After a collaborative chin-wag, we design and install a number of 2D/3D ‘floating’ decals. We are using the basic ‘drop shadow’ technique but in real-time. By cutting round and removing the head /body from the card, we are left with a template stencil to lay down the ‘shadow’ in grey spray paint. Once dry, we then lay the head/body down over it, ensuring the shadow matches the lghting on the picture which should not cover the shadow entirely (see pic).
They do look freaky, especially when placed in areas with limited spatial range.