We have added ITP as a dE measurement... 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 ITP 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 ITP 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 ITP multiplied by 240, to scale to be compatible with dE2000.

In this way the graphs are comparable as you look at them.

We are still not sure if this is exactly what real users will want, and we are not sure of ITP's viability as a dE measurement?

We are most definitely not fans of dE ITP...

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.

Additionally, ITP is fixed to a 10,000 nits scale, so displays with lower peak brightness will not report a visually accurate representation of the way the HVS sees any colour or brightness errors on a given display.

dE00 and dE76 correctly use the measured peak brightness as part of their dE calculation, so provide a better match the the way to HVS works as the available contrast (peak brightness of a given display) varies.

Basically, in our view, and until it can be proved otherwise, ITP 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 ITP, but nothing has been forthcoming as yet...)

Steve

Steve Shaw

Mob Boss at Light Illusion