Drawn on a piece of paper, the spokesman said it was "full of the pop art motifs we have now come to recognise, including Warhol's signature bright red lips, made using lipstick, and a typically pop art blocked background, coloured with green and orange felt-tip pen".

BBC: Warhol sketch

Like pre-existing blue-screen techniques it involves filming a scene against an aquamarine blue-coloured background.

BBC: Blue and green-screen effects pioneer Petro Vlahos dies

Grids of multi-coloured letters and numbers illuminate the background, itself computer-enhanced to resemble a pitch-black sky.

ECONOMIST: New German photography

The tunnel is brightly coloured to optimise the contrast between the subject's clothing and background, making it easier for the researchers to capture a 3D image as they walk.

BBC: Biometrics cybersecurity research gets government cash