In this study, we found that variations in image brightness, clarity, and contrast all affected the ability of IRIS-Fractal to accurately segment a digitized image of the retinal vasculature and, thereby, the measurements of
D f produced by IRIS-Fractal.
D f assessment was thus also sensitive to these image variations. Effect sizes for most of these image variations (contrast expansion, small decrease in image brightness) were low to moderate (Cohen
d, 0.21–0.46). Increased blur and decreased brightness both impaired the ability of IRIS-Fractal to accurately segment a retinal image, resulting in spuriously low
D f. However, these reductions in measured
D f were generally of small magnitude, except for the lowest range of pixel brightness values and greatest amounts of blur (
Table 1). Conversely, an increase in image brightness was not associated with any significant change in the reported
D f measures, nor was a small amount of image blurring. Moderately improving contrast improved detection of vessels and measurement of
D f by a small but significant amount (
D f difference, 0.0028;
P = 0.005,
d = 0.21), considering that the images studied were all reasonable quality to begin with. The increase in measured
D f seen after contrast expansion is not due to the change in brightness of the images, as the median pixel value of the contrast expansion set, 137.3, is within the range of that produced by the brightness increases that we examined and which were found to have no effect. Similarly, the effect on
D f caused by the blurring of an image is not related to any change in brightness, as each image-blur set had a median pixel value similar that of to the others and also to the reference set.