How can I use a difference matte to change the opacity of another layer?

Hi, Dan. This is what I'm doing so far:

I have 3 layers:

- Lens flare: Just a shape layer of a red circle as a placeholder for where the lens flare PNG will be.

- Base Video: Just the normal video of me talking.

- Candles: A pre-comp with a mask around the candles. The mask is named "Candles". It's a pre-comp that also contains a layer named "Reference Frame", which is a time-remapped freeze-frame of a frame where the candles aren't obstructed, and I have another layer, also named Video, which is just the normal video, but with a Difference Matte applied, with the source being the Reference Frame, so it detects the differences between the regular video and the freeze-frame.

As stated, in the Candles pre-comp, the differences between "Video" and "Reference Frame" are displayed in a black and white matte, so when my hand goes over the candles, the ordinarily, mostly black candles, are obscured by my white hand.

Right now, I'm using an expression to sample the candles, so that the more white the area is (meaning that there is more obstruction of the candles), the lower the opacity of the Lens Flare layer drops.

Here is the expression:

// Variables - Customize as needed

// Layer and mask settings
candleLayerName = "Candles";   // Name of the candle layer
maskName = "Candles";          // Name of the mask on the candle layer

// Slider controls from the Lens Flare layer
onThreshold  = effect("On Threshold")("Slider");   // Lower brightness bound for full opacity
offThreshold = effect("Off Threshold")("Slider");  // Upper brightness bound for zero opacity
maxOpacity   = effect("Max Opacity")("Slider");    // Maximum opacity value

// Calculate the mask's bounding box for sampling

candleLayer = thisComp.layer(candleLayerName);
mask = candleLayer.mask(maskName);
maskVertices = mask.maskPath.points();

xMin = yMin = Number.MAX_VALUE;
xMax = yMax = Number.MIN_VALUE;

for (var i = 0; i < maskVertices.length; i++) {
  var vertex = maskVertices[i];
  xMin = Math.min(xMin, vertex[0]);
  yMin = Math.min(yMin, vertex[1]);
  xMax = Math.max(xMax, vertex[0]);
  yMax = Math.max(yMax, vertex[1]);
}

sampleWidth = xMax - xMin;
sampleHeight = yMax - yMin;
sampleSize = [sampleWidth, sampleHeight];
samplePoint = [xMin + sampleWidth/2, yMin + sampleHeight/2];

// Sample brightness within the mask area

hsl = rgbToHsl(candleLayer.sampleImage(samplePoint, sampleSize, true, time));
brightness = hsl[2] * 100;  // brightness from 0 to 100

// Determine opacity based on brightness thresholds

// Initialize opacity level
opacityLevel = 0;

// If brightness is below the On Threshold, use max opacity
if (brightness <= onThreshold) {
  opacityLevel = maxOpacity;
} 
// If brightness is above the Off Threshold, set opacity to 0
else if (brightness >= offThreshold) {
  opacityLevel = 0;
} 
// Otherwise, interpolate between thresholds
else {
  // Map brightness from [onThreshold, offThreshold] to [maxOpacity, 0]
  var range = offThreshold - onThreshold;
  var positionInRange = brightness - onThreshold;
  var percentage = positionInRange / range;
  opacityLevel = maxOpacity * (1 - percentage);
}

opacityLevel;

Thanks so much! However, I went into labor when I saw the price of the license for the software.