I. Introduction
Any abnormality in the retina or ocular condition, such as glaucoma or diabetic retinopathy, can impair a person’s vision. Glaucoma is the second leading cause of irreversible vision loss worldwide, after cataracts [1]. Approximately 12% of all cases of blindness worldwide can be attributed to retinal abnormalities. The structure of retinal blood vessels is critical for the diagnosis of such abnormalities. The identification and localization of retinal vessels enable the differentiation of the diverse vasculature structure of the retina from the background of the fundus image. This allows clinicians to interpret potentially problematic retinal anatomical structures such as abnormal lesions, macula, and optic disc [2–4]. Even the color of the retina changes throughout life and can be used as a biomarker for a variety of diseases, including diabetes and stroke prediction [2–4].