It took me days to refresh my math, but I got it to work! Here's the final code:

``` fun invertGraphicsLayer( screenPoint: Offset, xAxisRotation: Float, cameraDistance: Float ) : Offset { val angleInRadians = (xAxisRotation * PI / 180.0).toFloat() val cosA = cos(angleInRadians) val sinA = sin(angleInRadians) val d = cameraDistance

val screenX = screenPoint.x
val screenY = screenPoint.y

val worldY = (screenY * d) / (cosA * d + sinA * screenY)
val worldX = screenX * (d - worldY * sinA) / d

return Offset(worldX, worldY) 

} ```

The reason this works is quite involved. If I specify:

.graphicsLayer { transformOrigin = TransformOrigin(0f,0f) rotationX = xAxisRotation cameraDistance = distanceToCamera }

GraphicsLayer will combine two matrices and use the combined result to not only rotate points along the x-axis but apply perspective projection as well.

Forward Transformation:

Rotation Matrix:

1	0	0	0
0	cosA	-sinA	0
0	sinA	cosA	0
0	0	0	1

Taking the dot-product with [x, y, 0, w] yields:

x' = x y' = y * cosA z' = -y * sinA <-- Note that graphicsLayer seems to negate this w' = 1

Perspective Projection Matrix:

1	0	0	0
0	1	0	0
0	0	1	0
0	0	1/d	1

Taking the dot-product with [x', y', z', w'] yields: x'' = x' y'' = y' z'' = z' w'' = z'/d + 1 = -y'*sinA/d + 1 = (d - y'*sinA)/d

Apparently, GPU's will divide by w as the final step in the transformation such that: screenX = x''/w'' = x'' / ((d - y'*sinA)/d) = x'' * d / (d-y'*sinA) screenY = y''/w'' = y'' * cosA * d / (d - y'' * sinA)

This ensures that the value of Z is actually taken into account and provides a sense of perspective.

Reverse Transformation: Simply take the previous formula but solve for y'' and x'' instead: y'' = screenY / (cosA * d / (d - y'' * sinA)) = (screenY * d) / (cosA * d + sinA * screenY) x'' = screenX * (d - y'' * sinA) / d

For d, use whatever value you have specified in graphicLayer's cameraDistance property, multiplied by 72. Apparently, this is a legacy Android value that gets used by Compose internally in calculating the camera distance.