I have objects on a desk, a have a man behind the desk, I have created a walk behind area so the the man does not appear to stand on the desk but behind it, my problem is that the objects also obey the walkbehind area and have now disappeared from my view.
Objects do not need a walkbehind area; they already cover everything "behind them" (meaning higher than their bottom-most pixel). Try deleting the walkbehind area and see if your setup works (it should). You can also set an objects baseline manually or at runtime.
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This will only work if your desk IS an object, and not part of the background scene.
If your desk isn't an object, set the baselines of the objects on top of the desk to a higher value than the baseline of the walkbehind.
Say the walkable area around the desk has a hole spanning across y-coordinates 130-140. Set the walkbehind's baseline to 133 and the objects' baseline to 136.