These photographs appear to show a very large cylindrical object recently installed in Victoria Square, Hull.
The shape and size of the object is so completely out of keeping with the rest of the scene that you may question whether it was actually present at all. At first sight the object appears to have been digitally superimposed on a photographic image of Victoria Square. But the photographs are real and not re-touched; the object really was installed in the square. So why does it seem unreal? Is it because the object is so large and smooth relative to the buildings and other objects in the scene that it does not seem to be part of the scene? This must be part of the explanation but I think there is also something else going on, which relates to the unconscious assumptions that our brain makes when we try to make sense of visual scenes.
The views of the blade in the photographs above show the right-hand side as drawn in the diagram. The blade's trailing edge is at the top pointing up to the sky, so its surface is slightly concave just below the top edge. Consequently the top third of the blade is in the shadow of the daylight overhead and therefore darker, as is the bottom third (which faces the ground). The middle section of the blade appears lighter because it faces slightly upward as shown in the diagram. This symmetrical sequence of shading across the blade (dark-light-dark) is commonly seen in views of cylinders when they are lit from the side facing the viewer, so we tend to interpret the shape of the blade in this way. However we also unconsciously infer that the rest of the scene is lit by daylight from above. The apparent inconsistency in lighting between the side-lit object and the top-lit scene contributes to the false impression that the object was not present in the scene; that the photograph is a composite of two different scenes. With some conscious effort you may be able to re-interpret the images of the blade in the way I describe, so that it fits into the scene more convincingly. Many more images have appeared on the web (Twitter: #Blade) and in the print media. The illusion can be experienced while viewing the blade at the site as well as in photographic images, at least if one eye is closed to remove shape cues based on stereo vision.
The photograph below reveals the true shape of the righthand side of the turbine blade (circular at the base but flattened and wing-shaped along its length).
The blade was made by turbine manufacturer Siemens and placed placed in Victoria Square as an art installation to mark the start of Hull’s year as the UK City of Culture 2017. As Hull City of Culture (https://www.hull2017.co.uk/whatson/events/blade/) states:
"Conceived by artist Nayan Kulkarni, Blade has been created for Look Up, a programme of temporary artworks created for the city’s public spaces and places.
It uses one of the first B75 rotor blades made in Hull and changes its status to that of a readymade artwork. At 75 metres it is the world’s largest, handmade fibreglass component – cast as a single element."
It will remain there until the end of March, so if you are in the area it is well worth visit.
Several scientific studies have also demonstrated that our perception of objects and scenes relies on hidden assumptions about shape and light source direction in real-world images. See, for example:
Langer, M. S., & Bülthoff, H. H. (2001). A prior for global convexity in local shape-from-shading. Perception, 30(4), 403-410.