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fine changes in intensity, filters of different sizes are used. Five different 
filters have been detected in the human visual system with filter widths 
of 4, 9, 17, 35, 63 pixels. 
Realizing the problems with area correlation, methods Using 
characteristic points have been proposed and implemented (/1/, /8/). 
Characteristic points represent a high gray value variance in all 
directions, and they can be determined with interest operators. Due to the 
high degree of nonlinearity in the mathematical model with a small radius 
of convergence, the approximations must be very good, on the order of | 
pixel size. This raises the question on how the human visual system copes 
with the problem of approximations. We are obviously capable of setting 
the fixation point at an average disparity level of that part of the the 
object at which we are looking. The depth information obtained from the 
coarsest channel is used in the next finer channel to prevent the matching 
‘of zero crossings that are not conjugent points since this would lead to a 
point in the object space that is not on a surface (false target problem). 
Hence in a multi- channel implementation no approximations are needed. 
Stereo Matching with Zero Crossings 
In the original implementation of the model of the human visual system as 
developed by Marr and Poggio, Grimson determinded the zero crossings in 
the filtered image along epipolar lines (see, eg,/3/) This requires 
resampling the original image such that the horizontal scanlines are in 
fact epipolar lines. The matching process then proceeds in an iterative 
manner, from coarse to fine channels. The search space for corresponding 
zero crossings is reduced in this approach from the entire image to one 
scan line and within the scan line to the range of twice the channel width, 
We, under consideration. With an average disparity, do. obtained from the 
computations from the coarser channel, the zero crossings are found 
within the interval [x + d, : Wes yl. In case several zero crossings are 
found within this interval, a disambiguation procedure considering surface 
continuity determines the most probable match. Recent modifications of 
the original implementation extend the search space to include several 
scan lines (/2/). This compensates for local distortions caused Dy the 
camera. 
-605.-