Because image pixels are the consequence of the discrete repre-
sentation of images and are not natural entities, therefore,whenwe
estimate the illuminant, partial image regions always is unable to
produce a robust estimate of the illuminant [33]. By segmenting
image into the pixel groups with gray and texture constancy, the
superpixel can be formed. Superpixels partition an image into
regions which are approximately uniform in size and shape. Super-
pixels are becoming increasingly popular in many computer vision
applications [34]. The advantage of superpixels is analyzed and
shown in applications, such as object recognition [35] and segmen-
tation [36]. Fig. 1 shows superpixel segmentation, in which the
image is divided into superpixels and each superpixel shows the
same visual appearance, which can cause substantial speed-up of
subsequent processing. Therefore, the careful choice of the super-
pixel method and its parameters for the particular application are
crucial. We use TurboPixels [37] to extract superpixels from an
image, in which one superpixel is roughly uniform in texture and
gray, so that the boundaries of regions are preserved. In order to
encode gray, texture and spatial information into superpixels, we
describe each superpixel j by a 7-dimensional wavelet feature vec-
tor Fj =(f1, f2, ... , f7), in which Fj is the average wavelet value of all