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ICtCp

Author Steve
Scene
#1 | Posted: 3 Mar 2018 13:08 | Edited by: Steve 
We have added ICtCp as a dE measurement within LightSpace... but, our view is it is not at all good as a way to define dE.

Here's some background info on our thinking.

dE1976 and dE2000 are based on L*a*b* and are a measure of perceptual uniformity.
This is not the same as JND (Just Noticeable Difference).
From historical published work, it has been concluded that the average dE2000 that represented a JND is 2.3 (there is a ref on Wikipedia), and that is the value we use in LightSpace.

But, the real value of dE2000 for a JND is very dependent on the actual colour, as the L*a*b* is not completely perceptually uniform, just LOCALLY uniform.

However, that would be very difficult to show in a graph – hence our use of a 2.3 JND target line.

Dolby have concluded that a correctly calculated ICtCp value needs to be multiplied by 240 in order for it to have a similar magnitude to a comparable dE2000 value.
(And we do tend to think this is about right as well, but have absolutely no idea why any absolute measured value should require a multiplier to make it 'viable'...).

But, Dolby also say that a dE2000 of 3 is about a JND!
We say its 2.3, based on standard industry acceptance, but Dolby say 3?
This may be experimentally true, but we are not confident that it is in any way correct.
(But this fact has no bearing on the LS implementation – it is just a huge difference, and as we know most calibrations aim for about 1 or below dE.)
So, to target a JND, the ICtCp value needs to be multiplied by 730 (240 x 3).
As we totally disagree with a value of 3 for a JND we have not implemented this multiplier.

So, this is what we have done within LightSpace:

We have added RED lines to the dE and dE-Dist graphs.
This is dE_ICtCp multiplied by 240, to scal to be compatible with dE2000.
In this way the graphs are comparable as you look at them.
All text values of dE_ICtCp are absolute, so look way lower than dE2000.

We are still not sure if this is exactly what real users will want, and we are not sure of ICtCp's viability as a dE measurement?
We are most definitely not fans of dE_ICtCp...
It is very poorly documented, still changing, and in no way a standard.
It is way out of scale when compared to dE2000/dE1976.
Worst of all it in no way takes account of the white reference, which is an integral part of L*a*b* and so dE2000/1976.

Basically, in our view, and until it can be proved otherwise, ICtCp is not a good metric to verify calibration accuracy against.
(We are very open to input that explains we have misunderstood the technical data behind ICtCp, but nothing has been forthcoming as yet...)

Steve

Author aldombr
Mono
#2 | Posted: 5 Mar 2018 17:44 
Here is a Dolby presentation where they talk about primaries mainly:

https://www.youtube.com/watch?v=cuWUvCr40GA

Steve, what formula do yo suggest to use when calibrating, say, a Sony x300, in HDR mode?

Author Steve
Scene
#3 | Posted: 6 Mar 2018 01:33 
Yep, we've seen that video.
They really don't explain a lot in detail, especially items such as not taking account of reference white, which is obviously an issue.
And there is no real explanation as to why you should need to multiply by 240?
Why not use a system that provides valid data directly?
Overall, a lack of info, as there is within the available documentation.

Regardless, it's been added to LS, and we'll keep an eye on changes for the specification (not that it is a specification...)

But I am not sure what you mean by "what formula do you suggest to use when calibrating, say, a Sony x300, in HDR mode?"
Can you explain further.

Steve

Author aldombr
Mono
#4 | Posted: 7 Mar 2018 17:44 
What metric to use to calculate AE. Dolby presentation suggested dE2000 is not good in highly saturated areas.

Author Steve
Scene
#5 | Posted: 7 Mar 2018 17:55 | Edited by: Steve 
At the end of the day, that is down to you...
(I assume yo mean dE, not AE?)
But, we do not concur with the suggestion that ICtCp is better, as we have stated.
And we are also not sure dE2000 fails in areas of high saturation.
We wait to see that proven.

As we say, this is all open to discussion, but as yet we are not fans...

But, remember that dE measurements/values have NOTHING to do with calibration.
They are just a measurement metric used after calibration, and have no impact on the calibration accuracy at all.

Steve

Author Steve
Scene
#6 | Posted: 24 Nov 2018 11:38 | Edited by: Steve 
As a follow-up, there has been further discussion on the use of a 720 multiplier, rather than 240 (3x240), to provide ICtCp figures that are 'better matched' for calibration verification based on JNDs (Just Noticeable Differences).

We do not agree with this.

The use of a 240 multiplier is to provide a better match to dE00.
The use of 720 is to attempt to scale the ICtCp values to be equivalent to a JND value of 3.
We view that as being incorrect, as a JND is universally agreed to be 2.3.

We presently use a 240 multiplier when the ICtCp data is displayed in LightSpace's dE graphs, to aid comparison with the alternate dE values.
When the ICtCp data is shown as text we display the underlying 'raw' ICtCp value.

However, Dolby have now released a 'Dolby Vision Best Practices Guide' for Dolby Vision Certified Mastering Facilities Colorgrading Systems and Monitors.
This document states Dolby are adopting the 720 multiplier for verification of master grading monitors.
We will therefore do the same, as LightSpace needs to be compatible with Dolby's standard.

Steve

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