Thanks Aharon, but will that really look the same? I was thinking 3D Fog does something more along the lines of a Blend crossfade. If I use a Luma Matte won’t I get the same sort of additive transparency issues you get if you try to do a crossfade with transparency instead of the Blend effect.
Perhaps I could use Blend and sample the value of the greyscale ZDepth layer with an expression to control the degree of the Blend.
John