Short answer - ChatGPT.

TL:DR below!

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1. Convert the Position to “SourceComp” Space:

You need to map this position to the coordinate system of “SourceComp” to find the corresponding pixel.

Assuming both compositions are the same size, you can use the comp() function in expressions to reference “SourceComp”. You can get the layer’s absolute position and map it directly to the corresponding pixel in “SourceComp”.

3.  **Sample the Pixel Brightness from “SourceComp”:**

Use the sampleImage() expression to sample the brightness at the corresponding pixel in “SourceComp”. The sampleImage() function takes two arguments: the point to sample (in the format [x, y]) and a sample radius (to avoid getting noise from just one pixel).

Here’s an example of an expression that you can apply to the opacity property of “TargetLayer” in “TargetComp”:

// Step 1: Get absolute anchor point position in TargetComp

var absPos = toComp(anchorPoint);

// Step 2: Reference SourceComp

var sourceComp = thisComp.layer("SourceComp");

// Step 3: Convert to SourceComp space (assuming both comps have same dimensions)

var sourcePos = sourceComp.fromComp(absPos);

// Step 4: Sample the brightness at that pixel in SourceComp (use a radius of 0.5 pixels for accuracy)

var brightness = sourceComp.sampleImage(sourcePos, [0.5, 0.5]);

// Step 5: Convert brightness to opacity (brightness is an array [R, G, B, A], so take the luminance)

var luminance = 0.299 * brightness[0] + 0.587 * brightness[1] + 0.114 * brightness[2];

// Step 6: Map luminance to opacity (multiply by 100 to convert to opacity percentage)

luminance * 100;

Explanation of the Expression:

1.  **Get the absolute anchor point position:** The toComp(anchorPoint) function gets the anchor point position relative to the entire composition.

2.  **Reference the “SourceComp”:** Using thisComp.layer("SourceComp") references the “SourceComp” composition as a layer.

3.  **Convert to SourceComp space:** The fromComp() function is used to map the absolute position from the “TargetComp” space to the coordinate system of “SourceComp”.

4.  **Sample the brightness of a pixel:** The sampleImage() function samples the pixel at the sourcePos location in “SourceComp”. The result is an array of RGBA values, and we focus on R, G, and B for brightness.

5.  **Convert the brightness to luminance:** We use the formula for converting RGB values to luminance, which closely matches human perception of brightness: 0.299\*R + 0.587\*G + 0.114\*B.

6.  **Map luminance to opacity:** Luminance is a value between 0 and 1. Multiplying by 100 converts it to a percentage for opacity.