Processes for detecting a road plane area that can be traveled or detecting an obstacle that is present in a traveling environment in guiding a mobile object such as an automobile or the like are roughly classified into processes using a laser radar, ultrasonic waves, a milliwave radar, etc., and processes using images.
The detecting processes using a laser radar and a milliwave radar are problematic in that the apparatus used are generally expensive and a sufficient spatial resolution cannot be achieved. The detecting processes using ultrasonic waves are problematic in that it is difficult to make measurements in distant ranges and the spatial resolution is low.
The processes using images are grouped into single-eyed processes and double-eyed processes. Heretofore, many of the processes using images employ an image obtained from a single eye, i.e., a single viewpoint. They are assumed to be used in a well-maintained environment such as an expressway or the like, and detect a traveling area by detecting a white line on the road (for example, a separation line or a center line). However, in general roads and parking lots where the presence of white lines or the like is not guaranteed and various road plane colors and patterns are present, it is difficult to stably distinguish between areas that can be traveled and obstacles, only from a density pattern obtained from a single-eyed image.
The double-eyed processes, i.e., processes using a stereo image, make it possible to recognize a traveling environment more stably because they can make use of a three-dimensional structure of the traveling environment in principle. Particularly, since an area that can be traveled can be regarded essentially as a plane in space, there has been proposed a process of calculating a projection transformation matrix between images, transforming one of the images into a two-dimensional projection image according to the projection transformation matrix, overlapping the transformed image onto the other image, and detecting whether the image represents a plane area or not or an obstacle or not, based on the overlapping (see Patent Document 1, Non-patent Document 1).
For detecting another object from a mobile object such as an automobile or the like, it is desirable to detect the object in as short a period of time as possible and perform a real-time processing process. However, the conventional image-processing-based stereo matching process requires a lot of calculations and suffers a low processing rate.
In view of the above drawbacks, there has been proposed a stereo matching process for extracting image edges from a stereo image and thereafter selecting straight edges having a predetermined length or greater to recognize an object (for example, see Patent Document 2).    Patent Document 1: Laid-Open Patent Publication 2000-293693 (Japan);    Patent Document 2: Laid-Open Patent Publication 2004-30453 (Japan); and    Non-patent Document 1: Heung-Yeung Shum, Richard Szeliski, “Panoramic Image Mosaics” Technical report, 1997, MSR-TR-97-23, Microsoft Research.