In the example I showed, transparency, controlled by an alpha mask, is critical in defining the shape of the object: it defines its edges. FrankT's cubes are 'solid' in the Z-depth render because the Z-depth map defines their relative positions.

In this example, quickly cobbled together in Vue, there's a figure with transmapped hair (aargh! sorry April, should have used one of yours!), and two cubes in the background, one solid, one glass.

Everything in the Z-depth render is as it should be: the alpha mask for the hair has been recognised; the cubes have the correct tones to record their positions in the Z-plane.

Using the Z-depth render to create depth of focus effects - or any of the Position channels for other effects - is highly compromised if alpha masks are ignored.

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Windows 10 x64 Pro - Intel Xeon E5450 @ 3.00GHz (x2)

PoserPro 11 - Units: Metres

Adobe CC 2017