Comparing ObjectPosition with SceneDepth to determine occlusion of Billboard

I’m attempting to create a corona billboard (Unlit / Additive / no depth test) for a light actor that compares the ObjectPosition with the SceneDepth. If the ObjectPosition is further away than SceneDepth, set the billboard’s Opacity to zero. If it’s closer, set it to one.

It works great when the billboard is roughly in the middle half of the screen, correctly occluding (being made transparent) if objects pass in front of it. But as it approaches the edges of the screen it becomes occluded even though it should be visible.

I’m guessing I may be converting the ObjectPosition to incorrect UV coordinates for the SceneDepth test?

Found the solution: