A ruler detection method for auto-adjusting scales of shoeprint images

Digital shoeprint comparison often requires the calibration of the image resolution so that features, such as patterns in shoeprints, can be compared on the same scale. To enable scaling, a shoeprint photograph can be taken with a forensic ruler in the same frame to obtain the pixel distance between two nearby graduations. However, manually measuring the number of pixels is a time-consuming process. Additionally, the measurement process might not be conducted accurately when the image is noisy or there is distortion in the ruler. In this study, we present an automated ruler detection method for adjusting the image scale. We show that this method can accurately estimate the image scale with a mean absolute percentage error of 3%. We also conducted automated shoeprint retrieval experiments on scale-unadjusted shoeprint images to show how the automated image scaling might be used in a common forensic process. Our results from these experiments show an increase in the retrieval performance from 0.735 to 0.929 at $S_1$ by employing this approach to adjust the shoeprint image scales.