we use gradient of
the edge to measure the degradation.
Prior to the feature extraction, the edges of the document
image are detected using the Canny operator [6]. A gradient
search algorithm is applied to each edge pixel px;y to find
the pixel such that the gray difference between two pixels is
bigger than a predefined threshold with the shortest distance.
The searching algorithm is summarized in Fig. 2
The average distance of all edge pixels within a page is
calculated according to: