What are feature of a shadow

1) The saturation component of the shadow should be low.

2) Under the Lambertian assumption, an input image I arises from a product of two intrinsic images: the reflectance image and the illumination image. Invariance to illuminant colour and intensity means that such images are free of shadows, as well, to a good degree. Since shadows are mostly due to removal of some of the lighting, such an image also has shadows removed.