. They "meant well" but simply didn't use an appropriate comparable situation when you get right down to it.

The danger of a 10 minute or 1 hour video is that it can't tell you all this stuff.

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post #14 of 54 Old 09-30-2013, 04:18 PM
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thanks for sharing the insights! Very useful information, especially about keeping mind the relative sensitivites of R G B when making grayscale adjustments.

I suppose one strategy for maintaining gamma would be, after first using the eye to set brightness and contrast, to keep a record of the luminances at each step (say from 10% up, since 0% is unreliable).

You could then try maintain the target luminance, at each level, when making your adjustments. And, as you say, when doing the very low levels, put some trust in your eyes.
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post #15 of 54 Old 09-30-2013, 04:40 PM
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Quote:
Originally Posted by spacediver View Post

thanks for sharing the insights! Very useful information, especially about keeping mind the relative sensitivites of R G B when making grayscale adjustments.

I suppose one strategy for maintaining gamma would be, after first using the eye to set brightness and contrast, to keep a record of the luminances at each step (say from 10% up, since 0% is unreliable).

You could then try maintain the target luminance, at each level, when making your adjustments. And, as you say, when doing the very low levels, put some trust in your eyes.

You do 100% first, and you don't pay any attention to luminance.

Then every other point is done in relation to target luminance if you have a multi-point system.

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post #16 of 54 Old 09-30-2013, 04:46 PM
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People say a lot of dumb stuff about gamma... I mean a whole boatload of stupidity is out there floating around and it gets repeated so many times that people assume it is true.

There is one argument that we have to use 2.4-2.5 gamma because that is the gamma of the (CRT) displays that were used for mastering video and new mastering monitors mimic the performance of those legacy CRTs that are no longer used. This is a STUPID statement because the gamma of the display is nearly meaningless as long as it is higher than the gamma you are actually using, The software used for mastering controls the gamma that is displayed regardless of the native gamma of the display (assuming your display is calibrated!). So even if your monitor's native gamma is 2.5, your software can EASILY change that to 2.2 or 2.3 and virtually everything mastered since DVD has been mastered on workstations setup somewhere in the range of 2.2-2.3. The actual visible difference between 2.2 and 2.3 gamma is very small. Few people could tell one from another without measurements. I use 2.25 for my gamma target all the time.

Stupid thing #2 -- gamma should be changed with viewing conditions. This is mostly stupid... the movie was mastered between 2.2 and 2.3 gamma, it should always be played-back with that gamma. All you need for a room with light is more light from the display. You do NOT want to make the black level higher just because there is light in the room. And you do NOT want to change gamma just because there is more light in the room. The reason people have this "room conditions somehow correlate to gamma" mindset is because for a LONG time all we had was CRT displays and they were not capable as getting as bright as we might want them in rooms with a lot of light. So, one way to make the picture brighter on displays that just don't have enough light capacity to make images bright enough in a room with some light in it is to use a numerically lower gamma because everything between the white point and black point will get brighter, but white and black stay where they are. Today, a plasma TV MIGHT have this issue (though it won't be as bad as CRTs in that respect) and front-projection systems might need that level of help also. But it is NOT done to make the picture look better, it is only done to make the picture sufficiently bright, LCDs and most newer plasmas can get PLENTY bright enough to view in rooms with some fairly bright light without resorting to lower (numeric) gammas to get even brighter images. So the whole concept that somehow a brighter room demands a lower (numeric) gamma is just stupid. 2.2-2.3 is the mastering gamma and should always be the gamma you use when you view the movie regardless of room lighting... unless you have a video display that just doesn't get very bright and you have no choice but use a lower (numeric) gamma. It's a crutch to use a lower gamma, not something done in the name of better images.
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post #17 of 54 Old 09-30-2013, 04:51 PM
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Joel,

When you let CalMAN auto-measure grayscale, it measures 100% first, then goes to 0% or 5% or whatever you set as your minimum measured point, and measures up from there to 90% or 95% or whatever step you set that is closest to 100%.

Is there a reason for not using 100-90-80-70-60-50-etc instead of 100-0-10-20-30-etc.? I assume the method you chose was just the result of a coin toss or whatever as I don't think it should matter, but have wondered about this for a while.

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post #18 of 54 Old 09-30-2013, 04:55 PM
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Re. gamma while calibrating... even if your 100% level varies a bit from adjusting gains, it won't change gamma enough to be visible if gamma was reasonable before you tweaked the gain controls (a little). Obviously, large changes to gain controls could make a big gamma difference, but that would be a very unusual situation.

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post #19 of 54 Old 09-30-2013, 05:17 PM
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Quote:
Originally Posted by Doug Blackburn View Post

Joel,

When you let CalMAN auto-measure grayscale, it measures 100% first, then goes to 0% or 5% or whatever you set as your minimum measured point, and measures up from there to 90% or 95% or whatever step you set that is closest to 100%.

Is there a reason for not using 100-90-80-70-60-50-etc instead of 100-0-10-20-30-etc.? I assume the method you chose was just the result of a coin toss or whatever as I don't think it should matter, but have wondered about this for a while.

It had more to anything to do with the sequencing of patterns on DVD discs, since most of the discs out there started at 10% and went up, we thought matching up with that would make the most sense. But measuring 100% first so that we wouldn't have to re-scale the measurements.

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post #20 of 54 Old 09-30-2013, 05:19 PM
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You can think of gains as RGB contrast controls and cuts as RGB brightness controls.

Contrast and brightness can effect gamma in small, but in no way should be used to adjust gamma. Adjust the controls correctly and let gamma fall where it may. Then go adjust gamma with gamma controls (10-point RGB or a gamma setting).

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post #21 of 54 Old 09-30-2013, 06:28 PM
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On the Samsung e450 if I put gamma on -1 the brightest whites go more torch white and makes dark screens too dark.And make gamma less consistent. The abl almost always brightens gamma on this tv. I find using a brightness setting around 5-10 above where dithering starts reduces black crush and brightens it up especially on dark dim screens where you can't hardly see anything(last part may be a samsung only thing).This tv Out of the box blacks were black..I'm sure every tv including same models are different and white balance settings change things too.
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post #22 of 54 Old 09-30-2013, 07:54 PM
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Gamma does not control how bright white is. It controls how dark midtones are, how slowly shadows gain luminance on the way to midtone levels, and how steep the slope is through the highlights, say from 65% to 99%. The steeper that highlight slope (high gamma numbers), the more unnatural highlights will look, shadows will stay dark much too high up the luminance scale, and the midtones will be darker than they should be.

-1 is a common and reasonable setting for Gamma on Samsung displays, though you might need -2, 0, or +1 depending on your setup and calibration. Without a meter, you are just GUESSING what the right/best gamma setting is. Of course, if the images look strange, you likely don't have the best setting. THOUGH... you can have situations where some other inappropriate combination of settings makes what would normally be a good gamma setting look bad. But you can't really know what other settings might be off without a meter and calibration software and the knowledge to use them. There really aren't any shortcuts or eyeball methods to this, There's an excellent chance that if -1 looks really bad, something somewhere else is not right.

You cannot rely on manufacturer numeric gamma settings either. If you happen to have a TV that has gamma settings labeled 2.0, 2.1, 2.2, 2.3, 2.4... when you actually measure those settings, you may find that 2.4 or 2.0 actually produces a gamma close to 2.2.

Also keep in mind that when we say the display's gamma is 2.2 or whatever, that's either an average of the whole grayscale or just one single point along the grayscale. What you really don't want to see is for 90% to have a gamma of 2.6 while 80% has a gamma of 1.8. Your images will look very strange indeed. If you average 1.8 and 2.6, you get 2.2 but your images won't look right. So you really MUST pay some mind to gamma and not end up with some strange gamma variations that overcome the nice low grayscale errors. Keep in mind that most calibration software calculates grayscale errors only using xy coordinates so Luminance (what controls gamma) is not included in the error calculation. Some calibration software allows you to calculate errors WITH luminance included, but you may have to manually select that option. You don't HAVE to have the error calculation include luminance, but you'd better be looking at a point-by-point gamma graph to make sure you haven't got some big gamma swings. I set my gamma target for 2.25 and if I can keep every grayscale point between 2.2 and 2.3 I'm really happy. Keeping gamma between 2.1 and 2.4 is the best a lot of displays can accomplish without 10 or 20 point grayscale controls, though there are a few that are VERY linear and adjust well with just offset and gain controls.

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post #23 of 54 Old 09-30-2013, 08:13 PM
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Also keep in mind that when we say the display's gamma is 2.2 or whatever, that's either an average of the whole grayscale or just one single point along the grayscale.

I always thought that gamma referred to the exponent of the function that describes the change in luminance over stimulus levels. So if you measure your curve, and fit it to a gamma function, the exponent is what we refer to.

But it seems I'm mistaken here (and indeed, I always wondered why people would talk about gamma at different luminances, when gamma, to me, is a property of the entire curve).
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post #24 of 54 Old 09-30-2013, 08:48 PM
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Originally Posted by spacediver View Post

I always thought that gamma referred to the exponent of the function that describes the change in luminance over stimulus levels. So if you measure your curve, and fit it to a gamma function, the exponent is what we refer to.

But it seems I'm mistaken here (and indeed, I always wondered why people would talk about gamma at different luminances, when gamma, to me, is a property of the entire curve).

You are smart to be thinking of the entire curve.

The issue is in the real world a display does not respond uniformly to stimulus, meaning at different points the curve measures at different gamma values. While you might target a single number, when you talk about the displays response, it's much more complicated than what can be communicated with a single number.

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post #25 of 54 Old 09-30-2013, 08:57 PM
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Is the idea something like this?

Take two ideal gamma curves, 2.2 and 2.3. Each of these curves fits that gamma perfectly, so they are 2.2 and 2.3 throughout both curves, respectively.

Now take a third curve in the real world that we measure. We find that at 20% stim, the luminance matches what the 2.2 ideal curve's value is at 20%. At 60% we find that the luminance matches what the 2.3 ideal curve's value is at 60%.

Is that what is meant by a curve having a gamma of 2.2 at 20%, and 2.3 at 60%?
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post #26 of 54 Old 10-01-2013, 05:40 PM
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Maybe... it depends on the context.

You just about never see a perfect gamma curve from any video display device. Excellent real-world response can be thought of as your ideal 2.3 and 2.2 curves showing the desired gamma range, while the ACTUAL gamma of the device would not be a smooth curve, but each measured point would be somewhere between the 2 ideal curves, bouncing around, usually randomly, within that "range". It's more common to have each measured grayscale point to have a different calculated gamma than it is for them to all have the same calculated gamma.

All you need for a gamma calculation is the 100% white level and a black level (when it cannot be measured, which is getting increasingly common, you can just use an appropriately small number like .001 or even .0001 for black). When you know those 2 numbers, you can calculate the gamma of any intermediate point. You can't calculate gamma for 100% or 0% though since they are variables in the equation. Though you COULD calculate gamma for 99.5% and 0.05% using a real or interpolated number based on your black and white numbers.

The gamma luminance curve is defined by "reversing" the equation... in this case the curve is not a variable, it's a "known" so you use your 100% white measurement and your black measurement (or predetermined black level like .001), plug your desired gamma curve into the calculation, like 2.2, and the output of the equation is a curve on a graph instead of a single number. When you measure a real-world video display, you just about never get a perfect gamma curve, so you really must average your measured point gammas. So if 10% is 2.2 gamma, and 20% is 2.3 gamma, and 30% is 2.2 gamma, and 40% is 2.3 gamma... etc. all the way to 100% (well all the way to 99.5% or something close to 100%) your end result is not a 2.2 or 2.3 gamma curve, but it would be pretty fair to say the average gamma for that display is 2.25.

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post #27 of 54 Old 10-01-2013, 06:43 PM
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edit: deleted, see next post.
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post #28 of 54 Old 10-01-2013, 07:03 PM
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Ok so is this what's going on?

Below, the five blue curves represent five ideal functions, ranging from 1.8 to 2.6

The red curve represents a simulated measured curve with an average gamma of 2.2 (I've added some gaussian noise). Notice at around 30% luminance, the red curve matches the second blue curve (gamma = 2.0), but at around 68% luminance, the red curve matches the fourth blue curve (gamma = 2.4).

So would we say that our measured curve has a gamma of 2.0 at 30%, and a gamma of 2.4 at 68% (with an average gamma of 2.2)?

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post #29 of 54 Old 10-02-2013, 04:23 PM
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No, you wouldn't say that. You'd say "look at this graph"... using words to describe a complex relationship like a graph is dangerous, especially when you have the graph.

Yeah, it might kinda sorta follow one curve or another at different places, but that's not likely to happen in the real world.

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post #30 of 54 Old 10-02-2013, 05:05 PM
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Quote:
Originally Posted by Doug Blackburn View Post

No, you wouldn't say that. You'd say "look at this graph"... using words to describe a complex relationship like a graph is dangerous, especially when you have the graph.

Yeah, it might kinda sorta follow one curve or another at different places, but that's not likely to happen in the real world.

but a lot of color management software does indeed display that in reality, the gamma doesn't track perfectly across the entire range of luminance.
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