Removing Shadows using Flash/Noflash Image Edges

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 ambient image edge map and inverting Poisson's equation fills in the shadow. In this way, we arrive at an image with the advantages of the ambient-only image-warmth, no flash effects such as disturbing illumination dropoff with distance, pixel saturation etc.-but no shadows