Why did the blade in Hull's Victoria Square look so strange?

These images appear to be unconvincing digital superimpositions of a large cylindrical object on photographs of Victoria Square in Hull. 

I visited the square and saw the object for myself; the photographs are real and not re-touched, the object really was installed in the square. So why does it seem to be superimposed in these photographs? Perhaps it is because the object is so large and smooth compared to the buildings and other objects in the scene that it does not seem to fit. This may 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.

Although the object in the photographs appears to be shaped like a cylinder, it is actually a relatively flat, 75-metre long blade from a wind turbine. At its lower edge as seen in the photographs the blade has a rounded, convex profile, but the upper edge is much sharper and turns over slightly toward the viewer to create a slightly concave shape near the top. As a result, the top third of the blade is in the shadow of the daylight overhead and therefore relatively dark, as is the bottom third (which faces the ground). The middle section of the blade appears light because it faces slightly upward. This kind of shading profile is normally seen on cylindrical-shaped objects that are lit from the side, so we tend to perceive the shape of the blade in this way. 

Of course the rest of the street scene is lit by natural daylight from above, not from the side.  So the apparent inconsistency in lighting between the blade (side-lit) and the rest of the scene (top-lit) makes it appear completely out of place. It creates the false impression that the blade was not present in the scene; that the photograph is a composite of two different scenes. Many more images have appeared on the web  and in the print media. Whenever they show the same side of the blade as in the photographs above, there is a tendency to misinterpret its shape and so interpret the lighting as inconsistent. The illusion could even be experienced while viewing the blade at the site, at least if one eye was closed to remove shape cues based on stereo vision.

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."

The Blade was removed from Victoria Square in March at the end of its exhibition period, and Siemens has yet to decide where its permanent home will be.

A more detailed discussion of this interpretation of the Blade is now published in the journal i-Perception.