Developing a new method to assess poor visibility level on roads by digital images

Abstract

This study proposes a new index for estimating visibility under poor-visibility conditions by using digital images from a road monitoring camera. The method allows representation of the amount of visual information based on human spatial frequency characteristics. A laboratory experiment was performed in 2002 under artificial fog. We examined the weighted intensity of power spectra (WIPS) to determine its applicability as a visibility assessment value. The magnitude of WIPS represented the difference in spatial frequencies within the image based on human contrast sensitivity function. WIPS was calculated from the following image-processing procedure. As an initial step, a box measuring 128 × 128 pixels was cut from the original image. The spatial frequency of the cutout image was calculated using two-dimensional Fourier transform, and the power spectrum of the cutout image was calculated. We totaled the WIPS at five spatial frequencies (cycles per degree, cpd): 1.5 cpd, 3 cpd, 6 cpd, 12 cpd and 18 cpd. WIPS were compared with the subjective visibility assessment values (SVAV) given by test subjects. There was a clear relationship between WIPS and total SVAV: WIPS increased as total SVAV increased. These results suggest that WIPS might be an appropriate index for estimating visibility using digital images.