What is the "correct" gamma to use? - Page 3 - AVS Forum
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post #61 of 82 Old 01-25-2007, 03:40 PM
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dithering is a technique used to hide banding. If you need to have a value of #.5 you do it by spatially/temporally alternating # and #+1. If you are of suitable viewing distance and frame rate it blends together as if it was the in-between value - it hides the sharp edge of the band that the eye can critically see. Maybe lumagen does this for 8b output I don't know - but the point remains the 8b digital outputs ARE a bottleneck.

DLP does both spatial&temporal dithering near black because of a lack of bandwidth - TI has improved on DLP with successive generations of DMD and colorwheel designs. But there are those who choose other technologies because they considering this dithering to be a psychovisual abomination (of course maybe just because they read about it not because they actually perceive it)

You need a good source material to detect banding - the AVIA greyscale ramps are themselves dithered but AVIA PRO is not. Getting perfect digital transmission of the values on the media to the display is sometimes difficult to achieve - but when you do the image just snaps into place. But if your display is 10000K with 3.0 gamma and you have no choice other than a scalar to calibrate - then you live with banding/dithering.
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post #62 of 82 Old 01-25-2007, 03:48 PM
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I simply can't yet explain why studio CRT-based monitors, using digital signal processing, alter their native gamma figure, which is nominally 2.5, to 2.2

Still confused - did Eric say this or did Poynton?
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post #63 of 82 Old 01-25-2007, 04:41 PM
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Quote:
Originally Posted by PencilGeek View Post

It looks absolutely amazing. The VP uses 10-bit processing from head to tail; and uses a parametric equation for implementing the gamma from the points you select; and there's plenty of math algorithms that can be used for curve-fitting between the adjacent points to create an absolutely smooth curve throughout.

Good for you. Maybe the processor you have specifically checks for 1-point difference in adjacent areas and tries to retain the same 1-point difference in output signal.

On the other hand, I don't get how 8 bits is enough to achieve smooth gradients AND high contrast ratio at the same time, because 1.01^219 = 8.84. Poynton himself makes an example of how many data points, spread with 1% difference, needed for not-too-spectacular CR 100:1, and it comes to 463. This number cannot fit 8 bits! What is the trick? If the luminosity value is scaled (multiplied) to achieve higher full contrast ratio, then how 1% difference is ensured?
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post #64 of 82 Old 01-25-2007, 05:00 PM
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Quote:
Originally Posted by krasmuzik View Post

You need a good source material to detect banding - the AVIA greyscale ramps are themselves dithered but AVIA PRO is not. Getting perfect digital transmission of the values on the media to the display is sometimes difficult to achieve - but when you do the image just snaps into place. But if your display is 10000K with 3.0 gamma and you have no choice other than a scalar to calibrate - then you live with banding/dithering.

Using Sencore VP403 signal generator ramp pattern. Don't know if they dither or not.


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post #65 of 82 Old 01-25-2007, 07:21 PM
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Quote:
Originally Posted by krasmuzik View Post

You should not be using a scaler to correct the display unless you have a higher bit output - available on analog or a fully implemented HDMI1.3 chain.

So can you recommend any device that will allow you to adjust gamma curves with this higher bit output via vga/rgbhv? Do these devices just adjust the overall curve or does it allow for adjustments at different IRE's?

Also sightly off topic, but are there also devices which allow you to change RGB% at different IRE's so that you can get a completely flat response?

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post #66 of 82 Old 01-25-2007, 09:30 PM
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Quote:
Originally Posted by Gino AUS View Post

So can you recommend any device that will allow you to adjust gamma curves with this higher bit output via vga/rgbhv? Do these devices just adjust the overall curve or does it allow for adjustments at different IRE's?

Also sightly off topic, but are there also devices which allow you to change RGB% at different IRE's so that you can get a completely flat response?

Yes, yes, yes. That's exactly what a Lumagen video processor does.


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post #67 of 82 Old 01-25-2007, 10:55 PM
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Yes?? Overall gamma curve adjustment or tweaking at different IRE's or both??

Are there any other processors that do this aswell? I'm looking at upgrading from the VP50 within the next few months.

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post #68 of 82 Old 01-26-2007, 12:33 AM
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Quote:
Originally Posted by Gino AUS View Post

Yes?? Overall gamma curve adjustment or tweaking at different IRE's or both??

Are there any other processors that do this aswell? I'm looking at upgrading from the VP50 within the next few months.

Lumagen has a 2, 5 or 11 point gamma "EQ" (No of points is user selectable) for EACH input. You can move the adjustment points individually to any "IRE point" you like in 0.5 increments. Each point also allows for different values for luma (average) and R,G,B separately (in 0.1 IRE increments). In conclusion; With a ~100$ colorimeter and a Lumagen vp, you can get to D65 with whatever gamma you like (and full FTB) within Delta 1 without breaking a sweat. I just love my HDQ!

// Lyckman
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post #69 of 82 Old 01-26-2007, 01:32 AM
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I do the same with my Lumagen. What I'd really like from them though (perhaps with the Radiance) is dynamic adjustment - ie you calibrate to a specific curve, say 2.2 gamma, and then the Lumagen could allow you to adjust from there with a click of the remote in 0.1 steps, eg, to 2.1, 2.0, 1.9 gamma ... this would allow you to really easily optimise the curve for your viewing environment. Right now what I do is program the A,B,C,D memories with a range of gamma curves and then flip between them until I find the best curve.

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post #70 of 82 Old 01-26-2007, 01:45 AM
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Sounds like the Lumagen HDQ is what I need... are there any others though that have this feature? How far away is the Radiance?

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post #71 of 82 Old 01-26-2007, 01:47 AM
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"would allow you to really easily optimise the curve for your viewing environment."

I thought you had a cave!

I found a display gamma of 2.5 to suit my "cave" and CRT pj best..
I agree though. A transfer function would be nice.. and if it could calculate the correct levels for any adjusted R,G,B at the new "points"... that would be even nicer..

// Lyckman
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post #72 of 82 Old 01-26-2007, 05:17 AM
 
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Quote:
Originally Posted by krasmuzik View Post

I simply can't yet explain why studio CRT-based monitors, using digital signal processing, alter their native gamma figure, which is nominally 2.5, to 2.2

Still confused - did Eric say this or did Poynton?

Kras,
That quote was from Dr. Soneira who measured a professional Sony CRT display to have a "ruler straight" gamma 2.20 all the way down to 1%.
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http://www.displaymate.com/ShootOut_Part_2.htm
The Gamma for the Sony CRT agrees perfectly with the 2.20 standard value. (CRT monitors from Ikegami, another major brand of professional studio monitors, also have a Gamma of 2.20 according to their Director of Engineering.)

Gamma' primary considerations in the creation and calibration of displays are to minimize quantization on a lossy (8 bit) channel, to linearize the display response, and to account for the change in perception of the observer to maintain rendering intent.

I think it would be helpful to consider that ideal response (2.20) for pro CRTs are a costly feature, and involve more than just pre-emphasis, much of the $35k price tag is for a laboratory grade vacuum tube design specifications.

Also the transfer function (OETF) for consumer displays are so far from the theoretical inverse of the Rec 709 encoding, that it totally irrelevant, especially near black. The performance for virtually all displays in this region is poor, the encoded content near black is marginal as well, coupled with a small calibration error on either end will make content there irrelevant.

BTW the Rec 709 "linear tail" is there to reduce coding gain so to suppress noise in the (camera) source. One would not want to emphasize (add gain) and undo something that was implemented to mask unwanted artifacts.
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post #73 of 82 Old 01-26-2007, 08:56 PM
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OK, so for what it's worth, I've come to a conclusion as to what is the "correct" gamma for my setup. Here's my setup.

1. Plasma TV.
2. Light controlled room with lights dimmed, but not completely off.
3. Using ColorHCFR with a DisplayLT.

I've managed to get as flat a greyscale as I'm going to get (all but one point with deltaE less than 2 and the outlier is still <3)

I used the "Camera Gamma" setting with a target of 2.2 as my target and manage to get a near perfect fit on the curve.

So what does the picture look like you ask?

I've tested with a sampling of DVDs that comprise both dark scenes and bright scenes and am quite pleased with the results. This is a purely subjective evaluation. There is not banding that I have noticed, shadow detail is quite good while still maintaining decent blacks and detail in the bright sections is also good.

FWIW, the DVDs I used were: Underworld, Matrix Revolutions, and The Fifth Element.

I think I am done for a while.
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post #74 of 82 Old 01-30-2007, 03:48 PM
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If you use software like Colorfacts, it uses a specific calculation for gamma, but using other software like HCFR gives you a choice of gamma and the resulting gamma can different by a couple of points depending which gamma you choose (i.e CF gives 2, HCFR with optimised regression gives 2.22), so which calculation should we really be using?

CF tells you how it calculates its gamma, but it appears different to Rec 601 or 709, and I would have thought Rec 601/709 would be the better choice since that's the standard?

Gary

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post #75 of 82 Old 01-30-2007, 04:28 PM
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Afaik, CF compute it very similarly to what we call "Display Gamma + black comp".

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post #76 of 82 Old 01-30-2007, 04:41 PM
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Hi Patrice,

I found that display gamma and display gamma + black comp both gave similar readings to CF, but other options gave higher gamma results. For those of us less knowledgeable, it can be a little confusing since we need guidance as to which calculation we should use.

If we were aiming for a gamma of 2.2 and the gamma calc choice gave us 2, by changing nothing except the method of calculation we can get 2.2, but that hasn't improved the display or helped us find a way to get a higher gamma using the other calculation.

If someone said that with Rec 709 was the standard for image reproduction for HD, then I would imagine we would calculate the gamma to that standard too, so why would we use other methods? I did look at Poyntons calculation but maths isn't my strong point. It kind of reminds me of a scene from "The Day the Earth Stood Still".

Hope that makes sense and any replies appreciated.

Gary

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post #77 of 82 Old 01-30-2007, 05:59 PM
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Gamma update.

Watching "Alien" right now with my camera gamma of 2.2 and it kicks ass!
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post #78 of 82 Old 02-02-2007, 09:58 AM
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Anyone got any thoughts which on which gamma calculation we should use?

Gary

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post #79 of 82 Old 02-02-2007, 11:24 AM
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Quote:
Originally Posted by Gary Lightfoot View Post

Anyone got any thoughts which on which gamma calculation we should use?

Gary

We recommend using the standard-compliant function since that is what has been defined as the standard; right, wrong or indifferent. The target value using that standard depends a lot upon your taste and viewing environment. No matter which function you use, it should produce a smooth curve, without any excessive irregularities. Given that all of the curves are, generally speaking, relatively smooth over the bulk of the stimulus range, this smoothness test is relatively independent of calculation methodology.

Bill

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post #80 of 82 Old 02-02-2007, 11:43 AM
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Hi Bill,

So which standard is that exactly using the HCFR options for example (assuming you're familiar with the program)?

I assume the CFPro gamma isn't considered the standard (Rec 709 will give a higher number IIRC)?

Thanks

Gary

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post #81 of 82 Old 02-02-2007, 02:23 PM
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The standard option Bill refers to is called "Camera Gamma (Std offset)" in ColorHCFR.

--Patrice

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post #82 of 82 Old 02-02-2007, 03:04 PM
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Thanks Patrice.

How does that calculation compare to one using Rec 709?

Thanks

Gary

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