Automatic and accurate shadow detection from (potentially) a single image using near-infrared information

Shadows, due to their prevalence in natural images, are a long studied phenomenon in digital photography and computer vision. Indeed, their presence can be a hindrance for a number of algorithms; accurate detection (and sometimes subsequent removal) of shadows in images is thus of paramount importance. In this paper, we present a method to detect shadows in a fast and accurate manner. To do so, we employ the inherent sensitivity of digital camera sensors to the near-infrared (NIR) part of the spectrum. We start by observing that commonly encountered light sources have very distinct spectra in the NIR, and propose that ratios of the colour channels (red, green and blue) to the NIR image gives valuable information about impinging illumination. In addition, we assume that shadows are contained in the darker parts of an image for both visible and NIR. This latter assumption is corroborated by the fact that a number of colorants are transparent to the NIR, thus making parts of the image that are dark in both the visible and NIR prime shadow candidates. These hypotheses allow for fast, accurate shadow detection in real, complex, scenes, including soft and occlusion shadows. We demonstrate that the process is reliable enough to be performed in-camera on still mosaicked images by simulating a modified colour filter array (CFA) that can simultaneously capture NIR and visible images. Finally, we show that our binary shadow maps can be the input of a matting algorithm to improve their precision in a fully automatic manner.