Fig. 2.

The intensity-based (IB) algorithm for the automatic extraction of retinal surface vessels is based on the approach of Saleh and Eswaran.²⁶ In this approach, the RGB image is first converted into grayscale, then a morphological opening operator is applied to the grayscale image to emphasize the smaller retinal vessels. This image is processed in parallel to the original grayscale image through a series of steps designed to increase contrast and reduce the effects of uneven illumination and/or background noise. The outputs of the filtering stage are postprocessed and eventually combined, and a threshold applied before the image pixels are labeled vessel or background (see text for details).