the opening-by-reconstruction (f_obr) is shown in fig. 9.30(b) for comparison. fig.9.30(c) shows the standard opening (f_o). opening-by-reconstruction did a better job of extracting the background between horizontally adjacent keys. subtracting the opening-by-reconstruction fron the original image is called tophat-by-reconstruction, and is shown in fig. 9.30(d):