Primary Controls Pivot Values

[jan1]

I'm now intrigued by the less understood Pivot controls in the Primaries color tab.

From the manual it states that these controls (which exist for white point and black point) control the range of action for the track ball. That seems to make sense, similar to pivots for contrast.

However, in practice their behavior does seem to be as logical as I would assume. If you change the white point pivot, and then change the white trackball, nothing is different. Similar on the black side. However, if I change the black trackball and modify the white pivot I see some changes.

To figure out what is going on, I created the standard gradient so you can watch it in the rgb scope. From that it appears, that the white Pivot is simply added as a linear offset to the black point correction and vice versa. It doesn't really constrain the the range. I would have expected that if constrained the range that only a section of the slope would be impacted while the rest remained as is. And as a result the line should start to bend accordingly, either hard or soft (with knee soft). But that's not happening. Interestingly enough, once you have a white Pivot you can counteract it with the black Pivot at opposing values.

So I must be mis-understanding something. The manual only has a very terse description.

This is the scope of my test. You could achieve this result by simply dragging the white trackball up, and it would stretch the line to a steeper angle. Except this correction is a -11 on the black trackball, inverted by a -2 on the white pivot.

What's curious is that in this math, there doesn't seem to be any benefit to using Pivot, if you could achieve the result by modifying the opposing trackball to the same effect.

What am I missing?

 

[jan1]

Here are the controls for the above scope....

 

[Yoav Raz]

Hi Jan to my understanding 

the pivot is basically represent the black and white point. So if you change gamma and play with pivot black or white you will see the Chang. Not sure about the math. 
In the up coming version it will be more user friendly. 
cheers yoav

[agonzalez@sgo.es]

Hi Jan, 

It's exactly what Yoav says.

The pivots have two modes, depending on the Knee Soft, if its enabled or not. If it's not enable, the pivots doesn't "break" the curve into Black, Gamma and White as you probably expected, because it extrapolates the gamma adjustment affecting whites. It doesn't change the normal math of the traditional Primary correction. So instead of understanding these pivots as ranges (like in Bands), where you limit the different ranges of the trackball, here the white pivot reacts in fact as a real pivot, so the curve rotate at that point when you control the gamma, but again it extrapolate that change to the whites to keep the consistency of the curve.

When you enable the Knee Soft, the math changes. And then it reacts in a more "ranges" way, so the whites are not extrapolated from Gamma, they are a full independent control, which allows you to fix the position of the white and control the gamma in a separated way. This mode is really useful in many circumstances, but depending of the amount of softness of the Knee Soft (0 is no softness), you can "break" the curve in the white pivot, which is something that do not happen with the Knee Soft disabled. 

So basically, Primary has two modes: one with Knee Soft enabled and one with Knee Soft disabled. It affects Whites especially, and it is especially useful for HDR projects (that's the reason why we call HDR to the mode with the white point at 50 and the Knee Soft enabled). It's true that the 50 value is not a real value for PQ or HLG, but many clients have reported it works well in both scenarios.   

 

 

 

[jan1]

Hi Adrian,

Thanks for that explanation. I'll play with it again with the gamma in the mix. In Mistika 8.11 you have to switch the Color node to HDR to get kneesoft, correct? There is no other way to enable kneesoft in the current version?

[jan1]

Now I can see the difference it makes...

First scope is with gamma correction, and then white pivot applied. It brings it down, but leaves the upper range straight. Second scope is same gamma correction, but no white pivot, instead white trackball correction to extreme (-84).  This leaves the gamma curve across the whole range. White Pivot is also much more aggressive than white point.