Calculating overlying opacities…

[Xavier Gomez]

If you have two layers “TOP” and “BOT”, and set
top = layer(“TOP”).transform.opacity/100;
bot = layer(“BOT”).transform.opacity/100;

then the resulting opacity is: total = 100*(top+bot-top*bot)

So you have to “solve” total = layer(“B”).transform.opacity;
which does not always have a solution, because opacity is clamped to the range [0,100].

If you leave the top layer opacity free and set the expression in the bottom layer, it would be:

top = layer("TOP").transform.opacity/100;
other = layer("B").transform.opacity/100;
top == 1 ? value : 100*(other-top)/(1-top);

(when the opacity of the top layer is 100, the bottom layer is completely hidden – unless its shape/scale has changed).

Else if you want to control the bottom opacity, the expression for the top layer would be:
bot = layer("BOT").transform.opacity/100;
other = layer("B").transform.opacity/100;
bot == 1 ? value : 100*(other-bot)/(1-bot);

Xavier.

[Mike Abbott]

I was thinking I would need some ‘curve based’ function.

As the opacity slider value varies so does the relationship between the two layers. That’s the mathematical relationship I’m trying to get my head around.

I suspect the curve will tail off – so it will need to be clamped at either extreme…

Mike

https://mikeabbott.info
https://vantagegraphics.co.uk

[Dan Ebberts]

I think it would be like this:

s = clamp(thisComp.layer(“control”).effect(“Slider Control”)(“Slider”),0,100)/100;
(1-(1-s)*(1-s))*100

Dan

[Mike Abbott]

Dan,
Many thanks for your contribution – as always.

Your suggestion works OK for the extremes – but doesn’t elsewhere. I’m trying to maintain a matching opacity between the the stack of two ‘A’ shapes and the single ‘B’ shape *throughout* the opacity range from 0 to 100%.

https://mikeabbott.info
https://vantagegraphics.co.uk

[Xavier Gomez]

I had missed the slider thing, didnt see.
You have 3 opacities and just one slider, so it’s not clear what the slider is expected to do.

These 2 expressions assume that the slider value (range 0-100) gives the percentage opacity layer “A1” gets, and the second one adjusts the opacity for layer “A2”

A1:

B = thisComp.layer("B").transform.opacity;
pc = clamp(thisComp.layer("CONTROLLER").effect("A1 percent")("Slider"), 0, 100)/100;
B*pc;

A2:

B = thisComp.layer("B").transform.opacity/100;
pc = clamp(thisComp.layer("CONTROLLER").effect("A1 percent")("Slider"), 0, 100)/100;
A1 = B*pc;
A1 == 1 ? 0 : 100*(B-A1)/(1-A1);

Xavier

[Dan Ebberts]

It works for me. What issues are you seeing?

Dan

[Mike Abbott]

Dan,
Apologies for the delay in response – but I just wanted to say thanks for your help on this – yes, your solution did work, I was getting tripped up by some other complexities in the project in which I was using it.
It’s all sorted now – and I very much appreciate your help.

Mike

https://mikeabbott.info
https://vantagegraphics.co.uk