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Image pre-processing techniques; Block (A): Original image (Grade 0: no DR, Grade 1: mild NPDR, Grade 2: moderate NPDR, Grade 3: severe NPDR, and Grade 4: PDR), Block (B): Artifacts removal (i. Otsu Thresholding, ii. Contours Detection, and iii. ROI Extraction), Block (C): Noise removal, Block (D): Image enhancement (CLAHE in the Y channel), and Block (E): Final output image.
