Square with hole -- illumination gradient.

(wdg3): The original image.
(wdg3hst1): Intensity histogram of wdg3.
(wdg3thr1): Result of thresholding wdg3 at a pixel value of 80.
(wdg3thr2): Result of thresholding wdg3 at a pixel value of 120.
(wdg3adp1): Result of adaptive thresholding wdg3 using the mean-C as a local threshold, where C = 4 and the neighborhood size is 7×7.
(wdg3adp2): Result of adaptive thresholding wdg3 using the mean-C as a local threshold, where C = 8 and the neighborhood size is 140×140.