Click in the image to see the difference before lighting has been applied. Click again to return.

This illusion is unusual in that, unlike the others, it depends less on geometry than light. The contrast of warm lighting to define the underside of the stair and cool lighting to define the top is essential. Click in the image to see the unlit raw composite before any light and shade have been applied. You will see immediately how the illusion depends completely upon the lighting. Click again  to return to the full lighting. 

The room is constructed photographically from about sixty photographs used to texture and accessorise a pre-drawn geometrical construction. The people were later posed and photographed to fit their eventual positions.

This one of my favourite illusions because it so simple and effective.