how to get a number value that reflects the difference (screen mode) between 2 video layers

initial code was like this:

// -- SETTINGS —

var threshold = 0.95;     // how “white” must a pixel be? (0–1)

var sampleRadius = [thisLayer.width/2, thisLayer.height/2]; 

// ———------

// Sample the entire layer

var avgColor = thisLayer.sampleImage(

    thisLayer.sourceRectAtTime(time, false).center,

    sampleRadius,

    true,

    time

);

// Convert the average RGB to luminance (perceptual brightness)

var luminance = 0.2126*avgColor[0] + 0.7152*avgColor[1] + 0.0722*avgColor[2];

// Estimate % white based on luminance relative to threshold

var whiteness = clamp(luminance / threshold, 0, 1);

// Return percentage

whiteness * 100;

but it gave the error that sampleimage's first 2 parameters must be numbers..

so i googled some more and ended up with the form in the post above,

as you can see i am fumbling in the dark blindfolded.

asked chat gpt for some help, i got something like this:

// -- SETTINGS —
var threshold = 0.95; // how “white” must a pixel be? (0–1)
// not used: var sampleRadius = [thisLayer.width/2, thisLayer.height/2];
// ———------

// Sample the entire layer
var avgColor = thisLayer.sampleImage(position, radius = [.5, .5], postEffect = true, t = time)[0];

// Convert the average RGB to luminance (perceptual brightness)
var luminance = 0.2126*avgColor[0] + 0.7152*avgColor[1] + 0.0722*avgColor[2];

// Estimate % white based on luminance relative to threshold
var whiteness = clamp(luminance / threshold, 0, 1);

// Return percentage
whiteness * 100;

but at this last line it says "couldn't turn result into a numeric value" 

luminance looks like a number to me, whiteness looks like a number as well..

omg.

i found this other post here

https://community.adobe.com/t5/after-effects-discussions/how-can-i-get-high-and-low-value-of-a-histogram/m-p/6802660 

there's a solution there and i don't even know what to do with it, overly complex.

was really hoping for an expresison that just gets me a number 😞 

tried googling "how much of a layer is white in %" and to not much avail.

from what i can see in that link it gets the average color in a certain area.

i need a hard number.

currently googling for ways to count in % how much of a layer is of a certain color (white) - so i can work on the distortion layer directly, no more complicated operations between 2 different layers.

i hope.

I take it the glitchy effect doesn't include keyframes?

I think the simplest approach would be to use sample image().  This can return colour and alpha values from a layer at a specific point.  So if you glitch always makes part of the screen white, you should be able to get a value from that.

As always Dan Ebberts has a good write up: https://www.motionscript.com/design-guide/sample-image.html