Automatic and Accurate Shadow Detection using Near-Infrared Information

We present a method to automatically detect shadows in a fast and accurate manner by employing the inherent sensitivity of digital camera sensors to the near-infrared (NIR) part of the spectrum. Shadows are generally found in the dark parts of an image, be it color or NIR. We consider these dark pixels as shadow candidates. By observing that commonly encountered light sources have very distinct spectra in the NIR, we propose that the ratios of the color channels (red, green and blue) to the NIR give valuable information about impinging illumination, which we employ to assess the shadow candidate pixels.