Fig. 3.

Effect of the background on the centroid calculation. The first row shows a simulated photoreceptor at three different contrasts compared to its background: (a) high contrast, (b) intermediate contrast, and (c) low contrast. The calibration factor was calculated by shifting a small rectangular area [i.e., a region of interest (ROI)] one pixel at a time in the vertical direction. The second row shows shifting of x-coordinates of the centroid when ROI shifts in the vertical direction. The calibration factor for the high-contrast image (a) was 0.50, the calibration factor for the intermediate-contrast image (b) was 0.14, and the calibration factor for the low-contrast image (c) was 0.05.