Median-rational hybrid filters for image restoration

image (very few research reports about tracing eye-fundus blood vessels have appeared in the literature). Fig. 4 is the result of this tracing. Three blood vessels were tested, of which the trace starting points are labelled as A, B and C. When Figs. 3 and 4 are compared, the traced lines look fairly good. The crosses near D and G were processed :juccessfully, but the branches near E and F were not detected. Other branches were successfully detected. By checking the Gaussian filter output, we find that zero-crosses were observed at those branches near E and F, implying that those branches were somehow cut by the filter (probably owing to noise), suggesting thai zero-cross data only is not enough for boundary information.