How power law gamma calibration can lead to crushed blacks - Page 2

Sorry, I thought that after I'd asked.

That is assuming the mastering studio is using those monitors. When I talked to the master people at Sony, they are mastering to a 2.2 spec. I've heard some studios are using 2.4. You also have to take into account that gamma is dependent on how bright you are at 100 IRE, which you don't know what their reference was, and you have to take into account ambient, which you may not know.

Gamma sucks, always has always will. Nothing more frustrating if you're trying to be "absolutely accurate".

The trick is that unless you're including ambient light, display max and display min in you gamma formula you really don't have a chance or matching perceptually.

Granted if those variables aren't part of the formula, you can simply replicate them.

Theatre projection. Transfers to DVD and BD are very much video world rather than DCI.

For some more industry input I recommend asking the people on either the cml-post or TKcolorist mailing lists, both of which have been host to discussions about gamma and reference monitoring over the years.

At cinematography.net, join the cml-post list
At tig.colorist.org, join the tig mailing list

I think those are the places where you'll find most colourists and chief engineer types on the net.

It's hard to get people to stick their necks out and say "this is what we should be calibrating to", but it might be possible to find out what displays people are preferring to replace the Sony BVMs.

Manufacturers have a lot of sway here too - it would be interesting to see what transfer functions are used out of the box on the newer Sony, Dolby, JVC, Barco, Panasonic, Cinetal et al reference monitors - the problem is that there are so many to choose from now!

Best,
L

Sony's LCD monitors were 2.2 out of the box, simply due to the fact that they don't have the contrast ratio required for anything more. In fact, if you use the standard BT.1886 with an 0.1cd/m² Lb value, it comes out very close to 2.2 gamma.

Standard monitor brightness is 100cd/m² almost universally now. (traditionally it has ranged from around 80–120cd/m²)

And IRE is no longer valid, that's an old analogue term.


Well I can tell you that Sony's OLED monitors are using a flat 2.40 out of the box, and Barco's monitors use a flat 2.35 out of the box. (based on the older EBU recommendation)



Things are moving in that direction. However, the fact remains that regardless of how a film was mastered, for the best viewing experience at home, you should follow the standard BT.1886 formula, as it takes your display's capabilities into account. Running a flat 2.40 gamma on most flat panels is a good way to ruin the image for example, as they just don't have the contrast ratio required.

Until there is a widely adopted standard, you could debate this point endlessly. The fact is that very few displays out there are capable of emulating a broadcast CRT—they just don't have the contrast ratio for it, to start with. Compensating for that using the alternate BT.1886 function may match the appearance of a CRT with its brightness raised to that level, but the fact is that those CRTs are not used in that way.

As we move forward, and more CRTs are replaced, there is going to be far less of a need to try and emulate them. Even with gamma curves that emulate a CRT response, the fact is that a CRT is a more "dynamic" display, and the response changes with the image being displayed on it, to some degree.

when is the new/next standard for gamma going to come out and what will it look like versus the current power formula? what will it be called?

will CalMANv5 support the new formula? is there a reason 2.35 is currently being used for the power formula versus 2.2 or 2.4 or something else from 2.2-2.6?

Those are... interesting results for the alternate CRT-matching function. Are you sure they are correct?

I've also been plotting various transfer functions for comparison, but have been having trouble with that one. If I'm reading things correctly, b should be manually adjusted until L for V=0.0183 is equal to measurements taken from the reference display? What data did you use for the reference display?

Does the alternate CRT matching function work when Lw is different? (e.g. when you have your screen brightness at 120cd/m² rather than the 100cd/m² reference level)

I'm also confused because the paragraph preceding the alternate transfer function makes mention of Lb, but it is not actually referenced in the function itself. (it is in the reference electro-optical transfer function, however)

Yes BT.1886 is in CalMAN 5.

I had similar issues in reading the alt-EOTF function. Since effectively no one is using BT.1886 yet, I can't imagine the alt function is going to be used anywhere in the real world.

Due to very similiar concerns CalMAN 5 won't have Alt-1886 turned on at least initially.

It seems to me like the Reference 1886 transfer function is what is going to be adopted going forwards. The "alt-1886" function seems to be for edge-cases where someone needs to match a specific monitor in their grading suite for a very specific reason. (perhaps they were in the middle of a project and needed to get a new monitor due to hardware troubles?)

While I understand that someone may wish to try and emulate a CRT, it seems unlikely to happen moving forward, and is more for legacy support while the industry is making a transition to newer monitors.

Especially for end-users in the home, the reference transfer function seems ideal, when it also compensates for display contrast capabilities. I think it's a far better solution than we've ever had, where it's often been up to the calibrator's preferences when dealing with ambient light, and so on.

But will your perception of it be altered?

The big issue here is that the x,y,Y (XYZ) colorspace doesn't tell you anything about how saturated you will perceive the color. The perception of color is relative to the viewing environment. Even L*a*b* is only so good about compensating for environment.

If anything BT.1886 doesn't go far enough in environmental compensation.

Most displays should be capable of a relatively flat 2.4 gamma down to 5% or so. The problem is that when you do this, your only option is to clip all levels below that, which is unacceptable.

Your only options then, are to use a non-linear gamma that ramps up considerably near black, to use a lower gamma, or to use a compensated gamma like the BT.1886 transfer function.

While it may be less accurate, perceptually, the compensated gamma tends to look better.

If you want to be accurate to the mastering process, you need a display of equal or greater quality to the monitor being used, which doesn't yet exist for the home market.

My local-dimming full array LED backlit LCD does a pretty good job with a flat 2.4 gamma. Unlike many displays, the dimming algorithm is not used to boost contrast as high as possible, but rather it is designed to extend the dynamic range of the display. A 10%stim box will remain the same brightness regardless of APL, for example, unlike other displays where the brightness of that box would change with APL to maximise image contrast at the cost of accuracy. (even some Plasmas suffer from that sort of thing) It isn't perfect as blooming comes into play at times... but the same thing happened with CRTs.

I believe it absolutely is.

Crushed black are one of the most obvious errors, it's something even people are even causally acquainted with calibration readily notice.

Couple more hours for us left coasters.

For the math challenged among us, are the numbers in this table accurate? I notice that the 25% stimulus values are equal at 4.31.

Maybe if I understood the formula for calculation of the target Y values, I could create a spreadsheet to generate them. I guess that's what you did.

Also, I have a 9 year old Sony XBR HD (CRT) that I have not done any calibration of since it has little in the way of adjustments and is in the guest bedroom. But I could set it up with a pluge and get a set of calibration measurements to see how it measures up. See what the gamma curve looks like and use the Y values to compare to your suggested (calculated) empirical Y values above.

It's not a BVM of course but it is a Sony CRT ...

Thanks, Zoyd. Given that I have the same TV as you, you have tweaked yours for better blacks, and you didn't like the look if these settings, it appears you have saved me a lot if wasted time. So thanks TWICE.

When CalMAN 5 comes out, will it make sense to calibrate to this new standard or is it really there for future use? Does all current program material like HDTV programming and BD movies use power law gamma?