BRIEF DESCRIPTION OF THE DRAWINGS
The invention, together with additional objectives, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which:
FIG. 1 is a schematic view illustrating overall construction of a visibility range measuring apparatus for a vehicle according to an embodiment of the present invention;
FIG. 2 is a schematic view illustrating a state, where front images that fall within an image capturing range, which is set to include a roadside object while a vehicle is running, are repeatedly taken;
FIG. 3 is a block diagram showing a configuration of an image processor of the visibility range measuring apparatus;
FIG. 4A is an illustrative view showing a distant image that is taken when a roadside object is located at a large distance from a vehicle;
FIG. 4B is an illustrative view showing a near image that is taken when the roadside object, which is shown in the distant image, is located adjacent to the vehicle;
FIG. 5 is a schematic diagram that shows a relationship between a forward distance to a roadside object and edge intensity of a front image that captures the roadside object when the front image is taken from a vehicle;
FIG. 6 is a schematic diagram that shows a visibility range conversion table to calculate a visibility range from a relationship between an edge intensity difference and a travel distance of a vehicle;
FIG. 7 is a flowchart showing a flow of an operation of an image processor in the visibility range measuring apparatus;
FIG. 8 is a schematic diagram that shows a relationship between an edge intensity difference and a travel distance of a vehicle when a margin is allowed for the edge intensity difference, which corresponds to the travel distance of the vehicle, according to a first modification of the embodiment; and
FIG. 9 is a schematic diagram that shows a conversion table to calculate a visibility range from a relationship between a frequency component value difference and a travel distance of a vehicle, according to a second modification of the embodiment.