Sky subtraction process for a galaxy (AGC 4130) in our parent sample as an example. The objective galaxy AGC 4130, as the edge-on galaxy in this frame, is located at the bottom-right region of every picture. The four pictures show (a) the original SDSS DR7 fpC-image in the r band for the galaxy, (b) the object-masking file, (c) the sky background map from fitting, and (d) the final sky-subtracted image, respectively.