Removing Shadows using Flash/Noflash Image Edges

13 years 10 months ago

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Flash/noﬂash pairs have been used for noise-reduction in ambient-light images. But not explicitly studied is the problem of shadows in the ambient images. While shadows are lessened in a ﬂash image, other problems arise, and other shadows are produced. It is known that we can in fact produce a ﬂash-only (no ambient) image by subtracting the two images, but the result is not as pleasant as the ambient image, because of several artifacts due to the ﬂash. Here, we use the pure-ﬂash image to detect the ambient shadows. We argue that ﬁrst going to a “spectrally sharpened” color space, and then focusing on the difference in a log domain of the ﬂash image minus the ambient image, gives a very simple feature space consisting of two components — one in an illuminant-change 3-vector direction, and one along the gray axis. This space provides excellent separation of the shadow and nonshadow areas. Inserting edges from the ﬂash image within the ambient-shadow region into the...

Mark S. Drew, Cheng Lu, Graham D. Finlayson

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