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Antonio Torralba, William T. Freeman; Accidental Cameras. Revealing the scene outside the picture.. Journal of Vision 2013;13(9):1159. doi: 10.1167/13.9.1159.
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© ARVO (1962-2015); The Authors (2016-present)
There are many ways in which pictures are formed around us. The most efficient mechanisms are to use lenses or narrow apertures to focus light into a picture of what is in front. So a set of occluders (to form a pinhole camera) will let us see an image as we view a surface. For that case, an image is formed by intentionally building a camera. However, similar arrangements appear naturally by accidental arrangements of surfaces in the world. Often the observer is not aware of the faint images produced by those "accidental cameras" or they are misinterpreted as being shadows. Although pinhole cameras are the ones we are most familiar with, there are other types of arrangements that can be used to form images. An occluder blocks certain of the light rays, producing a diffuse shadow. In the cast shadow, there is more than just the silhouette of the occluder, there is also the negative image of the scene in front of the occluder. The occluder produces an anti-pinhole camera. If we were able to extract the light that is missing (that is the difference between when the object is absent from the scene and when the object is present) we would get an image. That image would be the negative of the shadow and it will correspond to the image produced by a pinhole camera with a pinhole with the shape of the occluder. Therefore, a shadow is not just a dark region around an object. A shadow is the negative picture of the environment around the object producing it. Understanding accidental cameras is important to explain illumination variations that would otherwise be incorrectly attributed to shadows. Understanding them is required for a complete understanding of the photometry of many images.
Meeting abstract presented at VSS 2013
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